Exchange energy for conduction electrons in (ZnMn)Se derived from spin-flip Raman scattering and magnetization

Spin-flip Raman scattering, magnetization, and susceptibility data for Zn_0.97Mn_0.03Se are reported. The exchange energy N_oα = 243 ± 10 meV for the conduction electrons is obtained from an analysis of the Raman and magnetization data. At large magnetic fields (H > 60 kOe), the spin-flip energy ΔE saturates at 14 meV. At low fields ΔE does not extrapolate to zero as H → 0, which is characteristic of scattering from donor-bound electrons. The low temperature magnetization curves are fit to a modified Brillouin function. The fit gives $\text{x}\text{?}\text{/x}\text{= 0.67}$ as the fraction of active magnetic ions, and an effective temperature T_eff = T + T_o with T_o = 1.1 K. The magnetic susceptibility follows a Curie-Weiss law between T = 150 and 280 K with a Curie-Weiss temperature θ = ?33 K.     

Magnetization behaviour of DyAl2 single crystals

We report the theoretical interpretation of the magnetization and the magnetocrystalline anisotropy of ferromagnetic DyAl₂ single crystals between 4.2 and 60 K and magnetic fields up to 15 T. Good agreement between theory and experiment is obtained by using three temperature independent parameters: the two crystal field parameters B₄ = (?0.50 ± 0.05) × 10^?4 meV, B₆ = ? (0.51 ± 0.05) × 10^?6 meV and the Curie temperature T_c = (62 ± 2) K.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature T _c ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, Δ_Γ = 5.1–6.5 meV, Δ_L = 3.8–4.8 meV, and Δ_S = 0.9–1.9 meV (at T ? T _c), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ～12, and ～20%, respectively. Andreev spectra have been fitted within the extended Kümmel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the Δ(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming Δ_Γ and Δ_L are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λ_ΓL ≈ λ_LΓ ? λ_SΓ, λ_SL.     

The excitonic absorption of CdS under pressure

Abstract

We have investigated the pressure dependence of the absorption edge of tlie E_A-E_B- and the E_C-excitons of 1pi thick CtlS(hex) crystal slabs. We obtaiurd dE_Θ/dP =dE_c/dP = 47±3 meV/GPa for T=300K and dE_A/dP= dE_B/dP = 43±3 meV/GPa for T=78K. With tliese values we were able to determine the crystal-field splitting (68f4 nieV) aid the spin-orbit splitting (26±2 meV). The resulting pressure shifts of tlie rxcitonic energies were in agreement witli a κ · p-calculation of tlie transition energies using tlie perturbation niatrix of G.E. Pikus for Δ_so ? Δ_c. The refractive Index n‖c was iiieasured for T=300K up to 2.2 GPa and described by Marples Model.     

The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH3NH3)2MnCl4, from 10 to 300k

The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH₃NH₃)₂MnCl₄, has been measured over the range 10K <T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with ΔH_t = 727±5 J mol^?1 and ΔS_t = 7.76±0.05 J K^?1 mol^?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔH_t = 116±1J mol^?1 and ΔS_t = 0.451±0.004 J K^?1mol^?1. These results are discussed in the light of recent measurements on (CH₃NH₃)₂CdCl₄, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type.In addition to the structural phase transitions, (CH₃NH₃)₂MnCl₄ also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Spin-wave scattering from A γ-Fe47Mn53alloy

Spin waves in the antiferromagnetic alloy γ-Fe_0.5Mn_0.5 have been studied at $\text{295° K(}\text{T}\text{T}{}_{\mathrm{N}}\text{= 0.63)}$ by the inelastic neutron scattering technique. We observed an isotropic dispersion and obtained a value for the spin-wave velocity of 255 ± 30 meV Å (3.88 ± 0.50 × 10⁶ cm/sec), which is the order of the spin-wave velocity in Cr (a typical itinerant antiferromagnet). The energy gap at q = 0 was found to be 7.0 ± 0.5 meV. These results suggest the existence of a long-range spin ordering in the conduction electrons of this alloy.     

Observation of bosonic resonances in GdO<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>F<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FeAs by intrinsic multiple Andreev reflection effect spectroscopy

The current–voltage characteristics and dynamic conductance spectra of Andreev arrays in nearly optimal GdO_{1 ? x} F _x FeAs superconductors with the critical temperature T _c = 46–50 K have been studied. A reproducible fine structure that accompanies large gap features and caused by boson emission during the process of multiple Andreev reflections has been observed. At T = 4.2 K, the bosonic mode energy ε₀ = (12 ± 2) meV has been obtained.     

An EPR study of the temperature dependence of the energy gap in ytterbium dodecaboride

An EPR study of ytterbium dodecaboride (YbB₁₂) showed the presence of an energy gap with a width of 2Δ=12 meV in the energy spectrum of this Kondo insulator. The temperature dependence of the energy gap was determined by interpreting the experimental data within the framework of the exciton dielectric model: Δ(T)=72 K at an absolute zero and Δ(T)=0 at ～115 K. The temperature dependence of the EPR line-width exhibits a feature at 13–15 K, which is indicative of a finite density of states inside the gap. This can be related to the presence of impurity states or bound polaron excitations in the electron spectrum of YbB₁₂.     

