Previous Specific Heat Research Articles

Reconciling scaling of the optical conductivity of cuprate superconductors with Planckian resistivity and specific heat

Materials tuned to a quantum critical point display universal scaling properties as a function of temperature T and frequency ω. A long-standing puzzle regarding cuprate superconductors has been the observed power-law dependence of optical conductivity with an exponent smaller than one, in contrast to T-linear dependence of the resistivity and ω-linear dependence of the optical scattering rate. Here, we present and analyze resistivity and optical conductivity of La2−xSrxCuO4 with x = 0.24. We demonstrate ℏω/kBT scaling of the optical data over a wide range of frequency and temperature, T-linear resistivity, and optical effective mass proportional to sim ln T corroborating previous specific heat experiments. We show that a T, ω-linear scaling Ansatz for the inelastic scattering rate leads to a unified theoretical description of the experimental data, including the power-law of the optical conductivity. This theoretical framework provides new opportunities for describing the unique properties of quantum critical matter.

Electronic structure and anharmonic phonon mode in BaIr2Ge7 with two-dimensional Ba-Ge networks studied by photoemission spectroscopy

We report the electronic structure of ${\mathrm{BaIr}}_{2}{\mathrm{Ge}}_{7}$ with two types of cage structure by means of angle-resolved photoemission spectroscopy (ARPES) and hard x-ray photoemission spectroscopy. ARPES spectra reveal the three-dimensional and multiband Fermi surfaces (FSs) originating from the hybridized Ir $5d$ and Ge $4p$ orbitals. The observed FSs show ${C}_{2}$ symmetry, reflecting the orthorhombic $Ammm$ crystal structure of ${\mathrm{BaIr}}_{2}{\mathrm{Ge}}_{7}$. The temperature dependence of the ARPES spectra exhibits the thermal spectral broadening, and the width of the spectral peak shows a concave-downward behavior with temperature. Considering the effect of anharmonic phonon modes, we have reproduced the temperature dependence of the electrical resistivity as well as the thermal spectral broadening. The resultant renormalized phonon frequencies ${\ensuremath{\omega}}_{\mathrm{r}0}^{(1)}=146.9\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${\ensuremath{\omega}}_{\mathrm{r}0}^{(2)}=70.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ are comparable to the Einstein temperatures estimated from the previous specific heat measurement. Our results suggest the existence of the weak anharmonic phonon modes in ${\mathrm{BaIr}}_{2}{\mathrm{Ge}}_{7}$.

Thermal transport properties, magnetic susceptibility and neutron diffraction studies of the (Cr100-xAlx)95Mo5 alloy system

Previous electrical resistivity (ρ) and specific heat (CP) studies on the ternary (Cr100-xAlx)95Mo5 alloy system suggested that it harbours two quantum critical points (QCPs). This study reports comprehensive investigations of this alloy system through Seebeck coefficient (S), thermal conductivity (κ), magnetic susceptibility (χ) and neutron diffraction (ND) measurements in the concentration range 0 ≤ x ≤ 8.6. The results of S and χ show that spin-density-wave (SDW) antiferromagnetism is suppressed to temperatures below 2 K for concentrations in the range 1.4 ≤ x ≤ 4.4. Plots of dS/dT in the limit as T→ 2 K depict two minima, i.e. just above x = 1.4 and 4.4. This parameter has been used as a key indicator of quantum critical behaviour (QCB) in Cr alloys. Analyses against the Nordheim-Gorter relationship demonstrates a positive slope for the incommensurate (I) SDW alloys and a negative slope for the commensurate (C) SDW alloys. Extrapolations of these two slopes intercept at a concentration of 3.2 at.% Al indicating the occurrence of band structure modifications when Al is added into the Cr95Mo5 base alloy. The Lorenz number (L) for the alloys with x = 0 and 0.5 shows interesting anomalous behaviour associated with band structure effects and SDW magnetic ordering. ND measurements as a function of temperature confirm that alloys with x < 1.4 order in the incommensurate (I) SDW phase whilst alloys with x > 4.4 show commensurate (C) SDW order. Power law fits of the form TN∝1.40−x0.35±0.05 for the ISDW to P phase transition and TN∝x−4.400.63±0.03 for the P to CSDW phase transition rendered the critical exponents 0.35±0.05 and 0.63±0.03 respectively. Overall the results of S, κ, χ and ND corroborate the existence of two QCPs at x ≈ 1.4 and 4.4.

