Here at the Maple Lab, we specialize in research on correlated electron systems. From superconductors, to heavy fermions, to topological superconductors, we grow, characterize, and measure under extreme conditions these cutting edge materials in the hope that someday the knowledge we acquire will revolutionize the way we use technology.
Conducting surface state in d-electron Kondo insulator FeSi
Superconductivity in layered BiS2-based Compounds
A novel family of superconductors based on BiS2-based superconducting layers were discovered in 2012. In short order, other BiS2-based superconductors with the same or related crystal structures were discovered with superconducting critical temperatures Tc of up to 10 K. Many experimental and theoretical studies have been carried out with the goal of establishing the basic properties of these new materials and understanding the underlying mechanism for superconductivity. In this selective review of the literature, we distill the central discoveries from this extensive body of work, and discuss the results from different types of experiments on these materials within the context of theoretical concepts and models. Read More
Weak hybridization and isolated localized magnetic moments in the compounds CeT2Cd20
The CeT2Cd20 (T = Ni, Pd) systems have a large separation between Ce ions, limiting the Ruderman-Kittel-Kasuya-Yosida interaction between the localized moments of these ions. However, these compounds were not found to order magnetically down to 0.138 K and under applied pressure of up to 2.5 GPa. This fragile magnetic order may have potential applications as a material for performing adiabatic demagnetization. Read More
Nodal to Nodeless Superconducting Energy-Gap Structure Change Concomitant with Fermi-Surface Reconstruction in the Heavy-Fermion Compound CeCoIn5
London penetration depth measurements were made on the system Ce1-xRxCoIn5 (R = La, Nd, Yb) down to T ≈ 50 mK. The power law behavior of the London penetration depth is Δλ(T) ~ Tn describes the Ce1-xRxCoIn5 system, where the value of n corresponds to the structure of the superconducting gap. In the case of CeCoIn5, n = 1, consistent with line nodes in the SC gap. As the rare earths (La, Nd, Yb) are doped into this system, TC is suppressed and n evolves. For La and Nd, n increases from 1 and saturates at about 2, which shows a change to a “dirty”, nodal superconductor. However, for Yb, n increases above 3, indicating an evolution to a nodeless SC gap. These SC gap changes occur at about the same doping range as the reported Fermi surface reconstruction, suggesting the two phenomena are closely related. Read More
Enhancement of superconductivity in La1-xSmxO0.5F0.5BiS2
The superconducting transition temperature Tc and superconducting volume fraction were found to increase with x in the La1-xSmxO0.5F0.5BiS2 samples. The solubility limit of Sm has a large value of x ~ 0.8 in La1-xSmxO0.5F0.5BiS2, and a continuous decrease in the a-axis and increase in the c-axis is observed with increasing x. Bulk superconductivity was observed in the samples according to magnetic susceptibility and specific heat measurements. No evidence for a structural phase transition was found in this study. The results demonstrate that the superconducting critical temperature Tc of tetragonal BiS2-based compounds is correlated with the lattice parameters and can be significantly enhanced by Sm substitution. This gives a promising way to further increase the Tc of BiS2-based superconductors by modifying the blocking layers through the substitution of heavier Ln lanthanides (Ln = Eu – Tm) or synthesizing the parent LnO1-xFxBiS2 compounds. Read More
Electrodynamics of the antiferromagnetic phase of URu2Si2
In this work, two ordered phases of URu2Si2 have been studied spectroscopically: the hidden order phase and the antiferromagnetic phase, which is induced by Fe and Os substitution. The two phases show few differences other than an overall smooth increase in the gap and the transition temperature with substitution of Fe and Os. In the ordered states the gap and the transfer of spectral weight are characteristic of density waves and consistent with a partial gapping scenario. The normal states are also very similar: they are Fermi-liquid-like with a scaling factor b ≈ 1.0 characteristic of a Fermi liquid dominated by resonant impurity scattering. Read More
Pressure studies of the quantum critical alloy Ce0.93Yb0.07CoIn5
Heavy-fermion compound Ce0.93Yb0.07CoIn5 was further studied through electrical transport measurements under pressure due to having a well-established magnetic field-induced quantum critical point. Key findings include the observation of a suppression of the antiferromagnetic ordering quantum fluctuations as pressure is increased demonstrated by the behavior of the linear T term in resistivity corresponding to a heavier quasiparticle; in contrast, the contribution to the √T resistivity term remains fairly stable with pressure, suggesting that the inelastic scattering from separate, light effective mass quasiparticles is unaffected by applied pressure. Read More
Pressure-induced phase transition in La1-xSmxO0.5F0.5BiS2
Electrical resistivity measurements on polycrystalline samples of the BiS2-based superconductors La1-xSmxO0.5F0.5BiS2 (x = 0.1, 0.3, 0.6, 0.8) were performed from 2 K to room temperature under applied pressures. In the normal state, semiconducting-like behavior is suppressed with increasing pressure. A reversible low-Tc to high-Tc superconductor phase transition was observed in all of the samples at a pressure that is proportional to the Sm concentration. With increasing Sm concentration, Tc is suppressed, and a larger pressure is necessary to induce the transition from the SC1 phase to the SC2 phase. It is also found that an optimal Tc could be tuned by decreasing the a lattice parameter in the SC1 phase at ambient pressure or by increasing a in the SC2 phase under pressure. These results indicate that the high-pressure behavior of Sm-substituted LaO0.5F0.5BiS2 is largely determined by Sm concentration or the lattice parameter a at ambient pressure. Therefore, the evolution of Tc under pressure for the parent compound SmO0.5F0.5BiS2 can be estimated, and we find that the SC1 and SC2 phases exhibit almost indistinguishable Tc values; this result suggests that applied pressure may not induce a phase transition in SmO0.5F0.5BiS2. Read More
