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R. A. Fisher

Publications and source records attributed to R. A. Fisher.

18 recordsLinked to original sources
arXiv2016

Strong Correlations, Strong Coupling and s-wave Superconductivity in Hole-doped BaFe2As2 Single Crystals

We present a comprehensive study of the low-temperature heat capacity and thermal expansion of single crystals of the hole-doped Ba1-xKxFe2As2 series (0<x<1) and the end-members RbFe2As2 and CsFe2As2. A large increase of the Sommerfeld coefficient is observed with both decreasing band filling and isovalent substitution (K, Rb, Cs) revealing a strong enhancement of electron correlations and the possible proximity of these materials to a Mott insulator. This trend is well reproduced theoretically by our Density-Functional Theory + Slave-Spin (DFT+SS) calculations, confirming that 122-iron pnictides are effectively Hund metals, in which sizable Hund's coupling and orbital selectivity are the key ingredients for tuning correlations. We also find direct evidence for the existence of a coherence-incoherence crossover between a low-temperature heavy Fermi liquid and a highly incoherent high-temperature regime similar to heavy fermion systems. In the superconducting state, clear signatures of multiband superconductivity are observed with no evidence for nodes in the energy gaps, ruling out the existence of a doping-induced change of symmetry (from s to d-wave). We argue that the disappearance of the electron band in the range 0.4<x<1.0 is accompanied by a strong-to-weak coupling crossover and that this shallow band remains involved in the superconducting pairing, although its contribution to the normal state fades away. Differences between hole- and electron-doped BaFe2As2 series are emphasized and discussed in terms of strong pair breaking by potential scatterers beyond the Born limit.

cond-mat.supr-con↗
arXiv2010

Tin telluride: a weakly co-elastic metal

We report resonant ultrasound spectroscopy (RUS), dilatometry/magnetostriction, magnetotransport, magnetization, specific heat, and $^{119}$Sn Mössbauer spectroscopy measurements on SnTe and Sn$_{0.995}$Cr$_{0.005}$Te. Hall measurements at $T=77$ K indicate that our Bridgman-grown single crystals have a $p$-type carrier concentration of $3.4 \times 10^{19}$ cm$^{-3}$ and that our Cr-doped crystals have an $n$-type concentration of $5.8 \times 10^{22}$ cm$^{-3}$. Although our SnTe crystals are diamagnetic over the temperature range $2\, \text{K} \leq T \leq 1100\, \text{K}$, the Cr-doped crystals are room temperature ferromagnets with a Curie temperature of 294 K. For each sample type, three-terminal capacitive dilatometry measurements detect a subtle 0.5 micron distortion at $T_c \approx 85$ K. Whereas our RUS measurements on SnTe show elastic hardening near the structural transition, pointing to co-elastic behavior, similar measurements on Sn$_{0.995}$Cr$_{0.005}$Te show a pronounced softening, pointing to ferroelastic behavior. Effective Debye temperature, $θ_D$, values of SnTe obtained from $^{119}$Sn Mössbauer studies show a hardening of phonons in the range 60--115K ($θ_D$ = 162K) as compared with the 100--300K range ($θ_D$ = 150K). In addition, a precursor softening extending over approximately 100 K anticipates this collapse at the critical temperature, and quantitative analysis over three decades of its reduced modulus finds $ΔC_{44}/C_{44}=A|(T-T_0)/T_0|^{-κ}$ with $κ= 0.50 \pm 0.02 $, a value indicating a three-dimensional softening of phonon branches at a temperature $T_0 \sim 75$ K, considerably below $T_c$. We suggest that the differences in these two types of elastic behaviors lie in the absence of elastic domain wall motion in the one case and their nucleation in the other.

cond-mat.mtrl-sci↗
arXiv2010

Doping evolution of superconducting gaps and electronic densities of states in Ba(Fe1-xCox)2As2 iron pnictides

