Marcelo Jaime

Critical examination of heat capacity measurements made on a Quantum Design physical property measurement system

Abstract We examine the operation and performance of an automated heat-capacity measurement system manufactured by Quantum Design (QD). QD’s physical properties measurement system (PPMS) employs a thermal-relaxation calorimeter that operates in the temperature range of 1.8–395 K. The accuracy of the PPMS specific-heat data is determined here by comparing data measured on copper and synthetic sapphire samples with standard literature values. The system exhibits an overall accuracy of better than 1% for temperatures between 100 and 300 K, while the accuracy diminishes at lower temperatures. These data confirm that the system operates within the ±5% accuracy specified by QD. Measurements on gold samples with masses of 4.5 and 88 mg indicate that accuracy of ±3% or better can be achieved below 4 K by using samples with heat capacities that are half or greater than the calorimeter addenda heat capacity. The ability of a PPMS calorimeter to accurately measure sharp features in Cp(T) near phase transitions is determined by measuring the specific heat in the vicinity of the first-order antiferromagnetic transition in Sm2IrIn8 (T0=14 K) and the second-order hidden order (HO) transition in URu2Si2 (TN=17 K). While the PPMS measures Cp(T) near the second-order transition accurately, it is unable to do so in the vicinity of the first-order transition. We show that the specific heat near a first-order transition can be determined from the PPMS-measured decay curves by using an alternate analytical approach. This correction is required because the latent heat liberated/absorbed at the transition results in temperature–decay curves that cannot be described by a single relaxation time constant. Lastly, we test the ability of the PPMS to measure the specific heat of Mg11B2, a superconductor of current interest to many research groups, that has an unusually strong field-dependent specific heat in the mixed state. At the critical temperature the discontinuity in the specific heat is nearly 15% lower than measurements made on the same sample using a semi-adiabatic calorimeter at Lawrence Berkeley National Laboratory.

High-temperature thermopower in La{sub {bold 2/3}}Ca{sub {bold 1/3}}MnO{sub {bold 3}} films: Evidence for polaronic transport

Thermoelectric power, electrical resistivity, and magnetization experiments, performed in the paramagnetic phase of La{sub 2/3}Ca{sub 1/3}MnO{sub 3}, provide evidence for polaron-dominated conduction in colossal magnetoresistance materials. At high temperatures, a large, nearly-field-independent difference between the activation energies for resistivity {rho} and thermopower {ital S}, a characteristic of Holstein polarons, is observed, and ln{rho} ceases to scale with the magnetization. On approaching {ital T}{sub {ital c}}, both energies become field dependent, indicating that the polarons are magnetically polarized. Below {ital T}{sub {ital c}}, the thermopower follows a law {ital S}({ital H}){approximately}1/{rho}({ital H}) as in nonsaturated ferromagnetic metals. {copyright} {ital 1996 The American Physical Society.}

Superconductivity and magnetism in a new class of heavy-fermion materials

We report a new family of Ce-based heavy-fermion compounds whose electronic specific heat coefficients range from about 400 to over 700mJ/mol-Ce K2. Crystals in this family form as CenTmIn3n-2m, where T = Rh or Ir, n = 1 or 2, and m = 1, with a tetragonal structure that can be viewed as m-layers of CeIn3 units stacked sequentially along the c-axis with intervening m-layers of TIn2. Ambient and high-pressure studies show that the quasi-2D layers of CeIn3 produce unconventional superconducting and magnetic ground states. This family should enable new understanding of the relationship between magnetism and superconductivity in heavy-fermion materials and more generally of why heavyfermion superconductivity prefers to develop in one structure type and not another.

Anisotropic superconductivity in epitaxial MgB2 films

High-quality epitaxial MgB2 thin films prepared by pulsed laser deposition with T c = 39 K offer the opportunity to study the anisotropy and robustness of the superconducting state in magnetic fields. We measure the in-plane electrical resistivity of the films in magnetic fields to 60T and estimate the superconducting upper critical field Hρ c (0) ≈ 24 ± 3 T for field oriented along the c-axis, and Hρ ab (0) ≈ 30 ± 2 T for field in the plane of the film. We find the zero-temperature coherence lengths ξc(0) ≈ 30 A and ξab(0) ≈ 37 A to be shorter than the calculated electronic mean free path l ≈ 100 ± 50 A, which places our films in the clean limit. The observation of such large upper critical fields from clean limit samples, coupled with the relatively small anisotropy, provides strong evidence of the viability of MgB2 as a technologically important superconductor.

Correlated enhancement of Hc2 and Jc in carbon nanotube doped MgB2

The use of MgB2 in superconducting applications still awaits the development of a MgB2-based material where current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc, is in the range 2.5–10 at.% depending on field and temperature. Record values of the upper critical field, Hc2(4 K) = 41.9 T (with extrapolated Hc2(0)≈44.4 T), are reached in a bulk sample with 10 at.% DWCNT content. The measured Hc2 versus T dependences for all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich and co-workers.