Investigation of superconducting and normal-state properties of the filled-skutterudite system PrPt4Ge12-xSbx
We have performed experiments on the pseudoternary alloy system PrPt4Ge12-xSbx to probe the unconventional superconductivity in PrPt4Ge12. We observed a suppression of Tc with x to x = 4. The electronic specific heat coefficient γ decreases with x in the superconducting region, indicating that the density of states may be an important factor in determining Tc. A Pr “rattling” mode with an Einstein temperature ϴE ≈ 60 K for 0 ≤ x ≤ 5 was derived from Rietveld refinement of XRD data and fits of an Einstein model to specific heat data; however, no correlation between Tc and ϴE was observed. The exponential T-dependence of the specific heat in the superconducting state for a sample with x = 0.5 suggests a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity with increasing x. Read More
Broken time-reversal symmetry in superconducting Pr1-xCexPt4Ge12
We report results of zero-field muon spin relaxation experiments on the filled-skutterudite superconductors Pr1-xCexPt4Ge12 to investigate the effect of Ce doping on broken time-reversal symmetry (TRS) in the superconducting state. In these alloys broken TRS is signaled by the onset of a spontaneous static local magnetic field Bs below the superconducting transition temperature. We find that Bs decreases linearly with x →0 at x≈0.4, close to the concentration above which superconductivity is no longer observed. The (Pr,Ce)Pt4Ge12 and isostructural (Pr,La)Os4Sb12 alloy series both exhibit superconductivity with broken TRS, and in both the decrease of Bs is proportional to the decrease of Pr concentration. This suggests that Pr-Pr intersite interactions are responsible for the broken TRS. The two alloy series differ in that the La-doped alloys are superconducting for all La concentrations, suggesting that in (Pr,Ce)Pt4Ge12 pair-breaking by Ce doping suppresses superconductivity. Read More
Quantum criticality and superconducting pairing in Ce1-xYbxCoIn5 alloys
Electrical transport measurements were made on the heavy-fermion system Ce1-xYbxCoIn5 under high fields and pressures. One particular observation was that the field-induced quantum critical point was completely suppressed at xnominal = 0.2. Also the superconducting critical temperature Tc and Kondo lattice coherence temperature Tcoh were both suppressed as x was increased. Of particular interest was the evolution of Tc and Tcoh as pressure was applied; what we observed was both parameters were both enhanced as pressure was increased. This is apparently conflicting with the theory for composite pairing where both parameters should behave opposite each other as pressure is applied. This particular question is very intriguing for further study (ref: Pressure studies of the quantum critical alloy Ce0.93Yb0.07CoIn5). Read More
Chemical Substitution and High Pressure Effects on Superconductivity in the LnOBiS2 (Ln = La-Nd) System
A large number of compounds which contain BiS2 layers exhibit enhanced superconductivity upon electron doping. Much interest and research effort has been focused on BiS2-based compounds which provide new opportunities for exploring the nature of superconductivity. Important to the study of BiS2-based superconductors is the relation between structure and superconductivity. By modifying either the superconducting BiS2 layers or the blocking layers in these layered compounds, one can effectively tune the lattice parameters, local atomic environment, electronic structure, and other physical properties of these materials. Some of the recent progress on research of the effects of chemical substitution in BiS2-based compounds, with special attention given to the compounds in the LnOBiS2 (Ln = La-Nd) system, are reviewed in this article. Strategies which are reported to be essential in optimizing superconductivity of these materials will also be discussed. Read More
Structure and physical properties of RT2Cd20 (R = rare earth, T = Ni, Pd) compounds with the CeCr2Al20-type structure
Eleven new RNi2Cd20 (R = Y, La-Nd, Sm, Gd, Tb) and RPd2Cd20 (R = Ce, Pr, Sm) were grown in high temperature, cadmium-rich solutions. These compounds formed as single crystals in the CeCr2Al20-type structure. Electrical resistivity, magnetization, and specific heat measurements were performed and the results discussed. These new X=Cd compounds allow for further investigations of correlated electron behavior in the RT2X20 family, which may not be present in the X=Al, Zn series of compounds. Read More
Investigation of magnetic order in SmTr2Zn20 (Tr = Fe, Co, Ru) and SmTr2Cd20 (Tr = Ni, Pd)
In a study of the caged 1-2-20 compounds SmTr2Zn20 (Tr = Fe, Co, Ru) and SmTr2Cd20 (Tr = Ni, Pd), the low temperature magnetic orders of these compounds is explored. Enhanced Sommerfeld coefficients and quadratic temperature dependence of low temperature resistivity suggest that SmRu2Zn20 and SmPd2Cd20 experience enhanced quasiparticle mass due to hybridization between localized 4ƒ and conduction electrons. It is suggested that SmRu2Zn20 is a member of a rare class of Sm-based heavy fermion ferromagnets. Read More
Chemical pressure tuning of URu2Si2 via isoelectronic substitution of Ru with Fe
Specific heat and neutron diffraction measurements are used to support the conclusion that chemical substitution of Ru with Fe acts as a “chemical pressure” in the URu2Si2 system. This chemical pressure effect of adding iron seems to mimic the effects of pressure upon the URu2Si2 system, providing a powerful way of measuring the pressure effects on the system at ambient pressures. Thus, this provides a means for exploring the mysterious “hidden order” of the URu2Si2 system. Read More
Conventional Magnetic Superconductors