An extensive calorimetric study of the normal- and superconducting-state properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x > 0.06), which illustrates the strong competition between magnetism and superconductivity to monopolize the Fermi surface in the underdoped region and the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All superconducting samples exhibit a residual electronic density of states of unknown origin in the zero-temperature limit, which is minimal at optimal doping but increases to the normal-state value in the strongly under- and over-doped regions. The remaining specific heat in the superconducting state is well described using a two-band model with isotropic s-wave superconducting gaps.

cond-mat.supr-con↗
arXiv2009

Calorimetric Evidence of Multiband Superconductivity in Ba(Fe0.925Co0.075)2As2

We report on the determination of the electronic heat capacity of a slightly overdoped (x = 0.075) Ba(Fe1-xCox)2As2 single crystal with a Tc of 21.4 K. Our analysis of the temperature dependence of the superconducting-state specific heat provides strong evidence for a two-band s-wave order parameter with gap amplitudes 2D1(0)/kBTc=1.9 and 2D2(0)/kBTc=4.4. Our result is consistent with the recently predicted s+- order parameter [I. I. Mazin et al., Phys. Rev. Lett. 101, 057003 (2008)].

cond-mat.supr-con↗
arXiv2008

Observation of a continuous phase transition in a shape-memory alloy

Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au_{0.52}Zn_{0.48} above and below their martensitic transition temperatures (T_M=64K and 45K, respectively). In each composition, elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q = (1.33,0.67,0) at temperatures corresponding to each sample's T_M. Although the new Bragg peaks appear in a discontinuous manner in the Au_{0.52}Zn_{0.48} sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near the transition temperature are in favorable accord with a mean-field approximation. A Landau-theory-based fit to the pressure dependence of the transition temperature suggests the presence of a critical endpoint in the AuZn phase diagram located at T_M*=2.7K and p*=3.1GPa, with a quantum saturation temperature θ_s=48.3 +/- 3.7K.

cond-mat.mtrl-sci↗
arXiv2008

Combined experimental and theoretical investigation of the premartensitic transition in Ni$_2$MnGa

Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni$_2$MnGa over the temperature range $100K < T < 250K$. All measurements detect clear signatures of the premartensitic transition ($T_\mathrm{PM}\sim 247K$) and the martensitic transition ($T_\mathrm{M} \sim 196K$). Temperature-dependent UPS shows a dramatic depletion of states (pseudogap) at $T_\mathrm{PM}$ located 0.3eV below the Fermi energy. First-principles electronic structure calculations show that the peak observed at 0.3eV in the UPS spectra for $T > T_\mathrm{PM}$ is due to the Ni-d minority-spin electrons. Below $T_\mathrm{M}$ this peak disappears, resulting in an enhanced density of states at energies around 0.8eV. This enhancement reflects Ni-d and Mn-d electronic contributions to the majority-spin density of states and is accompanied by significant reconstruction of the Fermi surface.

cond-mat.mtrl-sci↗
arXiv2007

Electronic Instabilities in Shape-Memory Alloys

Using a variety of thermodynamic measurements made in magnetic fields, we show evidence that the diffusionless transition (DT) in many shape-memory alloys is related to significant changes in the electronic structure. We investigate three alloys that show the shape-memory effect (In-24 at.% Tl, AuZn, and U-26 at.% Nb). We observe that the DT is significantly altered in these alloys by the application of a magnetic field. Specifically, the DT in InTl-24 at.% shows a decrease in the DT temperature with increasing magnetic field. Further investigations of AuZn were performed using an ultrasonic pulse-echo technique in magnetic fields up to 45 T. Quantum oscillations in the speed of the longitudinal sound waves propagating in the [110] direction indicated a strong acoustic de Haas-van Alphen-type effect and give information about part of the Fermi surface.

cond-mat.other↗
arXiv2007

Heat Capacity in Magnetic and Electric Fields Near the Ferroelectric Transition in Tri-Glycine Sulfate

