Anisotropy of the heat capacity of a mixed-state superconducting Nd<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> single crystal for various magnetic field orientations with respect to the crystallographic axes

The anisotropy in the superconducting properties of single-crystal Nd_1.85Ce_0.15CuO₄ was studied from measurements of the heat capacity within the temperature interval 2–40 K in zero magnetic field and in a magnetic field of 8 T. We report on the first observation of heat capacity jumps occurring at the superconducting transition for various magnetic field orientations with respect to the crystallographic axes and on a strong anisotropy of the magnetic contribution to heat capacity in magnetic fields oriented in the a-b plane and perpendicular to it. These measurements yielded the anisotropy in the electronic heat capacity coefficient γ_n(H) and in the superconducting transition temperature T_c(H). The angular dependence of the Sommerfeld coefficient γ_n in the a-b plane observed in a magnetic field of 8 T exhibits four-lobe symmetry and zero gap direction of the order parameter. A comparison of the results obtained on the Nd_1.85Ce_0.15CuO₄ single crystal with the data available for La_1.85Sr_0.15CuO₄ permits one to conclude that the mechanisms of superconductivity in the electron-and hole-doped superconductors are similar.     

Field and temperature dependences of the specific heat of the La1.85Sr0.15CuO4 superconductor

This paper reports on the temperature and field dependences of the specific heat of high-quality La_1.85Sr_0.15CuO₄ single crystals carried out at low temperatures in magnetic fields of up to 8 T for two magnetic field orientations, namely, along the [100] and [110] crystallographic axes. The field dependence of the electronic density of states (DOS) was found to be anisotropic for different magnetic field orientations in the a–b plane, with the electronic density being the lowest along the a axis (for H ∥ [100]) and maximum for the field inclined at 45° to the a axis (for H ∥ [110]). Electronic specific heat in a magnetic field was observed to depend linearly on temperature T and nonlinearly on the magnetic field H: C _DOS=bTH ^1/2. In a zero field, the electronic specific heat grows quadratically with temperature as C _DOS=αT ². Estimation of the maximum superconducting gap width from the experimentally determined values of the α coefficient of T ² and of the electronic DOS in the normal state yields Δ ₀=300 K. The observed features indicate that La_1.85Sr_0.15CuO₄ is a superconductor with d symmetry of the order parameter.     

Anisotropy of the upper critical field and specific heat in V3Si — a superconductor with cubic symmetry

The specific heat of a V₃Si single crystal (T _c=17 K, H _c2=20 T) in magnetic fields up to 8 T isinvestigated experimentally for three orientations of the field relative to the crystallographic directions — H∥〈001〉, H∥〈110〉, and H∥〈111〉. Both the upper critical magnetic field and the specific heat of the mixed state are observed to depend on the orientation of the magnetic field relative to the crystallographic directions (anisotropy): The critical field reaches its maximum value and the specific heat its minimum value in a field along the 〈001〉 direction. The anisotropy scale in both phenomena increases as the magnetic field and reaches 3% in a 6 T field. The interrelationship of the upper critical field anisotropy and the specific-heat anisotropy in type-II superconductors is studied. It is shown that the anisotropy of the specific heat in the mixed state in weak fields can serve as a criterion for nontrivial pairing. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 26–29 (10 January 1999)     

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La_1.85Sr_0.15CuO₄ are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)     

Magnetostriction of the spin-Peierls compound CuGeO3

The longitudinal and transverse magnetostriction λ of the spin-Peierls compound CuGeO₃ in the b-c plane is measured in magnetic fields up to 20 T both above and below the transition temperature T _sp=14.3K. It is found that for a given crystallographic direction the value of magnetostriction is weakly dependent on the magnetic field direction. In the uniform U phase at T⩾T _sp, λ is negative and approximately equal in the b and c directions, while in the dimerized D phase at T<T _sp, λ is positive and λ _b >λ _c . At low temperatures, λ increases sharply at the magnetic-field-induced transition from the dimerized to the magnetic M phase. The experimental data allow estimation of the stress derivatives of the antiferromagnetic intrachain exchange interaction parameter and of the stress dependence of the critical field of the D-Mphase transition. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 156–159 (10 August 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Influence of magnetic field on the electronic specific heat of the organic metal (BEDT-TTF)2KHg(SCN)4

