Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

Specific heat of La(Fe<Subscript>0.873</Subscript>Co<Subscript>0.007</Subscript>Al<Subscript>0.12</Subscript>)<Subscript>13</Subscript> compound in antiferromagnetic and ferromagnetic states

The low-temperature specific heat C _p of La(Fe_0.873Co_0.007Al_0.12)₁₃ compound has been measured in two states: (i) antiferromagnetic (AFM) with a Néel temperature of T _N = 192 K and (ii) ferromagnetic (FM). The FM order appears at T = 4.2 K in a sample exposed to an external magnetic field with induction B _C ≥ 2.5 T and is retained for a long time in a zero field at temperatures up to T*_C = 23 K. The coefficient γ_FM in the low-temperature specific heat C = γT + βT ³ in the FM state differs quite insignificantly from that (γ_AFM) in the AFM state. Contributions to the low-temperature specific heat, which are related to a change in the elastic and magnetoelastic energy caused by magnetostrictive deformations, are considered.     

Heavy fermion behaviour of the nonmagnetic CeCu4Al compound

We present measurements of the temperature dependence of the electrical resistivity, the thermopower and the specific heat of the hexagonal compound CeCu₄Al. At high temperatures, the electrical resistivity is characterized by a nearly temperature independent behaviour, followed by a continuous increase below 100K. No maximum has been found down to 1.7 K. The thermopower shows a positive maximum at about 30 K. As in CeCu₆ no negative values are observable in the range from 4.2 K up to a room temperature. The specific heat data between 7 and 15 K reveal a γ value around 280 mJ mol^-1 K^-2. Below this temperature range the specific heat c_p/T shows a rapid rise and crosses the value of 1 J mol^-1 K^-2 at about 1.45 K.     

Incommensurate magnetic structure and crystal field in ErNi2B2C Studied by 166Er Mössbauer spectroscopy and specific heat measurements

¹⁶⁶Er Mössbauer absorption spectra have been recorded in the superconductor ErNi₂B₂C (T _c = 10 K), in zero field and in the temperature range 1.4 K–40 K. The spectra in the magnetically ordered phase (T _N ? 5.5 K) are characteristic of an incommensurate magnetic structure with a maximum Er³⁺ moment of 8.2 μB. We show that the magnetic transition is first order, with a small temperature range where magnetically ordered and paramagnetic domains coexist. The analysis of the magnetic and quadrupolar hyperfine interactions below and above T _N suggests that the Er³⁺ ground doublet is close to |J = 15/2; J _z = ± 1/2 > and shows that there is a low lying (? 10 K) excited doublet. The measured Er³⁺ electronic relaxation rate 1/T₁ shows an anomaly at 10 K (T _c suggesting that the conduction electrons that are exchange coupled to the 4f spin take part in the formation of the superconducting state. We also present specific heat data for Er _x Y_{1 ? x} Ni₂B₂C and Er_0.2Lu_0.8Ni₂B₂C which also evidence a low lying doublet and from which we propose a modified (improved) Er³⁺ crystal field level scheme (0, 9, 62, 71, 73, 181, 212 and 216 K) where the energies of the higher levels are consistent with published neutron scattering data.     

Superconductivity and calorimetric studies of Pr1.85Ce0.15CuO4+δ under different annealing conditions

Polycrystalline samples of electron-doped Pr_1.85Ce_0.15CuO_4+δ have been prepared under different annealing conditions and investigated by means of X-ray-diffraction, oxygen content analysis, electrical resistivity, magnetic susceptibility and low temperature specific heat measurements. X-ray-diffraction patterns show that samples contain a single T′ phase. The superconducting transition temperatures T_cm taken with the onset of diamagnetism in magnetic-susceptibility measurements are 20 and 19.5 K for sample annealed in flowing Ar gas and in vacuum (∼10⁻³ torr), respectively. The data of the samples, which are annealed in flowing Ar gas, show clear evidence for an αT² term at zero magnetic field in superconducting electronic specific heat, and are consistent with d-wave superconductivity. However, this behavior is not observed in the other sample, which is annealed in vacuum. These results indicate that different heat treatments affect the oxygen content, homogeneity, superconducting transition temperature T_c, superconducting volume fraction, and the superconducting pairing symmetry of Pr_1.85Ce_0.15CuO_4+δ.     