Specific-heat measurements are reported near the Curie temperature ($T_C$~= 320 K) on tri-glycine sulfate. Measurements were made on crystals whose surfaces were either non-grounded or short-circuited, and were carried out in magnetic fields up to 9 T and electric fields up to 220 V/cm. In non-grounded crystals we find that the shape of the specific-heat anomaly near $T_C$ is thermally broadened. However, the anomaly changes to the characteristic sharp $λ$-shape expected for a continuous transition with the application of either a magnetic field or an electric field. In crystals whose surfaces were short-circuited with gold, the characteristic $λ$-shape appeared in the absence of an external field. This effect enabled a determination of the critical exponents above and below $T_C$, and may be understood on the basis that the surface charge originating from the pyroelectric coefficient, $dP/dT$, behaves as if shorted by external magnetic or electric fields.

cond-mat.stat-mech↗
arXiv2006

Specific Heat of Na0.35CoO2,1.3H2O: Effects of Sample Age; Non-Magnetic Pair Breaking, Two Energy Gaps, and Strong Fluctuations in the Superconducting State

The specific heats of three samples of Na0.35CoO2,1.3H2O show an evolution of the superconductivity, and its ultimate disappearance, with increasing sample age. An overall increase in pair-breaking action, which occurs preferentially in an electron band with a small energy gap, produces a shift in the relative contributions of two electron bands to the superconducting condensation. The similarity of the time scale for these changes to that recently reported for structural changes in the CoO2 layers and the formation of O vacancies suggests a relation between the two effects and an explanation for the strong sample dependence of the properties of this material more generally. The onset of the transition to the vortex state is independent of magnetic field, suggesting the presence of unusually strong fluctuation effects.

cond-mat.supr-con↗
arXiv2004

Tunable Thermal Expansion Behavior in the Intermetallic YbGaGe

We investigate the effects of carbon and boron doping on the thermal expansion in the hexagonal (P63/mmc) intermetallic YbGaGe. X-ray powder diffraction was used to measure the lattice constants on pure and doped (C or B at nominal levels of 0.5 %) samples from T~10 K to T~300 K. Also measured were resistivity, specific-heat, and magnetic susceptibility. While the pure YbGaGe samples exhibit positive thermal volume expansion, (V300K-V10K)/V300K = 0.94%, the volume expansion in the lightly C and B-doped samples, contract and tend towards zero volume expansion. Such a strong response with such light doping suggests that the underlying mechanism for the reported zero volume expansion is substitutional disorder, and not the previously proposed valence fluctuations.

cond-mat.mtrl-sci↗
arXiv2004

Heat Capacity of Na$_{0.3}$CoO$_{2}$$\cdot$1.3H$_{2}$O, a New Two-Gap Superconductor: Comparison with the Heat Capacity of MgB$_2$

The superconducting-state heat capacity of Na$_{0.3}$CoO$_{2}$$\cdot$1.3H$_{2}$O shows unusual, marked deviations from BCS theory, at all temperatures. At low temperatures the heat capacity has the $T^2$ dependence characteristic of line nodes in the energy gap, rather than the exponential temperature dependence of a fully gapped, conventional superconductor. At temperatures of the order of one fifth of the critical temperature and above, the deviations are strikingly similar to those of MgB$_2$, which are known to be a consequence of the existence of substantially different energy gaps on different sheets of the Fermi surface. A two-gap fit to the Na$_{0.3}$CoO$_{2}$$\cdot$1.3H${_2}$O data gives gap amplitudes of 45% and 125% of the BCS value, on parts of the Fermi surface that contribute, respectively, 45% and 55% to the normal-state density of states. The temperature of the onset of the transition to the vortex state is independent of magnetic field, which shows the presence of unusually strong fluctuations.

cond-mat.supr-con↗
arXiv2003

Experimental electronic heat capacities of $α-$ and $δ-$Plutonium; heavy-fermion physics in an element

