Specific Heat Anomalies Research Articles

Texture in the Superconducting Order Parameter ofCeCoIn5Revealed by Nuclear Magnetic Resonance

We present a ^{115}In NMR study of the quasi two-dimensional heavy-fermion superconductor CeCoIn_5 believed to host a Fulde-Ferrel-Larkin-Ovchinnkov (FFLO) state. In the vicinity of the upper critical field and with a magnetic field applied parallel to the ab-plane, the NMR spectrum exhibits a dramatic change below T*(H) which well coincides with the position of reported anomalies in specific heat and ultrasound velocity. We argue that our results provide the first microscopic evidence for the occurrence of a spatially modulated superconducting order parameter expected in a FFLO state. The NMR spectrum also implies an anomalous electronic structure of vortex cores.

Cooling rate dependence of specific heat in systems out of equilibrium

The anomaly of specific heat in systems out of equilibrium, especially the measurement procedure dependence of specific heat, is investigated by means of free energy landscape. Introducing measurement procedure which is based on experimental method, we propose a calculation method of specific heat in systems out of equilibrium and find an abrupt change in specific heat between annealed and quenched states. For longer observation time the change in specific heat occurs at lower temperature and becomes sharper. For slower cooling of a system the transition temperature becomes lower. This cooling rate dependence of the transition temperature is consistent with experiments and thus the abrupt change in specific heat can be regarded as the glass transition which is thermally identified.

Explanation of the Glasslike Anomaly in the Low-Temperature Specific Heat of Incommensurate Phases

An explanation of the glasslike anomaly observed in the low-temperature specific heat of incommensurate phases is proposed. The key point of this explanation is to properly account for the phason damping when computing the thermodynamic magnitudes. The low-temperature specific heat of the incommensurate phases is discussed within three possible scenarios for the phason dynamics: no phason gap, static phason gap, and a phason gap of dynamical origin. Existing NMR and inelastic scattering data indicate that these scenarios are possible in biphenyl, blue bronze K(0.30)MoO(3), and bis (4-chlorophenyl) sulfone respectively. Estimates of the corresponding low-temperature specific heat are in reasonable agreement with the experiments.

Superconductivity in boron-doped diamond

Here we report the experimental study of a new superconductor, B-doped diamond. Subjecting a mixture of amorphous boron with graphite to a high-temperature and high-pressure treatment, we synthesized polycrystalline boron-doped diamond samples, whose sizes were suitable for specific heat measurements. For this kind of sample, we detected two specific heat anomalies at temperatures (∼4 K) closely corresponding to temperatures at which the resistance and magnetic susceptibility anomalies take place. The most probable origin of these specific heat anomalies is a bimodal B distribution in polycrystalline diamond. The correlation of physical properties, such as specific heat, electrical resistivity, and magnetic susceptibility, near the superconducting transition temperature is clear evidence of bulk superconductivity in these boron-doped diamond samples. Specific features in the low frequency range of Raman spectra correlate with superconducting properties of diamond. These features help to select heavily boron-doped CVD diamond films (DF) grown on (111) natural diamond, which exhibit superconductivity.

High-pressure phase diagrams of CeRhIn5 and CeCoIn5 studied by ac calorimetry

The pressure–temperature phase diagrams of the heavy fermion antiferromagnet CeRhIn5 and the heavy fermion superconductor CeCoIn5 have been studied under hydrostatic pressure by ac calorimetry and ac susceptibility measurements using diamond anvil cells with argon as the pressure medium. In CeRhIn5, the use of a highly hydrostatic pressure transmitting medium allows for a clean simultaneous determination by a bulk probe of the antiferromagnetic and superconducting transitions. We compare our new phase diagram with the previous ones, discuss the nature (first or second order) of the various lines, and the coexistence of antiferromagnetic order and superconductivity. The link between the collapse of the superconducting heat anomaly and the broadening of the antiferromagnetic transition points to an inhomogeneous appearance of superconductivity below Pc ≈ 1.95 GPa. Homogeneous bulk superconductivity is only observed above this critical pressure. We present a detailed analysis of the influence of pressure inhomogeneities on the specific heat anomalies which emphasizes that the observed broadening of the transitions near Pc is connected with the first-order transition. For CeCoIn5 we show that the large specific heat anomaly observed at Tc at ambient pressure is suppressed linearly at least up to 3 GPa.

Specific heat at the magnetic order transitions in RbFe (MoO [formula omitted]) [formula omitted

We measured the low temperature specific heat of antiferromagnetic RbFe(MoO 4 ) 2 single-crystal samples, by means of a microcalorimetric technique, in magnetic fields up to 15 T applied along the ab plane. Peaks in the specific heat vs. temperature were observed at all the magnetic phase transitions present in this system below 4 K. The temperature at which specific heat anomalies were observed are in agreement with earlier neutron scattering results. Additionally, we found that the transition into the paramagnetic state upon heating in a magnetic field of 10 T or larger decreases in increasing fields, extrapolating to zero temperature at an approximate field of 19 T.