Long-range magnetic order typically acts to destroy superconductivity. However, there are a host of “conventional magnetic superconductors” in which the interplay between superconductivity and magnetism give rise to some exotic and interesting behavior including reentrant superconductivity, magnetic field induced superconductivity, and the coexistence of antiferromagnetic order and superconductivity. This article addresses the effect of magnetic impurities on Tc in binary materials that led to the idea of a magnetic exchange interaction between the localized spins of the magnetic impurity ions and the spins of the conduction electrons. This is followed by a discussion of several classes of superconducting materials including ternary and quaternary compounds. Read More
Pressure-induced enhancement of superconductivity and suppression of semiconducting behavior in LnO0.5F0.5BiS2 (Ln = La, Ce) compounds
The superconducting transition temperature Tc observed for some of the high-Tc cuprates are known to respond dramatically to pressure. We found that the BiS2-based superconducting materials, which form in a layered crystal structure similar to the cuprate materials also exhibit large jumps in their values of Tc. For the LaO0.5F0.5BiS2 and CeO0.5F0.5BiS2 compounds we found that by applying pressure, we could induce a phase transition from a low-Tc phase to a high-Tc phase. In LaO0.5F0.5BiS2, there is a threefold enhancement of superconductivity as Tc rapidly jumps from 3.3 K at ambient pressure to 10 K at a critical pressure of PT ~ 1 GPa. Read More
Enhancement of superconductivity near the pressure-induced semiconductor-metal transition in the BiS2-based superconductors LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd)
This is a follow up study to our original investigation of the BiS2-based layered superconducting materials under pressure in which we observed a pressure-induced phase transition and an associated enhancement of superconductivity in the two LnO0.5F0.5BiS2 (Ln = La, Ce) compounds. We performed similar measurements of electrical resistivity for the LnO0.5F0.5BiS2 (Ln = Pr, Nd) compounds under pressure in which we observed a similar pressure dependence of Tc characterized by a rapid increase in Tc within a small range of pressure ~ 0.3 GPa from a low-Tc phase to a high-Tc phase. We observed that for the four LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd) compounds, both the critical pressure PT and the size of the jump in Tc both scale with the lanthanide element in LnO0.5F0.5BiS2. Read More
Evolution of critical pressure with increasing Fe substitution in the heavy-fermion system URu2-xFexSi2
Recently, it has been shown that the isoelectronic substitution of smaller ions of Fe for Ru in the heavy-fermion compound URu2Si2 acts as a chemical pressure Pch in which the evolution of T0 with Fe concentration x is remarkably consistent with the pressure dependence of T0. In this article, we report on the combined effect of Fe substitution and the application of pressure in the URu2-xFexSi2 system with (x = 0.025, 0.05, 0.10, 0.15 and 0.20). We found that the critical pressure was reduced from Pc = 1.2 GPa at x = 0.025 to Pc = 0 GPa at x = 0.15. Furthermore, by converting the concentration of Fe to a chemical pressure, Pch(x), we consistently found that the induced HO to LMAFM phase transition occurred at various combinations of x and P such that Pch(x) + P = 1.5 GPa. Read More
Equipped with various sample synthesis and characterization tools, our lab has the capabilities of making single crystalline, polycrytalline, and thin film samples. The low temperature and high pressure facilities enable us to measure:
- electrical resistivity and ac magnetic susceptibility down to 10 mK and up to 11 T, under nearly hydrostatic pressure up to 3 GPa or non-hydrostatic pressure up to 100 GPa
- dc magnetic susceptibility down to 0.3 K and up to 7 T
- specific heat down to 0.5 K and up to 9 T
- magnetoresistance, thermalconductivity, thermoelectric power, Hall effect down to 1.8 K and up to 9 T
The tetra arc furnace is used to grow samples using the Czochralski method. The sample is spun and four electrodes are used to melt the sample homogenously. The homogenous melt is slowly pulled up using the pulling rod, allowing the crystal to form.
The optical floating zone image furnace is used to grow crystals using the floating zone method. This is done by moving a liquid zone through the material.
Arc furnaces use electrodes with high currents to melt elements into polycrystal samples.
One method of growing new crystals is through a flux growth. In this type of sample preparation, the constituent components of a crystal are heated to high temperature in order to melt together, with one constituent being in excess, and thus forming the “flux”. These high temperature furnaces allow us to heat materials to temperatures above 1000 K
It is often desirable to isolate samples from the gases in the air to avoid a reaction. To this end, samples and mixtures of samples are often sealed into quartz tubes under vacuum. The oxygen/hydrogen torch reaches temperatures hot enough to seal the quartz tubes allowing the tubes to be placed into a furnace where the elements inside can be melted into a new crystal.
The pulsed laser deposition system is used to create thin film samples. This is done by using a pulsed laser on a target polycrystal to generate a plasma plume which is then attracted to a substrate, allowing a thin film to be grown on the substrate.
Glove boxes are used to work with materials in an enviornment free from the various gases in the air. Sensitive materials can be stored and manipulated in these boxes, whcih feature gloves built-in to allow researchers to handle the samples.
The gold sputtering machine creates a plasma of gold that is attracted towards the sample. This causes a thin coat of gold to be applied to the sample. This, along with masking, is used to create electrically conductive areas on the sample which wire leads can be wired to.
Fume hoods are used to safely deal with chemicals that may produce dangerous gases. These are typically used in our lab to clean samples or pieces of equipment in acid baths.
Nomarski microscopy is used to increase the contrast in a sample using the inteference of light through the sample and the optical path length of the light.