Two-Band Calculations on the Upper Critical Field of Sc2Fe3Si5

The ternary compound Sc2Fe3Si5 has attracted much attention because of the various anomalous physical properties. The specific heat experiment and energy band structure calculation suggest that Sc2Fe3Si5 is a two-gap superconductor. Based on this, we analyze the upper critical field for superconducting Sc2Fe3Si5 crystals using the two-band Ginzburg-Landau theory. A two-parameter variational approach is adopted to obtain the upper critical field in arbitrary direction. The temperature and angular dependences of the upper critical field are plotted. The results reproduce the experimental data in a very broad temperature range and strongly support previous specific heat data and theoretical calculation, pointing to the existence of two energy gaps in Sc2Fe3Si5. The anisotropy of the upper critical field is also studied and is about 2, in accordance with the experimental result. Moreover our calculations indicate that Sc2Fe3Si5 has rather a three-dimension character, in agreement with the energy band calculation.

Low-temperature thermal expansion measurements in PrV2Al20

We have measured thermal expansion of PrV2Al20 and LaV2Al20 from room temperature down to 2 K, using a capacitance dilatometer. Linear thermal expansion ΔL/L along [111] direction decreases monotonically on cooling in both materials. The extracted 4ƒ electrons contribution of the linear thermal expansion coefficient a clearly shows a broad peak at ∼ 30 K which may correspond to the crystal electric field excited state at 40 K suggested in the previous specific heat study.

An intermediate state between the kagome-ice and the fully polarized state in Dy$_2$Ti$_2$O$_7$

Dy$_2$Ti$_2$O$_7$ is at present the cleanest example of a spin-ice material. Previous theoretical and experimental work on the first-order transition between the kagome-ice and the fully polarized state has been taken as a validation for the dipolar spin-ice model. Here we investigate in further depth this phase transition using ac-susceptibility and dc-magnetization, and compare this results with Monte-Carlo simulations and previous magnetization and specific heat measurements. We find signatures of an intermediate state between the kagome-ice and full polarization. This signatures are absent in current theoretical models used to describe spin-ice materials.Received: 17 May 2015, Accepted: 12 June 2015; Edited by: A. Vindigni; Reviewed by: M. Perfetti, Dipartimento di Chimica, Universitá di Firenze, Italy; DOI: http://dx.doi.org/10.4279/PIP.070009Cite as: S A Grigera, R A Borzi, D G Slobinsky, A S Gibbs, R Higashinaka, Y Maeno, T S Grigera, Papers in Physics 7, 070009 (2015)This paper, by S A Grigera, R A Borzi, D G Slobinsky, A S Gibbs, R Higashinaka, Y Maeno, T S Grigera, is licensed under the Creative Commons Attribution License 3.0.

Structural and Magnetic Phase Transitions near Optimal Superconductivity in BaFe2(As(1-x)Px)2.

We use nuclear magnetic resonance (NMR), high-resolution x-ray, and neutron scattering studies to study structural and magnetic phase transitions in phosphorus-doped BaFe2(As(1-x)P(x)2. Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x=0.3. However, we show that the tetragonal-to-orthorhombic structural (T{s}) and paramagnetic to antiferromagnetic (AF, TN) transitions in BaFe2(As(1-x)Px)2 are always coupled and approach T{N}≈T{s}≥T{c} (≈29 K) for x=0.29 before vanishing abruptly for x≥0.3. These results suggest that AF order in BaFe_{2}(As(1-x)Px)2 disappears in a weakly first-order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.

Anomalous magnetoresistance in nanocrystalline gadolinium at low temperatures

The results of a detailed investigation of electrical resistivity, ρ(T) and transverse magnetoresistance (MR) in nanocrystalline Gd samples with an average grain size d = 12 nm and 18 nm reveal the following. Besides a major contribution to the residual resistivity, ρr(0), arising from the scattering of conduction electrons from grain surfaces/interfaces/boundaries (which increases drastically as the average grain size decreases, as expected), coherent electron–magnon scattering makes a small contribution to ρr(0), which gets progressively suppressed as the applied magnetic field (H) increases in strength. At low temperatures (T ≲ 40 K) and fields (H = 0 and H = 5 kOe), ρH(T) varies as T3/2 with a change in slope at T+ ≃ 16.5 K. As the field increases beyond 5 kOe, the T3/2 variation of ρH(T) at low temperatures (T ≲ 40 K) changes over to the T2 variation and a slight change in the slope dρH/dT2 at T+(H) disappears at H ⩾ 20 kOe. The electron–electron scattering (Fermi liquid) contribution to the T2 term, if present, is completely swamped by the coherent electron–magnon scattering contribution. As a function of temperature, (negative) MR goes through a dip at a temperature Tmin ≃ T+, which increases with H as H2/3. MR at Tmin also increases in magnitude with H and attains a value as large as ∼15% (17%) for d = 12 nm (18 nm) at H = 90 kOe. This value is roughly five times greater than that reported earlier for crystalline Gd at Tmin ≃ 100 K. Unusually large MR results from an anomalous softening of magnon modes at T ≃ Tmin ≈ 20 K. In the light of our previous magnetization and specific heat results, we show that all the above observations, including the H2/3 dependence of Tmin (with Tmin(H) identified as the Bose–Einstein condensation (BEC) transition temperature, TBEC(H)), are the manifestations of the BEC of magnons at temperatures T ⩽ TBEC. Contrasted with crystalline Gd, which behaves as a three-dimensional (3D) pure uniaxial dipolar ferromagnet in the asymptotic critical region, ρH=0(T) of nanocrystalline Gd, in the critical region near the ferromagnetic-paramagnetic phase transition, is better described by the model proposed for a 3D random uniaxial dipolar ferromagnet.