Nature of the A phase in CeCu2Si2.

Neutron diffraction experiments have been performed on a magnetically ordered CeCu2Si2 single crystal exhibiting A-phase anomalies in specific heat and thermal expansion. Below T(N) approximately 0.8 K antiferromagnetic superstructure peaks have been detected. The propagation vector of the magnetic order appears to be determined by the topology of the Fermi surface of heavy quasiparticles as indicated by renormalized band-structure calculations. The observation of long-range incommensurate antiferromagnetic order as the nature of the A phase in CeCu2Si2 suggests that a spin-density-wave instability is the origin of the quantum critical point in CeCu2Si2.

Defective vortex lattices in layered superconductors with point pins at the extreme type-II limit

The mixed phase of layered superconductors with no magnetic screening is studied through a partial duality analysis of the corresponding frustrated $\mathrm{XY}$ model in the presence of weak random point pins. Isolated layers exhibit a defective vortex lattice at low temperature that is phase coherent. Sufficiently weak Josephson coupling between adjacent layers results in an entangled vortex solid that exhibits weak superconductivity across layers. The corresponding vortex liquid state shows an inverted specific-heat anomaly that we propose accounts for that seen in yttrium barium copper oxide. A three-dimensional vortex lattice with dislocations occurs at stronger coupling. This crossover sheds light on the apparent discrepancy concerning the observation of a vortex-glass phase in recent Monte Carlo simulations of the same $\mathrm{XY}$ model.

Defective vortex-lattices in layered superconductors with both point and correlated pins

The mixed phase of layered superconductors with no magnetic screening is studied through a partial duality analysis of the corresponding XY model in the presence of random pinning centers. Sufficiently weak Josephson coupling between adjacent layers results in an entangled vortex-solid that exhibits weak superconductivity across layers. The corresponding vortex-liquid state shows an inverted specific heat anomaly that is a precursor to the weak superconductor.

Double Kondo-lattice-like system in the ytterbium deuterides

Heat capacity measurements on the β-ytterbium deuteride samples with the composition D/Yb=2.71 and 2.46 have been carried out over a temperature range from 1.9 to 260 K. The low temperature specific heat anomalies at around 4 K in YbD 2.71 and at 6 K in YbD 2.46 are analysed in terms of two different Kondo-lattice systems which, depending on the hydrogen concentration, can be considered in the β-ytterbium deuterides. The specific heat anomaly in YbD 2.71 has been analysed as a non-magnetically doped Kondo semiconductor whereas anomaly at about 6 K in YbD 2.46 was interpreted as a result of the free carriers vanished owing to the bound electron–hole pairs.

Magnetic and transport properties of Fe-substituted manganites La0.67Ca0.33Mn1$minus;xFexO3

Specific heat, magnetic susceptibility and magnetoresistivity measurements have been performed on the polycrystalline samples of Fe-substituted manganites. The transition temperatures derived from the specific heat anomalies for x=0,0.01,0.06 were in agreement with the position of the maximum slope of AC susceptibility. For compositions with x=0.10 and 0.15 no transition was observed in the specific heat. A sharp increase in resistivity below 135 and 125K was observed in zero field for x=0.10, and 0.15, respectively.

Specific heat measurements of pyrochlore-type R 2Mo 2O 7 (R=Nd–Ho)

Specific heat measurements have been performed on pyrochlore-type R 2Mo 2O 7 (R=Nd–Ho) single crystals. For the ferromagnetic compounds with R=Nd–Gd ( T C≈50– 90 K) , Schottky-like specific heat anomalies corresponding to the level splitting for the 4f electrons of R 3+ are observed at low temperatures, in contrast to the specimens with R=Tb–Ho which show spin-glass-like order below T g≈20– 25 K . Low-temperature specific heat data of Sm 2Mo 2O 7 under a magnetic field of 10 T are presented.

Phase Transition in K 2 Fe(SO 4 ) 2 ·4H 2 O

In a Tutton's-salt like family with Fe atom, three compounds K 2 Fe(SO 4 ) 2 n H 2 O ( n = 2,4, or 6) are known. The phase transition in K 2 Fe(SO 4 ) 2 2H 2 O was already examined by the authors' group. Anomalies of both specific heat and dielectric constant were recently found on K 2 Fe(SO 4 ) 2 4H 2 O at 283 K. Structure analyses were performed, therefore, at 300 K (monoclinic, space group C 2/ m ; a = 11.853(1), b = 9.544(1), c = 9.951(1)å, g = 94.81(1), V = 1121.7(5)å 3 ; Z = 4) and 123 K ( P 2 1 / a, Z = 4).