The metallographic microscope is a very powerful microscope. It is able to pick up metallographic features, and is useful for making precise measurements of samples.
In order to ensure the right amounts of elements are mixed in a growth, it is necessary to carefully measure the mass of each material. These balances offer a high resolution to accurately determine the mass of samples.
In order to measure the mass of very small samples, it is necessary to use a very sensitive scale. Crystal samples prepared in our lab are often measured with the microbalance to determine their mass.
Diamond wheel saws are used to cut and shape samples for different measurements.
The wire saw is used to cut and shape samples. The wire saw is most effective at cutting brittle or fragile samples. It works by using the wire to drag an abrasive slurry quickly across the sample.
The mini lathe and mini mill can be used to precisely machine samples or materials to specific shapes when this is required.
Ball mills spin a platform filled with balls, which can be used to crush samples into a fine powdered form. It can be used to create nanoparticles and powder samples.
The Cold Isostatic Press is used to compact powder samples. It is capable of applying uniform pressure to the sample from all directions.
Vibromet polishers are able to create a very flat surface on a sample by polishing it with a rotating disk.
Our group has an extensive stock of elements, which we use in our crystal growing processes to create many new kinds of crystals to study.
Minifridge is one of our lab’s three dilution refrigerators. Dilution refrigerators work by using the phase separation of 3He-4He mixture to cool to extremely low temperatures. Minifridge is capable of reaching ~40 mK, and features an 11 T superconducting magnet.
Nautilus is one of our lab’s three dilution refrigerators. Dilution refrigerators work by using the phase separation of 3He-4He mixture to cool to extremely low temperatures. Nautilus is capable of reaching ~10 mK, and features an 8 T superconducting magnet.
Thor is one of our lab’s three dilution refrigerators. Dilution refrigerators work by using the phase separation of 3He-4He mixture to cool to extremely low temperatures. Thor is capable of reaching ~10 mK, and features an 9 T superconducting magnet. Thor also features a “sliding seal” which allows the cryostat to be removed and reinserted while the dewar is cold.
The 3He Calorimeter uses evaporative cooling of helium-3 to reach low temperatures. The calorimeter is able to measure specific heat of samples down to temperatures of 0.5 K, and features a 6 Tesla superconducting magnet.
The 3He Faraday magnetometer uses evaporative cooling of helium-3 to reach low temperatures. The Faraday magnetometer is able to measure magnetization of samples down to temperatures of 0.5 K, and features a 5.5 Tesla superconducting magnet.
A Superconducting QUantum Interference Device (SQUID) uses josephson junctions to precisely measure magnetic fields. Using this, our Quantum Design Magnetic Properties Measurement System (MPMS) allows us to accurately measure a sample’s magnetization from 2 K to 400 K. The MPMS also features a 7 Tesla magnet.
Our lab features two Quantum Design Dynacool Physical Properties Measurement Systems (PPMS), one of which is the very first Dynacool ever created by Quantum Design. These are highly automated systems which allow measurement of various properties of materials such as resistivity, specific heat, thermal conductivity, thermoelectric power and many others, by using a modular measurement system. The Dynacool itself uses a pulse tube cryocooler, allowing it to reach low temperatures without the need for an external source of liquid helium. The Dynacool has a temperature range of 1.8 K to 400 K and features a 9 Tesla superconducting magnet.
Our lab has two liquid helium cryostats, which are filled with liquid helium, then pumped on, to reach temperatures as low as 1.2 K. We use these to measure low temperature resistivity under pressure and at ambient pressure.
Helium is a precious resource, but important in cryogenic systems for reaching low temperatures. Our lab features an extensive helium recovery system which is hooked to our equipment and gives us the oppoortunity to recycle the helium we use. This system is attached to a Quantum Design ATL liquifier which liquifies the helium we recover allowing us to use the liquid helium in our experiments.
Diamond anvil cells are used to apply extreme pressure to samples by squeezing them between two diamonds. These setups can reach over 1 Mbar of pressure.
Bridgman anvil cells feature two opposing anvils that are pressed together to apply pressure to a sample. The Bridgman cells in our lab can reach pressures between 0.5 and 160 kbar.
Hydrostatic-piston cylinder cells apply pressure to a small capsule containing a liquid around the sample. This causes the hydrostatic pressure the liquid applies on the sample to increase. The cells in our lab can reach pressures of up to 25 kbar.
Our group has a long history of important achievements and contributions to the community of condensed matter physics. Some information of particular historical note is provided in the pages below.
The Woodstock of physics refers to the American Physical Society’s meeting on March 18, 1987 regarding high-temperature superconductors. The session, which was chaired by Prof. Maple, was added last-minute to their annual meeting, to discuss the new research into the recently discovered high-temperature superconductors, which remained superconducting at liquid nitrogen temperatures.
Excited scientists started lining up for the session well before the time it was scheduled to begin, and many had to watch from outside using camera feeds set up by the hotel. The session went well into the night, ending at 3:15 am, though many stayed longer to discuss the presentations. This meeting led to real excitement in both the scientific community and the mainstream media over the possibilities in high temperature superconductors. Many laboratories began searching for new breakthroughs to push the critical temperature of superconductivity even higher.