Spin waves in MnO; from 4°K to temperatures close to TN

Spin waves have been measured in MnO by inelastic scattering of neutrons from 4 °K to a temperature 0·25 °K below T_N. The 4 °K spectrum is interpreted in the frame of linear spin wave theory with effective exchange integrals J₁^? = 0·321 meV, J₁⁺ = 0·424 meV, J₂¹ = 0·446 meV and a phenomenological parameter for anisotropy D₁ = 0·059 meV. The effect of the actual dipole-dipole Hamiltonian is shown to give a priori a very good account of the lifting of the degeneracy of spin waves near the Brillouin zone center. The Bloch model for interacting spin waves and theories based on Green function approximations have been adapted to the MnO case, in order to compute the main properties (mean magnetization of a sublattice, anisotropic deformation, extra isotropic contraction and magnon spectrum) at many temperatures. Comparison of these with experimental results tends to favour a generalized Callen renormalization model, which gives an overall fit from 4 °K, to temperalures close to T_N.     

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.     

Orientational and positional disorder of ions and its freezing in the CsNO2-TlNO2 system2

The heat capacities of Cs_0.695Tl_0.305NO₂ (Specimen I) and Cs_0.385Tl_0.615NO₂ (Specimen II) have been measured between 14 and 350 K. Specimen I underwent a phase transition at (197.7 ± 0.1) K, with ΔS = (19.2 ± 1.5) JK^?mol^?, and specimen II at (214.5 ± 0.2) K, with ΔS = (5.4 ± 1.0) JK^?1mol^?1, respectively. Above the phase transition, an exothermic temperature drift due to phase separation was observed. Annealing of the sample at 203 K for 300 hr brought about complete phase separation. The solid solution system annealed at 203 K gave two heat capacity peaks at (203.3 ± 0.1) K, with ΔS = (13.8 ± 0.8) JK^?1 mol^?1, and (242.4 ± 0.2) K, with ΔS = (10.6 ± 1.3) JK^?1 for Specimen I, and at (203.0 ± 0.1) K with ΔS = (6.7 ± 0.5) JK^?1 mol^?1, and (257.5 ± 0.2) K with ΔS = (17.9 ± 1.7) JK^?1 mol^?1 for Specimen II. The phase diagram of the CsNO₂-TlNO₂ binary system was constructed on the basis of DTA, heat capacity and dielectric measurements. In the metastable phase, the existence of a residual entropy due to the freezing of a random distribution of Cs⁺₁ and Tl⁺ cations in addition to the orientational disorder of the NO₂^?1 ion was confirmed by a comparison of entropies of the stable and the metastable phases.     

Superconducting energy gap distribution in c-axis oriented MgB2 thin film from point contact study

We have analyzed about a hundred voltage-dependent differential resistance dV/dI(V) curves of metallic point contacts between c-axis-oriented MgB₂ thin film and Ag, which exhibit clear Andreev reflection features connected with the superconducting gap. About one half of the curves show the presence of a second larger gap. The histogram of the double gap distribution reveals distinct maxima at 2.4 and 7 meV, while curves with single-gap features result in a more broad maximum at 3.5 meV. The double-gap distribution is in qualitative agreement with the distribution of gap values over the Fermi surface calculated by H. J. Choi et al. (cond-mat/0111183). The data unequivocally show the presence of two gaps: Δ_S=2.45±0.15 meV and Δ_L=7.0±0.45 meV in MgB₂ with the gap ratio Δ_L/Δ_S=2.85±0.15. Our observations further prove a widely discussed multigap scenario for MgB₂, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one.     

Phonon coupled cooperative low-spin 1A1high-spin 5T2 transition in [Fe(phen)2(NCS)2] and [Fe(phen)2(NCSe)2] crystals

Heat capacities of [Fe(phen)₂(NCS)₂] and [Fe(phen)₂(NCSe)₂] were measured between 13⁵ and 375 K. A heat capacity anomaly due to the spin-transition from low-spin ¹A₁ to high-spin π₂ electronic ground state was found at 176·29 K for the SCN-compound and at 231·26 K for the SeCN-compound, respectively. Enthalpy and entropy of transition were determined to be ΔH = 8·60 ± 0·14 kJ mol^?1 and ΔS = 48·78 ± 0·71 J K^?1 mol^?1 for the SCN-compound and ΔH = 11·60 ± 0·44 kJ mol^?1 and ΔS = 51·22 ± 2·33 J K^?1 mol^?1 for the SeCN-compound. To account for much larger value of ΔS compared with the magnetic contribution, we suggest that there is significant coupling between electronic state and phonon system. We also present a phenomenological theory based on heterophase fluctuation. Gross aspects of magnetic, spectroscopic, and thermal behaviors were satisfactorily accounted for by this model. To examine closely the transition process, infrared spectra were recorded as a function of temperature in the range 4000 ? 30 cm^?1. The spectra revealed clearly the coexistence of the ¹A₁, and the ⁵T₂ ground states around T_c.     