Theoretical Study on the Upper Critical Field of a Layered Superconductor NbSe $$_2$$ 2

Based on two-band Ginzburg-Landau theory, we study the temperature and angular dependences of the upper critical field for superconducting NbSe $$_2$$ crystals. We introduce two variational parameters in our approach, and provide the explicit equations to determine the upper critical field in arbitrary direction. Our analyses strongly support previous scanning tunneling microscopy results and specific heat data, pointing to the existence of two energy gaps in this compound. Our results thus suggest that besides MgB $$_{2}$$ , NbSe $${_2}$$ is another example of two-gap superconductors.

Molecular flexibility and structural instabilities in crystalline l-methionine

We have investigated the dynamics in polycrystalline samples of l-methionine related to the structural transition at about 307K by incoherent inelastic and quasielastic neutron scattering, X-ray powder diffraction as well as ab-initio calculations. l-Methionine is a sulfur amino acid which can be considered a derivative of alanine with the alanine R-group CH3 exchanged by CH3S(CH2)2. Using X-ray powder diffraction we have observed at ~190K an anomalous drop of the c-lattice parameter and an abrupt change of the β-monoclinic angle that could be correlated to the anomalies observed in previous specific heat measurements. Distinct changes in the quasielastic region of the neutron spectra are interpreted as being due to the onset and slowing-down of reorientational motions of the CH3–S group, are clearly distinguished above 130K in crystalline l-methionine. Large-amplitude motions observed at low frequencies are also activated above 275K, while other well-defined vibrations are damped. The ensemble of our results suggests that the crystalline structure of l-methionine is dynamically highly disordered above 275K, and such disorder can be linked to the flexibility of the molecular thiol-ether group.

Thermoelectric power of the YbT2Zn20(T=Fe, Ru, Os, Ir, Rh, and Co) heavy fermions

The thermoelectric power, $S(T)$, of the heavy fermions YbT$_{2}$Zn$_{20}$ ($T$ = Fe, Ru, Os, Ir, Rh, and Co) has been measured to characterize their strong electronic correlations. A large, negative, local minimum in $S(T)$ with approximately -70 $\mu$V/K is found for all compounds. From the observed local minimum, the energy scales associated with both the Kondo temperature and the crystalline electric field splitting are deduced and compared to previous specific heat measurements. At low temperatures, $S(T)$ follows a linear temperature dependence, $S(T)$ = $\alpha T$, with an enhanced value of $\alpha$ which is characteristic for a heavy fermion state. In the zero temperature limit, the enhanced $\alpha$ value strongly correlates with the electronic specific heat coefficient, $C(T)/T$, due to the large density of states at the Fermi level.

An exotic ordered phase in the pyrochlore-like antiferromagnet Ni2(OH)3Cl studied by NMR

We investigated a three-dimensional frustrated 3d-electron magnet on a pyrochlore-like lattice Ni2(OH)3Cl with s = 1 by means of 1H-NMR measurements. Previous specific heat measurements reported that this material shows a phase transition at 4.0 K and also has an anomaly at around 20 K. Our 1H-NMR measurement in 0.73 T has demonstrated that both the spin-lattice relaxation rates T1−1 and the spectra show clear evidences of a magnetic long-range ordering below 4.0 K. However, they do not show any anomalies at 20 K. By contrast the width of the 1H-NMR spectra in 4.3 T starts to deviate from linearity to the magnetization below 20 K, which suggests that the magnetic anisotropy is induced by the magnetic field. In addition, T1−1 starts to decrease below 20 K, which indicates the suppression of spin fluctuation. We discuss the unusual spin state below 20K.