The specific heat of Y0.7Th0.3C1.58

The specific heat of the novel high temperature superconductor Y_0.7Th_0.3C_1.58 (T_c = 17.0 K) has been measured between 4 and 22 K. Unlike the other known high temperature superconductors (T_c > 16 K) which have either an A-15 or a NaCl-type structure, this material forms in the b.c.c., Pu₂C₃-type, structure. The Debye temperature, θ_D, is 346 K and the linear term coefficient, γ, of the specific heat has the value 4.66 mJ/mole-K². Thus the electronic density of states, N(0), which is proportional to γ, is quite low. The energy gap, 2Δ/kT_c, on the other hand has an anomalously high value of 5.8. Comparisons between these parameters of Y_0.7Th_0.3C_1.58 and those for some A-15 and NaCl-type superconductors are made.     

Phase transition in electronic manganite Ca0.85Sm0.15MnO3

The resistivity, the magnetic susceptibility, the magnetization, and the specific heat of electronic manganite Ca_0.85Sm_0.15MnO₃ were studied. The data obtained suggest that this compound undergoes phase transition into the insulator antiferromagnetic state at T _c ～115 K and displays negative magnetoresistance at T<T _c . A minor ferromagnetic component of 0.025µ_B in the magnetization of Ca_0.85Sm_0.15MnO₃ may be caused by the deviation of this composition from the exact stoichiometry Mn³⁺: Mn⁴⁺=1: 8. The Debye temperature Θ_D=575 K and the entropy of phase transition ΔS=5.1 J/(mol K) were derived from the temperature dependence of specific heat.     

Measurements of the heat capacity of diamond with different isotopic compositions

We report measurements of the heat capacity C_p of diamond with several isotopic compositions (¹²C_1−x¹³C_x) in the 20-280 K temperature range. The results, in particular the derivative of the specific heat with respect to the average isotopic mass, are compared with recent ab initio calculations. Agreement is good for T>150 K. At lower temperatures, the samples with isotopic compositions different from the natural one display an anomalous increase in C_p/T³ with decreasing temperature which is attributed to the presence of metallic inclusions stemming from the metal used as a solvent in the high-pressure high-temperature growth process.     

Specific heat of Na1−x V2O5 single crystals

Temperature dependences of the specific heat C and the magnetic susceptibility χ of Na_1?x V₂O₅ single crystals (x=0, 0.01, 0.02, 0.03, and 0.04) are studied. In NaV₂O₅, the transition to the spin-gap state (T _c =34 K) is accompanied by a sharp decrease in χ, while C exhibits a λ-shaped anomaly. At low temperatures, the specific heat of NaV₂O₅ is approximated by the sum of phonon ～T ³ and magnon ～exp(?Δ/T) contributions, which makes it possible to estimate the Debye temperature Θ_D=336 K and the gap in the magnetic excitation spectrum Δ=112 K. With the departure from stoichiometry, the anomalies observed in the behavior of χ and C are spread and shifted to lower temperatures. The low-temperature specific heat of nonstoichiometric samples is determined by the sum of phonon and magnon components and the contribution due to the presence of defects. The values of magnetic entropy characterizing the phase transitions in Na_1?x V₂O₅ are calculated.     

Specific heat of SbSI

The temperature dependence of the specific heat of vapor and flux grown single crystal samples of ferroelectric antimony sulfur iodide (SbSI) was measured over a temperature range from 230K to 400K. C_p^E at T_c of the electrical flux and vapor grown samples are 0.078 cal/gmK and 0.076 cal/gmK respectively. ΔS and ΔQ at T_c are evaluated. The samples measured were grown by two different methods[1,2]. Differences are observed between the measurements made on the samples, both in the magnitude of the specific heat and the temperature at which the ferroelectric transition takes place. These differences are caused by the different levels of oxygen impurities in the samples.     

