K.-Q. Wang

Specific heat up to 14 tesla and magnetization of a Bi2Sr2CaCu2O8 single crystal thermodynamics of a 2D superconductor

Abstract A 0.25 g single crystal of Bi 2 Sr 2 CaCu 2 O 8 with a sharp superconducting transition at 85 K is characterized by AC susceptibility, X-ray diffraction, Meissner effect, normal-state susceptibility, magnetization in the mixed state, and specific heat from 1 to 300 K. The residual linear term γ ∗ T in the specific heat at low T is small and field independent. High-resolution ( t c show that: (a) there is no mean-field specific heat jump at T c ; (b) a symmetrical logarithmic ‘divergence’ occurs at the transition; (c) its amplitude decreases with B -2 3 ; (d) its width scales with B 1 2 ; (e) T c does not depend on the field, and coincides with the crossing point of the magnetization curves M ∗ ( H, T ∗ ); (f) a crossing point C x (H, T x ) is found in the specific heat 10 K below T c ; (g) the effect of a field along the CuO planes is at least 50 times smaller than that of a field along the c axis; (h) no transition is observed at the irreversibility line. An empirical formula for ϑ( C/T )/ϑ H that accounts for C ( H, T ) and M ( H, T ) data over a wide temperature and field range is proposed.

Split superconducting transitions in the specific heat and magnetic susceptibility of YBa2Cu3Ox versus oxygen content

A homogeneous batch of YBa2Cu3Ox porous ceramics was studied in detail as a function of the annealing temperature Ta. X-ray diffraction patterns and optical micrographs show definitely one single phase. Well-separated double superconducting transitions are, however, observed; most clearly in the specific heat curves, except for Ta≈460°C. Initial values are Tc1=87 K and Tc2=92 K for Ta=300°C. Upon increasing the annealing temperature Ta to 500°C, Tc1 increases to 93.5 K while Tc2 remains almost constant. Beyond Ta=500°C, both Tc1 and Tc2 decrease. The essentially reversible splitting of the superconducting transition appears to be due to the complex kinetics of the oxygen uptake. The two critical temperatures Tc1 and Tc2 are attributed to the high oxygen content external layer of the grains and to the low oxygen content grain cores, respectively.

Preparation and superconducting properties of high-quality Bi-2212 ceramics

Abstract High-quality Bi-2212 ceramic samples were obtained by a solid-state reaction method. At the starting composition Bi 2.12 Sr 1.90 Ca 1.02 Cu 1.96 O y , X-ray diffraction, optical micrographs and scanning electron microscopy investigations show that the level of impurity phases is less than 1% by volume. Synthesis at a few degrees below the melting point favours grain growth. A final annealing treatment at controlled temperature and oxygen pressure, followed by rapid quenching, yields sharp superconducting transitions, as observed by measurement of the a.c. susceptibility and Meissner effect, and specific heat experiments. The Meissner fraction in 2 mT approaches 50%.

Specific heat up to 14 tesla of a YBa2Cu3O6.92 single crystal

Abstract A large porous YBa2Cu3O6.92 single crystal was synthesized using a recrystallisation technique. The superconducting transition at Tc = 92.4 K could be narrowed down to 0.14 K by quenching. We present high resolution specific heat measurements in magnetic fields from 0 to 14 T applied along the principal crystallographic directions. Thermal fluctuations near the phase transition in zero field are analysed in terms of critical behaviour. The Gaussian approximation is found to be unsuitable inside of a temperature window |T − Tc| ≈ 3 K. Measurements in magnetic fields indicate an anisotropy of ϵ = 7 ± 1 for for Hc2, a positive curvature of the Hc2(T) line near Tc, and a mixed-state linear term γmT increasing with H. The slope of the upper critical field at Tc, μ0∂Hc2,c/∂T, is found to be approximately -3.2 T/K both by direct determinations of Tc(H), and by the analysis of Gaussian fluctuations in H = 0. The scaling behaviour predicted either by the low-field 3D-XY or by the high-field critical approaches are compared.

Specific heat of Tl2Ba2CuO6 (“2201”) 90 K superconducting ceramics in magnetic fields up to 14 T

Abstract The specific heat of Tl 2 Ba 2 CuO 6 (“2201”) superconducting ceramics with T c ≈ 90 K is measured in magnetic fields of 0, 0.1, 1, 3, 5, and 14 T. The very small anomaly at the superconducting transition can be described in B = 0 by 2D fluctuations alone, without any measurable mean-field component. Its amplitude is found to decrease rapidly for small fields, and very slowly at large fields while its shape remains symmetrical. The temperature of the top of the anomaly ( “T c ” ) is field independent. These properties show that the dimensional nature of the phase transition in Tl 2 Ba 2 CuO 6 is more closely related to that of Bi 2 Sr 2 CaCu 2 O 8 than to that of YBa 2 Cu 3 O 7 . The behavior of the specific-heat anomaly with the field is thermodynamically consistent with previous magnetization measurements.