Two-band BCS mechanism of superconductivity in a Ba(Fe0.9Co0.1)2As2 high-temperature superconductor

Terahertz and infrared spectra of the conductivity, σ(ν), and dielectric constant, ɛ(ν), of a Ba(Fe0.9Co0.1)2As2 film (Tc = 20 K) have been analyzed together with previous specific-heat and angular resolved photoelectron spectroscopy data. It has been shown that the spectra σ(ν) and ɛ(ν) of Ba(Fe0.9Co0.1)2As2 in the superconducting phase at T = 5 K, as well as the magnetic field penetration depth, can be described well using the standard Bardeen-Cooper-Schrieffer (BCS) model with an additive contribution of electron and hole bands. It has been found that the measured temperature dependence of the magnetic field penetration depth in a wide temperature range 5 K < T < Tc can be described only with the introduction of interband pairing interaction. The coupling constant of electron and hole bands, λ1, 2 = 0.1, as well as the temperature dependences of superconducting gaps in the electron and hole subsystems, has been determined using the model of two-band superconductivity developed earlier for MgB2.

Thermoelectric power investigations of YbAgGe across the quantum critical point

The magnetic field and temperature dependences of the thermoelectric power (TEP) of the antiferromagnetically ordered heavy fermion compound YbAgGe are measured across the field-induced quantum critical point. These TEP measurements reproduce the earlier $H\text{\ensuremath{-}}T$ phase diagram and identify an additional domelike phase between $\ensuremath{\sim}45$ and $\ensuremath{\sim}70\text{ }\text{kOe}$. On the low-field side of this region, $Hg{H}_{c}\ensuremath{\sim}45\text{ }\text{kOe}$, the sign of the TEP changes from negative to positive; on the high-field side of this region, $H\ensuremath{\approx}70\text{ }\text{kOe}$, a non-Fermi-liquid state is evidenced as the logarithmic temperature dependence of $S(T)/T$, in agreement with previous specific heat results $C(T)/T\ensuremath{\propto}\ensuremath{-}\text{log}(T)$. For higher fields, $Hg70\text{ }\text{kOe}$, the observed large value of $\ensuremath{\alpha}$, $S(T)=\ensuremath{\alpha}T$, is indicative of the heavy fermion state and shows a correlation with $C(T)/T$.

151Eu-Mössbauer study of complex magnetism in Eu2PdSi3: Effect of Eu2+ substitution by Y3+ and of high pressure

With 151Eu-Mössbauer spectroscopy and other methods the complex magnetic properties of Eu2PdSi3, arising from the two crystallographically different lattice sites of the Eu2+ ions, have been already studied. Here we study the impact of magnetic dilution of the magnetic Eu2+ sites by non-magnetic Y3+ ions. A previous specific heat study has found reduced magnetic ordering temperatures with strong indication of disorder effects like in magnetic spin glasses. Here we provide from 151Eu-Mössbauer spectroscopy detailed information of the impact of Y3+ substitution on the magnetic properties of the two lattice sites, well distinguishable in the 151Eu-spectra. Since the substitution of the larger Eu2+ ions by the smaller Y3+ ions is connected with a lattice contraction, we also applied high pressure to the Eu2PdSi3 sample and observed drastic changes in the magnetic properties originating from a valence transition towards Eu3+ for one of the Eu sites.

Magnetic structures of the ternary silicide Nd6Ni1.67Si3

The ternary silicides RE6Ti1.67Si3 (RE = Ce, Pr, Nd, Gd, Tb and T = Co, Ni) crystallizing with the hexagonal Ce6Ni1.67Si3-type structure has been recently extensively studied. It has been shown that some of these compounds exhibit interesting magnetocaloric properties. In this work, the magnetic structures of the Nd6Ni1.67Si3 compound were determined by means of neutron powder diffraction. According to previous magnetization and specific heat measurements, this compound exhibits two successive magnetic transitions at 84K and 38K. The first transition corresponds to a ferromagnetic arrangement of the Nd-moments along the c-axis and the second one to a non-collinear ferromagnetic arrangement yielding a conical structure. The magnetic structures of Nd6Ni1.67Si3 are compared to those of the Nd6Co1.67Si3homologous ternary silicide.