Interactions of μAl acceptor impurity in weakly and heavily doped silicon

The interactions of the aluminum acceptor impurity in silicon are investigated using polarized negative muons. The polarization of negative muons is studied as a function of temperature on crystalline silicon samples with phosphorus (1.6×10¹³ cm^?3) and boron (4.1×10¹⁸ cm^?3) impurities. The measurements are performed in a magnetic field of 4.1 kG perpendicular to the muon spin, in the temperature range from 4 to 300 K. The experimental results show that, in phosphorus-doped n-type silicon, an _μAl acceptor center is ionized in the temperature range T>50 K. For boron-doped silicon, the temperature dependence of the shift of the muon spin precession frequency is found to deviate from the 1/T Curie law in the temperature range T ? 50 K. The interactions of a _μAl acceptor that may be responsible for the effects observed in the experiment are analyzed.     

Thermal and magnetic properties of DyB62 at low temperatures

Temperature dependences of heat capacity C_P(T) and magnetization M(T) of an icosahedral dysprosium boride (DyB₆₂) single crystal have been experimentally investigated in the temperature range of 2-300 K. The magnetic susceptibility χ(T) of DyB₆₂ follows Curie-Weiss law with a paramagnetic Curie temperature of −3.7 K, which implies that the antiferromagnetic interactions are dominant in this material and suggests the possibility of magnetic ordering at low temperatures. This conjecture is supported by the temperature dependence of heat capacity C_P(T), which decreases upon heating from 2 to 7 K. The heat capacity of DyB₆₂ at 2 K is analyzed as a sum of magnetic, Debye, two-level system and soft atomic potential components.     

Specific heat and magnetic susceptibility of single-crystalline ZnCr2−xAlxSe4 (x=0.15, 0.23)

A correlation between the second critical field H_c2 of the helix to paramagnetic transition and the magnetic specific heat C-peak was found in ZnCr_2−xAl_xSe₄ spinel single crystals with x=0.15, 0.23. The specific heat peak is anomalously sharp for all finite magnetic fields used here and this points to a first order magneto-structural transition (from cubic to tetragonal symmetry). The C(T)-peak is increasingly suppressed as the external field increases. Approaching the Neel temperature T_N, a broad ac-magnetic susceptibility peak is observed for zero dc-magnetic field. That peak does not show an energy loss and thus points towards a return to a second order type of transition. The magnetic contribution to the specific heat displays a sharp peak at T_N and is maximal at the spin fluctuation temperature T_sf=34 K. T_sf is related to the maximum of the magnetic susceptibility at T_m=40 K (at 50 kOe) in the spin fluctuation region, as evidenced by the entropy exceeding 90% of the entropy calculated classically for the complete alignment of the Cr spins, (2−x)R ln(2S+1). The X-ray photoelectron spectroscopy (XPS) data indicate that Al-substitution does not affect Cr³⁺ 3d³ electronic configuration.     

A.C. specific heat of terbium and dysprosium

The dependence of a.c. specific heat measurements upon the temperature modulation amplitude ΔT has confirmed the first-order nature of the antiferromagnetic-ferromagnetic transition in single crystal terbium (T_C～221K) and polycrystalline dysprosium (T_C～85K). No peak in a.c. specific heat is obtained for 2ΔT less than the temperature hysteresis at the transition. By comparison, the peaks expected in a.c. specific heat were observed at the higher-order antiferromagnetic-paramagnetic transition in both materials for ΔT amplitudes as low as～40 mK.     

Low temperature specific heat of KCl: OH with high OH concentration

We report on measurements of the specific heat (between 0.035 and 3 K) of two KCl crystals doped with 0.5 and 1 at.% OH^-. For the 1 at.% sample, the excess specific heat C due to OH^- varies roughly linearly between 0.035 and 0.6 K. A similar behaviour is also found for 0.5 at.% between 0.2 and 0.6 K while below 0.2 K, C ～ T^1.8. Thus, contrary to previous measurements, a T^{$\text{3}\text{2}$} dependence is not observed. We discuss our results in terms of strain and electric dipolar interactions.     