Footage of the entire session of the Woodstock of Physics is available below in 8 parts, courtesy of APS
- Research Highlights
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Conducting surface state in d-electron Kondo insulator FeSiThe compound FeSi has been the focus of intense research efforts due to its unusual electrical, magnetic, and lattice properties, which are not well understood. We discovered a semiconducting-to-metallic cross-over with decreasing temperature at ~19 K, which is not accompanied by any bulk features, in single-crystal samples of FeSi. The low-temperature metallic behavior can be significantly enhanced by reducing the width/thickness of the sample. The application of an external magnetic field easily suppresses the electrical resistivity at low temperatures but does not have a comparable effect on the temperature below which metallic conduction occurs. Since the surface conduction of FeSi is very robust under magnetic fields and is significant at only <19 K, the negative magneto-resistivity of the sample should be associated with the bulk properties of the samples.
Superconductivity in layered BiS2-based Compounds
A novel family of superconductors based on BiS2-based superconducting layers were discovered in 2012. In short order, other BiS2-based superconductors with the same or related crystal structures were discovered with superconducting critical temperatures Tc of up to 10 K. Many experimental and theoretical studies have been carried out with the goal of establishing the basic properties of these new materials and understanding the underlying mechanism for superconductivity. In this selective review of the literature, we distill the central discoveries from this extensive body of work, and discuss the results from different types of experiments on these materials within the context of theoretical concepts and models. Read More
Weak hybridization and isolated localized magnetic moments in the compounds CeT2Cd20
The CeT2Cd20 (T = Ni, Pd) systems have a large separation between Ce ions, limiting the Ruderman-Kittel-Kasuya-Yosida interaction between the localized moments of these ions. However, these compounds were not found to order magnetically down to 0.138 K and under applied pressure of up to 2.5 GPa. This fragile magnetic order may have potential applications as a material for performing adiabatic demagnetization. Read More
Nodal to Nodeless Superconducting Energy-Gap Structure Change Concomitant with Fermi-Surface Reconstruction in the Heavy-Fermion Compound CeCoIn5
London penetration depth measurements were made on the system Ce1-xRxCoIn5 (R = La, Nd, Yb) down to T ≈ 50 mK. The power law behavior of the London penetration depth is Δλ(T) ~ Tn describes the Ce1-xRxCoIn5 system, where the value of n corresponds to the structure of the superconducting gap. In the case of CeCoIn5, n = 1, consistent with line nodes in the SC gap. As the rare earths (La, Nd, Yb) are doped into this system, TC is suppressed and n evolves. For La and Nd, n increases from 1 and saturates at about 2, which shows a change to a “dirty”, nodal superconductor. However, for Yb, n increases above 3, indicating an evolution to a nodeless SC gap. These SC gap changes occur at about the same doping range as the reported Fermi surface reconstruction, suggesting the two phenomena are closely related. Read More
Enhancement of superconductivity in La1-xSmxO0.5F0.5BiS2
The superconducting transition temperature Tc and superconducting volume fraction were found to increase with x in the La1-xSmxO0.5F0.5BiS2 samples. The solubility limit of Sm has a large value of x ~ 0.8 in La1-xSmxO0.5F0.5BiS2, and a continuous decrease in the a-axis and increase in the c-axis is observed with increasing x. Bulk superconductivity was observed in the samples according to magnetic susceptibility and specific heat measurements. No evidence for a structural phase transition was found in this study. The results demonstrate that the superconducting critical temperature Tc of tetragonal BiS2-based compounds is correlated with the lattice parameters and can be significantly enhanced by Sm substitution. This gives a promising way to further increase the Tc of BiS2-based superconductors by modifying the blocking layers through the substitution of heavier Ln lanthanides (Ln = Eu – Tm) or synthesizing the parent LnO1-xFxBiS2 compounds. Read More
Electrodynamics of the antiferromagnetic phase of URu2Si2
In this work, two ordered phases of URu2Si2 have been studied spectroscopically: the hidden order phase and the antiferromagnetic phase, which is induced by Fe and Os substitution. The two phases show few differences other than an overall smooth increase in the gap and the transition temperature with substitution of Fe and Os. In the ordered states the gap and the transfer of spectral weight are characteristic of density waves and consistent with a partial gapping scenario. The normal states are also very similar: they are Fermi-liquid-like with a scaling factor b ≈ 1.0 characteristic of a Fermi liquid dominated by resonant impurity scattering. Read More
Pressure studies of the quantum critical alloy Ce0.93Yb0.07CoIn5
Heavy-fermion compound Ce0.93Yb0.07CoIn5 was further studied through electrical transport measurements under pressure due to having a well-established magnetic field-induced quantum critical point. Key findings include the observation of a suppression of the antiferromagnetic ordering quantum fluctuations as pressure is increased demonstrated by the behavior of the linear T term in resistivity corresponding to a heavier quasiparticle; in contrast, the contribution to the √T resistivity term remains fairly stable with pressure, suggesting that the inelastic scattering from separate, light effective mass quasiparticles is unaffected by applied pressure. Read More
Pressure-induced phase transition in La1-xSmxO0.5F0.5BiS2