1H-NMR Spin–Lattice Relaxation Rate of the Quantum Spin System (CH3)2CHNH3Cu(ClxBr1-x)3 with x=0 and 0.35

The spin–lattice relaxation rate T 1 -1 of 1 H-NMR has been measured in (CH 3 ) 2 CHNH 3 Cu(Cl x Br 1- x ) 3 with x = 0 and 0.35, in order to investigate their microscopic magnetism. Previous macroscopic magnetization and specific heat measurements suggested that these two exist in a singlet-dimer phase. The temperature dependence of T 1 -1 in an x = 0 system decreased exponentially toward zero, providing microscopic evidence of a gapped singlet ground state, which is consistent with results of macroscopic experiments. At the same time, in the x = 0.35 system, T 1 -1 showed a sharp peak structure at approximately 7.5 K but no splitting of 1 H-NMR spectra, indicative of magnetic ordering. We discuss the observed sharp peak structure in the x = 0.35 system with the soft mode toward the exotic magnetic ground state suggested by recent µSR experiments.

From unconventional insulating behavior towards conventional magnetism in the intermediate-valence compoundSmB6

In intermediate valence compounds, the subtle quantum equilibrium of two microscopic configurations is at the origin of a very rich physics. ${\text{SmB}}_{6}$ is a famous example as it displays an intriguing semiconducting behavior at low pressure and a magnetic phase at high pressure. New transport measurements on ${\text{SmB}}_{6}$ under hydrostatic conditions show that the two phenomena are intimately linked. The insulating state vanishes just at the pressure $(p=10\text{ }\text{GPa})$ where homogeneous long range magnetic order appears. A comparison to previous specific heat experiments clearly points out that a magnetic anomaly is observed in resistivity measurements when excellent hydrostatic conditions are realized. The particular sensitivity of ${\text{SmB}}_{6}$ to pressure inhomogeneities may reflect the strong anisotropy in the band structure.

1H-NMR Study of the Random Bond Effect in the Quantum Spin System (CH3)2CHNH3Cu(ClxBr1-x)3

The spin–lattice relaxation rate T 1 -1 of 1 H-NMR has been measured in (CH 3 ) 2 CHNH 3 Cu(Cl x Br 1- x ) 3 with x =0.88, which is reportedly a gapped system with a singlet ground state from the previous macroscopic magnetization and specific heat measurements, in order to investigate the bond randomness effect microscopically in the gapped composite Haldane system (CH 3 ) 2 CHNH 3 CuCl 3 . It was discovered that the spin–lattice relaxation rate T 1 -1 in the present system includes both fast and slow relaxation parts indicative of the gapless magnetic ground state and the gapped singlet ground state, respectively. We discuss the obtained results with the previous macroscopic magnetization and specific heat measurements together with the microscopic µSR experiments.

Low-temperature specific heat and possible gap to magnetic excitations in the Heisenberg pyrochlore antiferromagnetGd2Sn2O7

The ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ pyrochlore Heisenberg antiferromagnet displays a phase transition to a four sublattice N\'eel ordered state at a critical temperature ${T}_{c}\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The low-temperature state found via neutron scattering corresponds to that predicted by a classical model that considers nearest-neighbor antiferromagnetic exchange and long-range dipolar interactions. Despite the seemingly conventional nature of the ordered state, the specific heat ${C}_{v}$ has been found to be described in the temperature range $350\phantom{\rule{0.3em}{0ex}}\mathrm{mK}\ensuremath{\leqslant}T\ensuremath{\leqslant}800\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ by an anomalous power law ${C}_{v}\ensuremath{\sim}{T}^{2}$. A similar temperature dependence of ${C}_{v}$ has also been reported for ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, another pyrochlore Heisenberg material. Such behavior is to be contrasted with the typical ${T}^{3}$ behavior expected for a three-dimensional antiferromagnet with conventional long-range order which is then generally accompanied by an $\mathrm{exp}(\ensuremath{-}\ensuremath{\Delta}∕T)$ behavior at lower temperature where anisotropy effects induce a gap $\ensuremath{\Delta}$ to collective spin excitations. Such anomalous ${T}^{2}$ behavior in ${C}_{v}$ has been argued to be correlated to an unusual energy dependence of the density of states which also seemingly manifests itself in low-temperature spin fluctuations found in muon spin relaxation experiments. In this paper, we report calculations of ${C}_{v}$ that consider spin-wave-like excitations out of the N\'eel order observed in ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ via neutron scattering. We argue that the parametric ${C}_{v}\ensuremath{\propto}{T}^{2}$ does not reflect the true low-energy excitations of ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. Rather, we find that the low-energy excitations of this material are antiferromagnetic magnons gapped by single-ion and dipolar anisotropy effects, and that the lowest temperature of $350\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ considered in previous specific heat measurements accidentally happens to coincide with a crossover temperature below which magnons become thermally activated and ${C}_{v}$ takes an exponential form. We argue that further specific heat measurements that extend down to at least $100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ are required in order to ascribe an unconventional description of magnetic excitations out of the ground state of ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ or to invalidate the standard picture of gapped excitations proposed herein.