Phonon, Spin-Wave, and Defect Contributions to the Low-Temperature Specific Heat of α-FeOOH

The specific heat of α-FeOOH was measured in the temperature range 0.6 to 30 K by a combination of semi-adiabatic and isothermal techniques. A broad specific-heat anomaly was detected at approximately 1 K. The data above this anomaly were fitted to the equation C=B asw T 3 e −Δ/T +∑ B n T n, where ∑ B n T n represents the harmonic-lattice model for the lattice specific heat, and B asw T 3 e −Δ/T represents the magnetic specific heat. The e −Δ/T term that modifies the usual T 3 term for an antiferromagnetic spin-wave contribution accounts for a spin-wave gap in the spin-wave spectrum, where Δ is the width of the spin-wave gap in Kelvin. As expected, the root-mean-square deviation (RMS) of the fit decreased progressively with inclusion of additional lattice terms, but the Debye temperature, Θ D , and the spin-wave gap, Δ, remained essentially unchanged. The addition of a linear term, γT, decreased the RMS of the fit and eliminated the need for an excessive number of lattice terms. The γT contribution to the specific heat, rather than being electronic in nature, is associated either with a small number of Fe3+ vacancies, or with loosely bound particles vibrating in defect sites in the crystallite interfaces. The possibility of the existence of vacancies is implied by the excess water content in the sample (0.083 moles of H2O per mole of FeOOH) and the presence of the interfaces is indicated by the particle size (determined by BET surface area measurement) being larger than the individual crystallite size (determined by X-ray diffraction).

Specific heat anomaly at the glass transition

A general frame work is devised to obtain the specific heat of nonequilibrium systems described by the energy-landscape picture, where a representative point in the phase space is assumed to obey a stochastic motion which is governed by a master equation. The specific heat depends on the observation time and becomes quenched one for short observation time and annealed one for long observation time. In order to test its validity, the frame work is applied to a two-level system where the state goes back and forth between two levels stochastically. The specific heat is shown to increase from zero to the Schottky form as the observation time is increased from zero to infinity. The anomaly of specific heat at the glass transition is reproduced by a system with a model energy-landscape, where basins of the landscape form a one-dimensional array and jump rate between adjacent basins obeys a power-law distribution. It is shown that the glass transition can be understood as a transition from an annealed to a quenched system and that the glass transition temperature becomes lower when the observation time is increased.

First-order superconducting phase transition in CeCoIn5.

The superconducting phase transition in heavy fermion CeCoIn5 (T(c)=2.3 K in zero field) becomes first order when the magnetic field H parallel [001] is greater than 4.7 T, and the transition temperature is below T0 approximately 0.31T(c). The change from second order at lower fields is reflected in strong sharpening of both specific heat and thermal expansion anomalies associated with the phase transition, a strong magnetocaloric effect, and a steplike change in the sample volume. This effect is due to Pauli limiting in a type-II superconductor, and was predicted theoretically in the mid-1960s.

Thermal transport in chiral conformal theories and hierarchical quantum Hall states

Chiral conformal field theories are characterized by a ground-state current at finite temperature, that could be observed, e.g., in the edge excitations of the quantum Hall effect. We show that the corresponding thermal conductance is directly proportional to the gravitational anomaly of the conformal theory, upon extending the well-known relation between specific heat and conformal anomaly. The thermal current could signal the elusive neutral edge modes that are expected in the hierarchical Hall states. We then compute the thermal conductance for the Abelian multi-component theory and the W 1+∞ minimal model, two conformal theories that are good candidates for describing the hierarchical states. Their conductances agree to leading order but differ in the first, universal finite-size correction, that could be used as a selective experimental signature.

Specific heat of Nd 1− xSr xMnO 3 ( x∼0.5)

The specific heat of the single crystal of Nd 1− x Sr x MnO 3 ( x∼0.5) has been measured at 20 K< T<280 K in H<5 T. Two specific heat anomalies are observed at T C∼255 K and T N∼157 K ( x=0.5) and 164 K ( x=0.5+ δ with δ≃0.002). The former transition is attributed to the ferromagnetic order of Mn 3+ and Mn 4+ spins and the latter the charge and orbital orders at the ferromagnetic–antiferromagnetic (AF) transition. Double peak structure is seen at T N in the sample with x=0.5+ δ, but not for x=0.5. These two peaks are related to the phase segregation of the CE and A-type AF order phases in narrow x range of 0.5< x<∼0.52.

Specific heat study of the complex phase transitions in magnetism and dielectricity on triangular lattice antiferromagnets

The complex phase transitions involving dielectricity and magnetism on distorted triangular lattice antiferromagnets, "KNiCl3-family" crystals, which have the crystal structures derived from the prototype CsNiCl3, were studied through the heat capacity measurements. These crystals are classified into characteristic three groups from the viewpoint of magnetic and structural properties clarified so far. The results of the dielectric constant measurement as a function of temperature are also presented in detail. The present calorimetric study reveaed that the structural successive phase transitions rather than the magnetic transitions in these crystals are recognized more distinctly as the presence of specific heat anomalies at the respective phase transition points.

New approach to the anomalies of thermodynamic properties of mixed valence compounds

A new interpretation of a specific heat and thermal expansion anomalies is proposed for mixed valence compounds and applied to Sm1−xLaxB6 and Ce1−xLaxNi. These thermodynamic quantities are considered as a superposition of contributions resulting from lattice and electron (f- and conduction electron) excitation spectra.