Electrical resistivity measurements on polycrystalline samples of the BiS2-based superconductors La1-xSmxO0.5F0.5BiS2 (x = 0.1, 0.3, 0.6, 0.8) were performed from 2 K to room temperature under applied pressures. In the normal state, semiconducting-like behavior is suppressed with increasing pressure. A reversible low-Tc to high-Tc superconductor phase transition was observed in all of the samples at a pressure that is proportional to the Sm concentration. With increasing Sm concentration, Tc is suppressed, and a larger pressure is necessary to induce the transition from the SC1 phase to the SC2 phase. It is also found that an optimal Tc could be tuned by decreasing the a lattice parameter in the SC1 phase at ambient pressure or by increasing a in the SC2 phase under pressure. These results indicate that the high-pressure behavior of Sm-substituted LaO0.5F0.5BiS2 is largely determined by Sm concentration or the lattice parameter a at ambient pressure. Therefore, the evolution of Tc under pressure for the parent compound SmO0.5F0.5BiS2 can be estimated, and we find that the SC1 and SC2 phases exhibit almost indistinguishable Tc values; this result suggests that applied pressure may not induce a phase transition in SmO0.5F0.5BiS2. Read More
Investigation of superconducting and normal-state properties of the filled-skutterudite system PrPt4Ge12-xSbx
We have performed experiments on the pseudoternary alloy system PrPt4Ge12-xSbx to probe the unconventional superconductivity in PrPt4Ge12. We observed a suppression of Tc with x to x = 4. The electronic specific heat coefficient γ decreases with x in the superconducting region, indicating that the density of states may be an important factor in determining Tc. A Pr “rattling” mode with an Einstein temperature ϴE ≈ 60 K for 0 ≤ x ≤ 5 was derived from Rietveld refinement of XRD data and fits of an Einstein model to specific heat data; however, no correlation between Tc and ϴE was observed. The exponential T-dependence of the specific heat in the superconducting state for a sample with x = 0.5 suggests a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity with increasing x. Read More
Broken time-reversal symmetry in superconducting Pr1-xCexPt4Ge12
We report results of zero-field muon spin relaxation experiments on the filled-skutterudite superconductors Pr1-xCexPt4Ge12 to investigate the effect of Ce doping on broken time-reversal symmetry (TRS) in the superconducting state. In these alloys broken TRS is signaled by the onset of a spontaneous static local magnetic field Bs below the superconducting transition temperature. We find that Bs decreases linearly with x →0 at x≈0.4, close to the concentration above which superconductivity is no longer observed. The (Pr,Ce)Pt4Ge12 and isostructural (Pr,La)Os4Sb12 alloy series both exhibit superconductivity with broken TRS, and in both the decrease of Bs is proportional to the decrease of Pr concentration. This suggests that Pr-Pr intersite interactions are responsible for the broken TRS. The two alloy series differ in that the La-doped alloys are superconducting for all La concentrations, suggesting that in (Pr,Ce)Pt4Ge12 pair-breaking by Ce doping suppresses superconductivity. Read More
Quantum criticality and superconducting pairing in Ce1-xYbxCoIn5 alloys
Electrical transport measurements were made on the heavy-fermion system Ce1-xYbxCoIn5 under high fields and pressures. One particular observation was that the field-induced quantum critical point was completely suppressed at xnominal = 0.2. Also the superconducting critical temperature Tc and Kondo lattice coherence temperature Tcoh were both suppressed as x was increased. Of particular interest was the evolution of Tc and Tcoh as pressure was applied; what we observed was both parameters were both enhanced as pressure was increased. This is apparently conflicting with the theory for composite pairing where both parameters should behave opposite each other as pressure is applied. This particular question is very intriguing for further study (ref: Pressure studies of the quantum critical alloy Ce0.93Yb0.07CoIn5). Read More
Chemical Substitution and High Pressure Effects on Superconductivity in the LnOBiS2 (Ln = La-Nd) SystemA large number of compounds which contain BiS2 layers exhibit enhanced superconductivity upon electron doping. Much interest and research effort has been focused on BiS2-based compounds which provide new opportunities for exploring the nature of superconductivity. Important to the study of BiS2-based superconductors is the relation between structure and superconductivity. By modifying either the superconducting BiS2 layers or the blocking layers in these layered compounds, one can effectively tune the lattice parameters, local atomic environment, electronic structure, and other physical properties of these materials. Some of the recent progress on research of the effects of chemical substitution in BiS2-based compounds, with special attention given to the compounds in the LnOBiS2 (Ln = La-Nd) system, are reviewed in this article. Strategies which are reported to be essential in optimizing superconductivity of these materials will also be discussed. Read More
Structure and physical properties of RT2Cd20 (R = rare earth, T = Ni, Pd) compounds with the CeCr2Al20-type structureEleven new RNi2Cd20 (R = Y, La-Nd, Sm, Gd, Tb) and RPd2Cd20 (R = Ce, Pr, Sm) were grown in high temperature, cadmium-rich solutions. These compounds formed as single crystals in the CeCr2Al20-type structure. Electrical resistivity, magnetization, and specific heat measurements were performed and the results discussed. These new X=Cd compounds allow for further investigations of correlated electron behavior in the RT2X20 family, which may not be present in the X=Al, Zn series of compounds. Read More
Investigation of magnetic order in SmTr2Zn20 (Tr = Fe, Co, Ru) and SmTr2Cd20 (Tr = Ni, Pd)In a study of the caged 1-2-20 compounds SmTr2Zn20 (Tr = Fe, Co, Ru) and SmTr2Cd20 (Tr = Ni, Pd), the low temperature magnetic orders of these compounds is explored. Enhanced Sommerfeld coefficients and quadratic temperature dependence of low temperature resistivity suggest that SmRu2Zn20 and SmPd2Cd20 experience enhanced quasiparticle mass due to hybridization between localized 4ƒ and conduction electrons. It is suggested that SmRu2Zn20 is a member of a rare class of Sm-based heavy fermion ferromagnets. Read More
Chemical pressure tuning of URu2Si2 via isoelectronic substitution of Ru with FeSpecific heat and neutron diffraction measurements are used to support the conclusion that chemical substitution of Ru with Fe acts as a “chemical pressure” in the URu2Si2 system. This chemical pressure effect of adding iron seems to mimic the effects of pressure upon the URu2Si2 system, providing a powerful way of measuring the pressure effects on the system at ambient pressures. Thus, this provides a means for exploring the mysterious “hidden order” of the URu2Si2 system. Read More