Magnetostriction and magnetic texture to 100.75 Tesla in frustrated SrCu2(BO3)2

Strong geometrical frustration in magnets leads to exotic states such as spin liquids, spin supersolids, and complex magnetic textures. SrCu2(BO3)2, a spin-1/2 Heisenberg antiferromagnet in the archetypical Shastry–Sutherland lattice, exhibits a rich spectrum of magnetization plateaus and stripe-like magnetic textures in applied fields. The structure of these plateaus is still highly controversial due to the intrinsic complexity associated with frustration and competing length scales. We discover magnetic textures in SrCu2(BO3)2 via magnetostriction and magnetocaloric measurements in fields up to 100.75 T. In addition to observing low-field fine structure with unprecedented resolution, the data also reveal lattice responses at 73.6 T and at 82 T that we attribute, using a controlled density matrix renormalization group approach, to a unanticipated 2/5 plateau and to the long-predicted 1/2 plateau.

SiC and carbon nanotube distinctive effects on the superconducting properties of bulk MgB2

This work describes in detail the simultaneous enhancement of the upper critical field (Hc2) and the critical current density (Jc) of MgB2 bulk samples doped with nano-SiC particles, as well as single-walled and double-walled (dw) carbon nanotubes (CNTs). The magnetization properties were examined in a superconducting quantum interference device magnetometer, and four-probe transport measurements were performed using a 50T pulsed magnet to determine Hc2(T). We found that the Jc enhancement is similar in all doped samples at 5K but nano-SiC addition is more effective to improve the flux pinning in the high temperature range (T⩾20K); this improvement cannot solely be attributed to the C incorporation to the lattice but also to the presence of other types of defects (i.e., several kinds of nanoinclusions). CNTs produce a better C incorporation that is more effective to enhance Hc2 [i.e., dwCNT-doped samples reached a record Hc2(0)∼44T value for bulk MgB2]. All the Hc2(T) curves obtained for different types o...

Cascade of magnetic field induced spin transitions in LaCoO3.

We present magnetization and magnetostriction studies of LaCoO3 in magnetic fields approaching 100 T. In contrast with expectations from single-ion models, the data reveal two distinct first-order transitions and well-defined magnetization plateaus. The magnetization at the higher plateau is only about half the saturation value expected for spin-1 Co3+ ions. These findings strongly suggest collective behavior induced by interactions between different electronic configurations of Co3+ ions. We propose a model that predicts crystalline spin textures and a cascade of four magnetic phase transitions at high fields, of which the first two account for the experimental data.

Fermi surface reconstruction and multiple quantum phase transitions in the antiferromagnet CeRhIn5

L. Jiao, H. Q. Yuan, ∗ Y. Kohama, E. D. Bauer, J. -X. Zhu, J. Singleton, T. Shang, J. L. Zhang, Y. Chen, H. O. Lee, T. Park, M. Jaime, J. D. Thompson, F. Steglich, and Q. Si † Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou, Zhejiang 310027, China Los Alamos National Laboratory, Los Alamos, NM 87545 Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany Department of Physics and Astronomy, Rice University, Houston, TX 77005 (Dated: May 11, 2014)Significance Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of microscopic details of a specific material. An analogous description is lacking for phase transitions that are driven at absolute zero temperature by a nonthermal control parameter. Classification of quantum-driven phase transitions is a fundamental but open problem that arises in diverse contexts and multiple classes of materials. Here we report the first observation, to our knowledge, of a sharp Fermi surface reconstruction while applying a strong magnetic field to suppress an antiferromagnetic transition to zero temperature. These experiments demonstrate that direct measurements of the Fermi surface can distinguish theoretically proposed models of quantum criticality and point to a universal description of quantum phase transitions. Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of the specific microscopic details of a material. However, many current studies focus on materials that exhibit quantum-driven continuous phase transitions (quantum critical points, or QCPs) at absolute zero temperature. The classification of such QCPs and the question of whether they show universal behavior remain open issues. Here we report measurements of heat capacity and de Haas–van Alphen (dHvA) oscillations at low temperatures across a field-induced antiferromagnetic QCP (Bc0 ≈ 50 T) in the heavy-fermion metal CeRhIn5. A sharp, magnetic-field-induced change in Fermi surface is detected both in the dHvA effect and Hall resistivity at B0* ≈ 30 T, well inside the antiferromagnetic phase. Comparisons with band-structure calculations and properties of isostructural CeCoIn5 suggest that the Fermi-surface change at B0* is associated with a localized-to-itinerant transition of the Ce-4f electrons in CeRhIn5. Taken in conjunction with pressure experiments, our results demonstrate that at least two distinct classes of QCP are observable in CeRhIn5, a significant step toward the derivation of a universal phase diagram for QCPs.

AC measurement of heat capacity and magnetocaloric effect for pulsed magnetic fields

A new calorimeter for measurements of the heat capacity and magnetocaloric effect of small samples in pulsed magnetic fields is discussed for the exploration of thermal and thermodynamic properties at temperatures down to 2 K. We tested the method up to μ(0)H=50 T, but it could be extended to higher fields. For these measurements we used carefully calibrated bare-chip Cernox(®) and RuO(2) thermometers, and we present a comparison of their performances. The monotonic temperature and magnetic field dependences of the magnetoresistance of RuO(2) allow thermometry with a precision as good as ±4 mK at T=2 K. To test the performance of our calorimeter, heat capacity and magnetocaloric effect for the spin-dimer compound Sr(3)Cr(2)O(8) and the triangular lattice antiferromagnet RbFe(MoO(4))(2) are presented.