Specific heat measurements of a single crystal of the organic metal (BEDT-TTF)₂KHg(SCN)₄ have been carried out at low temperatures and under a magnetic field of up to 14 T. A jump in the specific heat of about 0.1 J/mol·K, which corresponds to the antiferromagnetic phase transition, has been observed. The magnetic field is found to decrease the transition temperature at any field orientation. The strongest effect was found to take place in the field direction along the highly conducting ac plane. Zh. éksp. Teor. Fiz. 113, 1058–1063 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Determination of signs of hyperfine coupling constants for an S = ½ system through E.P.R., ENDOR and double ENDOR

A systematic method of obtaining relative signs of hyperfine coupling constants is described. It applies to systems consisting of (a) a set of one or more nuclei coupled fairly strongly to the electron spin, and possessing a two-fold (or higher) axis of symmetry, together with (b) a set of weakly coupled nuclei defining superhyperfine transitions. ENDOR measurements for several E.P.R. hyperfine transitions, with the field oriented along the symmetry axis, give relative signs of hyperfine components for this direction. Signs for the other directions can then be obtained through ENDOR measurements on a single hyperfine transition at various field orientations. Additional double ENDOR measurements may be necessary for very weakly coupled nuclei. This method can complement double ENDOR studies in favourable cases. It is illustrated by the determination of signs of coupling constants of protons and of ⁷⁵As in the AsO₄ ^4- radical in KH₂AsO₄.     

On the electron mobility of δ layers in the presence of diamagnetic “ejection” of size-quantization levels

The Hall mobility of electrons is investigated as a function of the population of size-quantization subbands in the two-dimensional electron gas of a δ-doped layer in GaAs with constant total electron density N _s =3.2×10¹² cm⁻² (three initially filled subbands) at T=4.2 K. The population of the subbands is varied by diamagnetic “ejection” of size-quantization levels (i.e., pushing them over the Fermi level) by a magnetic field oriented parallel to the plane of the δ-doped layer. The measurements are made in magnetic fields making small angles (5°) with the plane of the doping. The magnetic field component normal to the plane was used to measure the Hall mobility and density. It is found that the measured Hall mobility as a function of the ejecting magnetic field has a distinct maximum. This maximum is due to an increase in the electron mobility in the first subband (the ground subband is assigned the index 0) and electron redistribution between subbands with in increasing ejecting magnetic field parallel to the plane of the δ layer. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 704–708 (10 December 1997)     

阻挫三角反铁磁AgCrO2螺旋自旋序的第一性原理研究

基于密度泛函理论的广义梯度近似(GGA)和投影缀加波(PAW)方法,分别从共线和非共线磁性结构出发,研究了自旋阻挫三角反铁磁AgCrO₂的基态、磁性以及电子结构,从理论计算的角度给出了基态磁性结构.计算结果表明:AgCrO₂具有120°螺旋自旋序反铁磁基态,其自旋螺旋面平行于(110)面或(11^-0)面;由于Cr离子间的自旋几何阻挫,导致沿晶体的a,b和a+b方向上均形成了螺旋自旋转动角为120°的 关键词： 第一性原理 交换相互作用 阻挫 反铁磁     

Triangular antiferromagnetic order in the honeycomb layer lattice of

ErCl₃ crystallizes in the AlCl₃-type layer structure. The crystal structure was refined in the paramagnetic state by powder neutron diffraction. The monoclinic lattice parameters at 1.5 K are a = 6.8040(3)?, b = 11.7456(5)?, c = 6.3187(3)? and . The space group is C2/m. Short-range, predominantly in-plane, magnetic ordering occurs above 350 mK up to several Kelvin. Below mK a three-dimensional antiferromagnetic order with a propagation vector of sets in. The magnetic structure of ErCl₃ was determined by powder and single-crystal neutron diffraction at temperatures down to 45 mK. The Er³⁺ ions are located on two-dimensional honeycomb layers in the a–b plane. There are two antiferromagnetically coupled triangular sublattices which form right- and left-handed helices along the c-axis. The magnetic moments are oriented in the a–b plane and amount to 3.3(1) at saturation. From the temperature dependence of the integrated neutron magnetic peak intensity a critical exponent (2) was derived for the magnetic phase transition. Received 1 December 1999 and Received in final form 21 July 2000     