Conventional Magnetic SuperconductorsLong-range magnetic order typically acts to destroy superconductivity. However, there are a host of “conventional magnetic superconductors” in which the interplay between superconductivity and magnetism give rise to some exotic and interesting behavior including reentrant superconductivity, magnetic field induced superconductivity, and the coexistence of antiferromagnetic order and superconductivity. This article addresses the effect of magnetic impurities on Tc in binary materials that led to the idea of a magnetic exchange interaction between the localized spins of the magnetic impurity ions and the spins of the conduction electrons. This is followed by a discussion of several classes of superconducting materials including ternary and quaternary compounds. Read More
Pressure-induced enhancement of superconductivity and suppression of semiconducting behavior in LnO0.5F0.5BiS2 (Ln = La, Ce) compoundsThe superconducting transition temperature Tc observed for some of the high-Tc cuprates are known to respond dramatically to pressure. We found that the BiS2-based superconducting materials, which form in a layered crystal structure similar to the cuprate materials also exhibit large jumps in their values of Tc. For the LaO0.5F0.5BiS2 and CeO0.5F0.5BiS2 compounds we found that by applying pressure, we could induce a phase transition from a low-Tc phase to a high-Tc phase. In LaO0.5F0.5BiS2, there is a threefold enhancement of superconductivity as Tc rapidly jumps from 3.3 K at ambient pressure to 10 K at a critical pressure of PT ~ 1 GPa. Read More
Enhancement of superconductivity near the pressure-induced semiconductor-metal transition in the BiS2-based superconductors LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd)This is a follow up study to our original investigation of the BiS2-based layered superconducting materials under pressure in which we observed a pressure-induced phase transition and an associated enhancement of superconductivity in the two LnO0.5F0.5BiS2 (Ln = La, Ce) compounds. We performed similar measurements of electrical resistivity for the LnO0.5F0.5BiS2 (Ln = Pr, Nd) compounds under pressure in which we observed a similar pressure dependence of Tc characterized by a rapid increase in Tc within a small range of pressure ~ 0.3 GPa from a low-Tc phase to a high-Tc phase. We observed that for the four LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd) compounds, both the critical pressure PT and the size of the jump in Tc both scale with the lanthanide element in LnO0.5F0.5BiS2. Read More
Evolution of critical pressure with increasing Fe substitution in the heavy-fermion system URu2-xFexSi2Recently, it has been shown that the isoelectronic substitution of smaller ions of Fe for Ru in the heavy-fermion compound URu2Si2 acts as a chemical pressure Pch in which the evolution of T0 with Fe concentration x is remarkably consistent with the pressure dependence of T0. In this article, we report on the combined effect of Fe substitution and the application of pressure in the URu2-xFexSi2 system with (x = 0.025, 0.05, 0.10, 0.15 and 0.20). We found that the critical pressure was reduced from Pc = 1.2 GPa at x = 0.025 to Pc = 0 GPa at x = 0.15. Furthermore, by converting the concentration of Fe to a chemical pressure, Pch(x), we consistently found that the induced HO to LMAFM phase transition occurred at various combinations of x and P such that Pch(x) + P = 1.5 GPa. Read More
- Facilities
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Equipped with various sample synthesis and characterization tools, our lab has the capabilities of making single crystalline, polycrytalline, and thin film samples. The low temperature and high pressure facilities enable us to measure:
- electrical resistivity and ac magnetic susceptibility down to 10 mK and up to 11 T, under nearly hydrostatic pressure up to 3 GPa or non-hydrostatic pressure up to 100 GPa
- dc magnetic susceptibility down to 0.3 K and up to 7 T
- specific heat down to 0.5 K and up to 9 T
- magnetoresistance, thermalconductivity, thermoelectric power, Hall effect down to 1.8 K and up to 9 T
Sample PreparationThe tetra arc furnace is used to grow samples using the Czochralski method. The sample is spun and four electrodes are used to melt the sample homogenously. The homogenous melt is slowly pulled up using the pulling rod, allowing the crystal to form.
The optical floating zone image furnace is used to grow crystals using the floating zone method. This is done by moving a liquid zone through the material.
Arc furnaces use electrodes with high currents to melt elements into polycrystal samples.
One method of growing new crystals is through a flux growth. In this type of sample preparation, the constituent components of a crystal are heated to high temperature in order to melt together, with one constituent being in excess, and thus forming the “flux”. These high temperature furnaces allow us to heat materials to temperatures above 1000 K
It is often desirable to isolate samples from the gases in the air to avoid a reaction. To this end, samples and mixtures of samples are often sealed into quartz tubes under vacuum. The oxygen/hydrogen torch reaches temperatures hot enough to seal the quartz tubes allowing the tubes to be placed into a furnace where the elements inside can be melted into a new crystal.
The pulsed laser deposition system is used to create thin film samples. This is done by using a pulsed laser on a target polycrystal to generate a plasma plume which is then attracted to a substrate, allowing a thin film to be grown on the substrate.
Glove boxes are used to work with materials in an enviornment free from the various gases in the air. Sensitive materials can be stored and manipulated in these boxes, whcih feature gloves built-in to allow researchers to handle the samples.
The gold sputtering machine creates a plasma of gold that is attracted towards the sample. This causes a thin coat of gold to be applied to the sample. This, along with masking, is used to create electrically conductive areas on the sample which wire leads can be wired to.
Fume hoods are used to safely deal with chemicals that may produce dangerous gases. These are typically used in our lab to clean samples or pieces of equipment in acid baths.