Specific heat, magnetic properties and critical behaviour of Mn2SiS4 and Fe2GeS4

The olivine Mn2SiS4 is antiferromagnetic (AF) up to 83 K, behaves as a weak ferromagnet (WF) from 83 to 86 K, and becomes paramagnetic (P) at higher temperatures. Its counterpart Fe2GeS4 undergoes similar transitions in a reversed order: WF up to 69 K, AF up to 144 K and then P. These transitions are characterized by susceptibility measurements and investigated by high resolution specific heat experiments. The transition to the paramagnetic state is of the critical type and belongs to the universality class of the Heisenberg model for Mn2SiS4 and the 3D Ising model for Fe2GeS4. The WF/AF or AF/WF transition is of first order with a latent heat ΔH = 0.09 J/mol for Mn2SiS4 and ΔH = 105 J/mol for Fe2GeS4. The lattice is found to be stiffer for the Mn thiosilicate than for the Fe thiogermanate.

Specific heat of YBa2Cu3O7−δ ceramics with single and double superconducting transitions in magnetic fields up to 14 T

Abstract The specific heat of porous YBa 2 Cu 3 O ≈7.00 ceramics with two well separated steps at the superconducting transition was measured in magnetic fields up to 14 T. The same measurements were performed for an YBa 2 Cu 3 O ≈6.93 ceramics from a similar batch annealed reversibly to obtain one single sharp transition. The results show that both components of the double transition, as well as the single transition, are transformed in the same way under the application of a field, both in position and width. It is concluded that both components of the split transition are of superconducting origin, and are related with the existence of two kinds of homogeneous domains, the typical sizes of which are large compared to the superconducting coherence length. Depending on the criterion chosen for the definition of T c ( H ), the angle-averaged critical-field slopes μ 0 ∂H c 2 / ∂T | Tc are found to range from −5.4 to − 10.1 T/K for all transitions. The slopes for both components of the split transition are equal within ±10%.

A dimensional characteristic in the specific heat of high temperature superconductors

Abstract The specific heat C of superconductors is strongly depressed just below the transition temperature by an increase of the magnetic field ΔB . A measurement of the function | ∂ ( C / T )/ ∂B | for different magnetic fields at T ≈ T c , which can be approximated by a Lorentzian of half-height width ΔT ( B ) αB n , is found to be a direct way of determining the dimensionality of a superconducting system in the LLL (lowest Landau level) approximation, with n= 1 2 D and n= 2 3 for 3D. No baseline subtraction is required. Examples are given using high resolution specific heat measurements between 0 and 16 tesla on YBa 2 Cu 3 O 7−δ (YBCO) and Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) single crystals. The temperature T cB ( B ) of the minimum of | ∂ ( C / T )/ ∂B | can be related to the mean-field critical temperature T c ( B ) in the framework of the theory of Tesanovic at high fields.

Specific heat of Bi2Sr2CaCu2Ox versus oxygen content (0≤B≤14T)

In order to characterize more precisely the superconducting transition of a quasi-2D superconductor, we quenched polycrystalline samples of Bi2Sr2CaCu2Ox from various locations of the (T; p(O2)) equilibrium diagram. A well-defined specific heat peak at Tc is observed with 0.01% resolution for the samples with the highest Tc. The samples with lower Tcs provide strong support to the normal-state background determined by an analysis of the data including phonon and fluctuation terms. We do not find any indication of a mean-field jump within experimental uncertainty, in stark contrast to YBa2Cu3O7. The application of a 14 Tesla field depresses the total specific heat by 0.54% only at T(peak)=92.5K, but this effect reduces to less than 0.03% already ±8K away from Tc. On passing the irreversibility line, no anomaly in the specific heat is detected at the 0.01% level.

Double superconducting transitions in YBa2Cu3Ox versus oxygen content

A homogeneous batch of YBa2Cu3Ox porous ceramics was studied in detail as a function of the annealing temperature Ta. X-ray patterns and optical micrographs show definitely one single phase. Well separated double transitions are however observed, most clearly in the specific heat curves, except for Ta≈460°C. Initial values are Tc1=87 K and Tc2=92 K for Ta=300°C. Upon increasing the annealing temperature to Ta=500°C, Tc1 increases to 93.5 K while Tc2 remains almost constant. Beyond Ta=500°C, both Tc1 and Tc2 decrease. The reversible splitting of superconducting transition appears to be due to the complex kinetics of oxygen uptake. Tc1 is attributed to the external layer of the grain and Tc2 to the grain cores.