A specific heat and magnetization study of highT c ceramic superconductors

From the temperature dependence of the specific heat of the semiconductor La₂CuO₄ and the high temperature superconductors La_1.8Sr_0.2CuO₄ (T _c =37.2 K) and YBa_1.9K_0.1Cu₃O_6.9 (T _c =91.5 K) in the range 1.5–30 K, a strong similarity of the lowfrequency part of their phonon density of states with a peak around 10 meV could be inferred. In the case of La_1.8Sr_0.2CuO₄ the thermodynamical critical field belowT _c has been determined and using the Rutger's formula and the BCS model, a Sommerfeld coefficient γ=9 mJ·mol^?1 K^?1 was obtained, which, taking into account recent results of band structure calculations leads to an electron-phonon enhancement factor γ=1.3, value compatible withT _c =36 K when using McMillan's formula forT _c . A systematic study of the magnetization offered evidence for strong flux trapping effects at higher fields and for Meissner shielding by superconducting Josephson currents in fields below 6 mT at 4.2 K.     

Heat capacity of Heusler alloys: Ferromagnetic Ni2MnSb,Ni2MnSn,NiMnSb and antiferromagnetic CuMnSb

We report the results of the investigation of the specific heat of the ferromagnetic Heusler Ni₂MnSn, Ni₂MnSb, NiMnSb and antiferromagnetic CuMnSb alloys. The low-temperature behaviour of the specific heat may be described as C=γT+βT³ for ferromagnetic compounds and as C=γT+δ T²+βT³ for antiferromagnetic CuMnSb. The values of the density of states from the heat capacity measurements are higher than those from electronic band structure calculations. Debye temperatures are in a good agreement with those obtained from thermal expansion measurements. The Grüneisen parameter is calculated for Ni₂MnSn and CuMnSb from the magnetic contribution to the specific heat in the vicinity of T_C or T_N.     

The low temperature specific heat of Lu-Cu-Y metallic glasses

The specific heat of a series of amorphous metallic alloys of the form Lu_xCu_0.37Y_0.63-x (x = 0, 0.1, 0.3 and 0.4) has been measured between 2 and 50 K, primarily in order to be able to determine the non-magnetic contributions of the specific heat in magnetic RE-Cu-Y amorphous alloys. The data at low temperature fit the simple form C_p = γT + βT³ from which values of γ and θ_D(0) have been determined. Consideration is given to the error that arises if Y is used rather than Lu or La in forming non-magnetic rare earth intermetallics for purposes of determining the non-magnetic contributions to the specific heat of magnetic samples. A simple procedure is described that allows a useful improvement in accuracy in estimating non-magnetic contributions below 20 K if Y is used. The method may also be useful if only a restricted range of compositions using Lu is possible.     

Magnetic and thermal properties of single-crystal NdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The neodymium ferroborate NdFe₃(BO₃)₄ undergoes an antiferromagnetic transition at T _N = 30 K, which manifests itself as a λ-type anomaly in the temperature dependence of the specific heat C and as inflection points in the temperature dependences of the magnetic susceptibility χ measured at various directions of an applied magnetic field with respect to the crystallographic axes of the sample. Magnetic ordering occurs only in the subsystem of Fe³⁺ ions, whereas the subsystem of Nd³⁺ ions remains polarized by the magnetic field of the iron subsystem. A change in the population of the levels of the ground Kramers doublet of neodymium ions manifests itself as Schottky-type anomalies in the C(T) and χ(T) dependences at low temperatures. At low temperatures, the magnetic properties of single-crystal NdFe₃(BO₃)₄ are substantially anisotropic, which is determined by the anisotropic contribution of the rare-earth subsystem to the magnetization. The experimental data obtained are used to propose a model for the magnetic structure of NdFe₃(BO₃)₄.