Nomarski microscopy is used to increase the contrast in a sample using the inteference of light through the sample and the optical path length of the light.
The metallographic microscope is a very powerful microscope. It is able to pick up metallographic features, and is useful for making precise measurements of samples.
In order to ensure the right amounts of elements are mixed in a growth, it is necessary to carefully measure the mass of each material. These balances offer a high resolution to accurately determine the mass of samples.
In order to measure the mass of very small samples, it is necessary to use a very sensitive scale. Crystal samples prepared in our lab are often measured with the microbalance to determine their mass.
Diamond wheel saws are used to cut and shape samples for different measurements.
The wire saw is used to cut and shape samples. The wire saw is most effective at cutting brittle or fragile samples. It works by using the wire to drag an abrasive slurry quickly across the sample.
The mini lathe and mini mill can be used to precisely machine samples or materials to specific shapes when this is required.
Ball mills spin a platform filled with balls, which can be used to crush samples into a fine powdered form. It can be used to create nanoparticles and powder samples.
The Cold Isostatic Press is used to compact powder samples. It is capable of applying uniform pressure to the sample from all directions.
Vibromet polishers are able to create a very flat surface on a sample by polishing it with a rotating disk.
Our group has an extensive stock of elements, which we use in our crystal growing processes to create many new kinds of crystals to study.
Low Temperature ResearchMinifridge is one of our lab’s three dilution refrigerators. Dilution refrigerators work by using the phase separation of 3He-4He mixture to cool to extremely low temperatures. Minifridge is capable of reaching ~40 mK, and features an 11 T superconducting magnet.
Nautilus is one of our lab’s three dilution refrigerators. Dilution refrigerators work by using the phase separation of 3He-4He mixture to cool to extremely low temperatures. Nautilus is capable of reaching ~10 mK, and features an 8 T superconducting magnet.
Thor is one of our lab’s three dilution refrigerators. Dilution refrigerators work by using the phase separation of 3He-4He mixture to cool to extremely low temperatures. Thor is capable of reaching ~10 mK, and features an 9 T superconducting magnet. Thor also features a “sliding seal” which allows the cryostat to be removed and reinserted while the dewar is cold.
The 3He Calorimeter uses evaporative cooling of helium-3 to reach low temperatures. The calorimeter is able to measure specific heat of samples down to temperatures of 0.5 K, and features a 6 Tesla superconducting magnet.
The 3He Faraday magnetometer uses evaporative cooling of helium-3 to reach low temperatures. The Faraday magnetometer is able to measure magnetization of samples down to temperatures of 0.5 K, and features a 5.5 Tesla superconducting magnet.
A Superconducting QUantum Interference Device (SQUID) uses josephson junctions to precisely measure magnetic fields. Using this, our Quantum Design Magnetic Properties Measurement System (MPMS) allows us to accurately measure a sample’s magnetization from 2 K to 400 K. The MPMS also features a 7 Tesla magnet.
Our lab features two Quantum Design Dynacool Physical Properties Measurement Systems (PPMS), one of which is the very first Dynacool ever created by Quantum Design. These are highly automated systems which allow measurement of various properties of materials such as resistivity, specific heat, thermal conductivity, thermoelectric power and many others, by using a modular measurement system. The Dynacool itself uses a pulse tube cryocooler, allowing it to reach low temperatures without the need for an external source of liquid helium. The Dynacool has a temperature range of 1.8 K to 400 K and features a 9 Tesla superconducting magnet.
Our lab has two liquid helium cryostats, which are filled with liquid helium, then pumped on, to reach temperatures as low as 1.2 K. We use these to measure low temperature resistivity under pressure and at ambient pressure.
Helium is a precious resource, but important in cryogenic systems for reaching low temperatures. Our lab features an extensive helium recovery system which is hooked to our equipment and gives us the oppoortunity to recycle the helium we use. This system is attached to a Quantum Design ATL liquifier which liquifies the helium we recover allowing us to use the liquid helium in our experiments.
High Pressure ResearchDiamond anvil cells are used to apply extreme pressure to samples by squeezing them between two diamonds. These setups can reach over 1 Mbar of pressure.
Bridgman anvil cells feature two opposing anvils that are pressed together to apply pressure to a sample. The Bridgman cells in our lab can reach pressures between 0.5 and 160 kbar.
Hydrostatic-piston cylinder cells apply pressure to a small capsule containing a liquid around the sample. This causes the hydrostatic pressure the liquid applies on the sample to increase. The cells in our lab can reach pressures of up to 25 kbar.
- History
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Our group has a long history of important achievements and contributions to the community of condensed matter physics. Some information of particular historical note is provided in the pages below.
Woodstock of PhysicsThe Woodstock of physics refers to the American Physical Society’s meeting on March 18, 1987 regarding high-temperature superconductors. The session, which was chaired by Prof. Maple, was added last-minute to their annual meeting, to discuss the new research into the recently discovered high-temperature superconductors, which remained superconducting at liquid nitrogen temperatures.
Excited scientists started lining up for the session well before the time it was scheduled to begin, and many had to watch from outside using camera feeds set up by the hotel. The session went well into the night, ending at 3:15 am, though many stayed longer to discuss the presentations. This meeting led to real excitement in both the scientific community and the mainstream media over the possibilities in high temperature superconductors. Many laboratories began searching for new breakthroughs to push the critical temperature of superconductivity even higher.Footage of the entire session of the Woodstock of Physics is available below in 8 parts, courtesy of APS
La Jolla Physics Reunion/Symposium - Videos
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