We have measured the heat capacities of $δ-$Pu$_{0.95}$Al$_{0.05}$ and $α-$Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduction of the electronic contribution to the heat capacity of $δ-$Pu$_{0.95}$Al$_{0.05}$; we find $γ= 64 \pm 3$ mJK$^{-2}$mol$^{-1}$ as $T \to 0$. This is a factor $\sim 4$ larger than that of any element, and large enough for $δ-$Pu$_{0.95}$Al$_{0.05}$ to be classed as a heavy-fermion system. By contrast, $γ= 17 \pm 1$ mJK$^{-2}$mol$^{-1}$ in $α-$Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of $δ-$Pu$_{0.95}$Al$_{0.05}$, one or both of which may be associated with the formation of the $α'-$ martensitic phase. We suggest that the large $γ$-value of $δ-$Pu$_{0.95}$Al$_{0.05}$ may be caused by proximity to a quantum-critical point.

cond-mat.str-el↗
arXiv2001

Phenomenological two-gap model for the specific heat of MgB_2

We show that the specific heat of the superconductor MgB_2 (MgB2) in zero field, for which significant non-BCS features have been reported, can be fitted, essentially within experimental error, over the entire range of temperature to T_c by a phenomenological two-gap model. The resulting gap parameters agree with previous determinations from band-structure calculations, and from various spectroscopic experiments. The determination from specific heat, a bulk property, shows that the presence of two superconducting gaps in MgB_2 is a volume effect.

cond-mat.supr-con↗
arXiv2001

Specific Heat of CeRhIn5: Pressure-Driven Evolution of the Ground State from Antiferromagnetism to Superconductivity

Measurements of the specific heat of antiferromagnetic CeRhIn5, to 21 kbar, and for 21 kbar to 70 kOe, show a discontinuous change from an antiferromagnetic ground state below 15 kbar to a superconducting ground state above, and suggest that it is accompanied by a weak thermodynamic first-order transition. Bulk superconductivity appears, apparently with d-wave electron pairing, at the critical pressure, 15 kbar; with further increase in pressure a residual temperature-proportional term in the specific heat disappears.

cond-mat.str-el↗
arXiv2001

Identification and Characterization of Two Energy Gaps in Superconducting MgB2 by Specific-Heat Measurements

This paper reports specific-heat measurements on superconducting MgB2 in magnetic fields to 9 T. In zero magnetic field the data can be fitted to a two-gap model, which is a generalization of a semi-empirical model for strong-coupled, single-gap superconductors (the alpha-model), to within the precision of the data. Both gaps close at Tc, with one gap larger and one smaller than the BCS weak-coupling-limit. (The two gaps have a ratio ~ 4:1.) Each gap accounts for ~ 50% of the normal electron density of states. The parameters characterizing the fits agree well with those from recent spectroscopic measurements and theory. In magnetic fields the superconductong anomaly is broadened as expected for an anisotropic, randomly-oriented superconductor.

cond-mat.supr-con↗
arXiv2001

Specific Heat of Mg ^{11}B_2

Measurements of the specific heat of Mg ^{11}B_2 (MgB2), from 1 to 50 K, in magnetic fields to 9 T, give the Debye temperature, $Θ$ = 1050 K, the coefficient of the normal-state electron contribution, $γ_n$ = 2.6 mJ mol^{-1} K^{-2}, and a discontinuity in the zero-field specific heat of 133 mJ mol^{-1} K^{-1} at $T_c$ = 38.7 K. The estimated value of the electron-phonon coupling parameter, $λ$ = 0.62, could account for the observed $T_c$ only if the important phonon frequencies are unusually high relative to $Θ$. At low $T$, there is a strongly field-dependent feature that suggests the existence of a second energy gap, about four times smaller than the major gap.

cond-mat.supr-con↗
arXiv2000

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 700 mJ/mole Ce-K2. Crystal 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 n-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 heavy-fermion superconductivity prefers to develop in one structure type and not another.

cond-mat.str-el↗