Thermal quenching of photoluminescence in GeSe2 glass,thin films and crystals

Investigations were carried out on the temperature dependence of the photoluminescence intensity and the emission band characteristics in GeSe₂ glasses, evaporated thin films and crystals at T >80 K, paying special attention to the fatigue taking place 20–50 s from the onset of excitation. From the temperature dependence of the initial and quasi-steady-state values of the PL intensity an activation energy 295?30 meV at T >210 K was deduced for the glass whereas two activation energies E₁=56±2 meV and E₂=96±3 meV were deduced for the crystalline GeSe₂. The strong fatigue observed in the evaporated thin films was attributed to their loose structure. The fast fatigue in the crystal was considered to result from the recombination of quasi-excitons through radiative and non-radiative channels. A configuration coordinate diagram involving a stable and a metastable state is proposed as a means of explaining this type of fatigue. The temperature dependence of the PL intensity is discussed in terms of a recently proposed model.     

Mößbauereffekt in Eisenstilben und FeCl2·H2O

Using Mößbauer effect measurements in the temperature range between 3 °K and 310 °K the magnetic fields at the nucleus in iron-stilbene, FeCl₂·H₂O and FeCl₃ are determined to beH _T=0=(250±10) kOe, (252±18) kOe and (468±10) kOe; a Néel-temperature ofT _N=(23±1) °K is measured for iron-stilbene. The electric quadrupole splittings atT=0 °K for iron-stilbene and FeCl₂ ·H ₂ O, ΔE=(+2.52±0.02) mm/sec and (+2.50±0.05) mm/sec, yield electric field gradients at the iron nucleus ofq _z=+9.7·10¹⁷ V/cm² and +9.6·10¹⁷ V/cm², whereq _z⊥H; Debyetemperatures of θ=162 °K and 188 °K are obtained. The energy of the excited 3d-electron levels in iron-stilbene is estimated to Δ₁=309 cm^?1 and Δ₂=618cm^?1 as deduced from the temperature dependence ofΔE. In contrast to the suggestion ofEuler andWillstaedt bivalence of the iron in ironstilbene is found; its composition is shown to be 4(FeCl₂ ·H ₂O)·stilbene.     

High-resolution heat capacity of SnCl2·2H2O single crystal

The heat capacity of SnCl₂·2H₂O single crystal was measured in the close vicinity of the phase transition temperature, T_c = 217.994 ± 0.01 K. Its anomalous part ΔC could be expressed as ΔC = A_± ⊥ (T ? T_c)/T_c⊥^-α_±, where α₊ = 0.492 ± 0.02, α_- = 0.492 ± 0.02, A₊ = 1.148 JK^?mol^?, and A_- = 1.155 JK^?mol^?. A quasi-isothermal absorption of the enthalpy amounting to 34 J mol^? was observed at T_c.     

Nb3Sn正常-超导转变时的比热反常

在16.0°K—20.3°K之间测量了Nb₃Sn样品的热容量。Nb₃Sn在临界温度附近的比热跳跃值ΔC=2.21(±5％)焦耳/克分子·度。样品的临界温度T_c=17.88°K,转变宽度ΔT_c≈0.2°K。ΔC值利用热力学关系式确定了Nb₃Sn在0°K时的热力学临界场H₀=5300奥斯特。利用本文的结果和文献上关于热膨胀系数的跳跃值Δα及?T/?P值验证了热力学关系式。扼要地描述了比热测量装置.     

Superconducting energy gap in Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals

Current-voltage characteristics of S-I-S tunnel break junctions fabricated from pure undoped Bi2223 single crystals (T _c =110 K) were measured. High quality of the crystals enabled production of good tunnel junctions with a low or almost zero leakage current and well developed gap structure in the tunneling spectra. The peak-to-peak energy gap values 2Δ_p-p in different crystals and the tunnel junctions ranged from 80 to 105 meV. The tunneling conductance in the superconducting state was normalized to that in the normal state and compared to a smeared BCS density of states. A simple fit of the data gave the average value of Δ=38.5 meV and reduced gap 2Δ/kT _c ?8, consistent with a very strong coupling mechanism.     

Investigation of the ferroelastic phase transition in the SrMgF4 pyroelectric crystal

A SrMgF₄ compound has been synthesized and a high optical-quality crystal has been grown. Optical-polarization observations, X-ray diffraction analysis, and the measurement of the birefringence Δn _i (t) in the SrMgF₄ crystal have been carried out in the temperature range of 90–1200 K. A second-order improper ferroelastic phase transition accompanied by birefringence anomalies and the symmetry change P112₁ (Z = 12) ? Cmc2₁ (Z = 4) has been discovered at T ₀ = 478 ± 1 K. The crystal remains pyroelectric in both phases. Considerable contributions of the fluctuations of the order parameter have been observed in the temperature ranges of (T ₀ ? T) < 15 K and (T ? T ₀) < 60 K.