La0.8Ca0.2MnO3/SrTiO3薄膜厚度对其结构及磁学性能的影响

采用多种X射线衍射技术和磁电阻测量技术研究了不同厚度的La_0.8Ca_0.2MnO₃/SrTiO₃ (LCMO/STO)薄膜的应变状态及其对磁电阻性能的影响.结果表明，在STO(001)单晶衬底上生长的LCMO薄膜沿［00l］取向生长.LCMO薄膜具有伪立方钙钛矿结构，随着薄膜厚度的增加，面内晶格参数增加，垂直于面内的晶格参数减小,晶格参数a和b相近，略小于c.LC 关键词： X射线衍射 微结构 应变 物理性能     

Field-induced confinement in (TMTSF)2ClO4 under accurately aligned magnetic fields

We present transport measurements along the least conducting c direction of the organic superconductor (TMTSF)₂ClO₄  performed under an accurately aligned magnetic field in the low temperature regime. The experimental results reveal a two-dimensional confinement of the carriers in the (a, b) planes which is governed by the magnetic field component along the b^′ direction. This 2-D confinement is accompanied by a metal-insulator transition for the c axis resistivity. These data are supported by a quantum mechanical calculation of the transverse transport taking into account in self consistent treatment the effect of the field on the interplane Green function and on the intraplane scattering time.     

Quadratic magnetoelectric effect and the role of the magnetocaloric effect in the magnetoelectric properties of multiferroic BaMnF<Subscript>4</Subscript>

The magnetoelectric effects in BaMnF₄ in magnetic fields up to 250 kOe are experimentally studied and theoretically analyzed using the magnetic symmetry of the crystal. The presence of quadratic magnetoelectric effect tensor components that correspond to the electric polarization components along the b and c axes indicates triclinic distortions in the monoclinic symmetry of the crystal. The anomalous dependence of the magnetic field-induced polarization (P _a (H _b )) can be related to the pyroelectric effect caused by magnetocaloric heating of the crystal. The measured torque curves point to a 9° deviation of the spin orientation from the b axis.     

Crystal structure of N-m-tolyl phthalimide

N-m-tolyl phthalimide, C₁₅NO₂H₁₁ crystallizes in the space group Cc with unit cell dimensions,a=8·54(1),b=19·89(2),c = 7·59(1)A, β=114·53(1)° andZ=4.V=1173(2)A³,D _m =1·35(1),D _c = 1·344 mg.m⁻³,M _r =237 λCoK_a=1·7903A. The structure was solved byMULTAN and refined to an R-factor of 0·116 for 632 counter reflections. The molecules are held together by van der Waal’s forces. The angle between the tolyl plane and the plane through the phthalimide group is 53·4(4)°. Contribution No. 607.     

Acoustic excitation of nuclear spin waves in the easy-plane antiferromagnetic material KMnF3

Ultrasound damping at T=4.2 K in single crystal easy-plane antiferromagnetic KMnF₃ is studied experimentally as a function of the magnitude and direction of a constant magnetic field H at frequencies of 640–670 MHz, corresponding to the frequencies of nuclear spin waves. Two experimental situations are examined: in the first, the vector H lies in the easy magnetization plane (001), and in the second, H forms an angle with (001). For longitudinal ultrasound waves propagating along the hard magnetization axis [001], it is found that the damping depends resonantly on the magnitude of the field H. In the first case a single damping maximum is observed, and in the second, two damping peaks that are well resolved with respect to the field. The angular dependence of the resonance damping signals on the direction of the constant magnetic field is found to have a 90° periodicity in all cases. The observed effects are explained by resonant ultrasonic excitation of nuclear spin waves. On the basis of an analysis of the magnetoacoustic interaction energy, it is shown that in the first case, nonzero oscillations of the antiferromagnetism vector L occur only in the basal plane, while in the second, oscillations of L occur both in the basal and a vertical plane, which are associated, respectively, with two branches of the nuclear spin waves. It is also shown that the 90° periodicity in the angular dependence of the damping signals is associated with a fourth order [001] axis. Zh. éksp. Teor. Fiz. 112, 1830–1840 (November 1997)     

Metamagnetism and magnetostriction of the Ising antiferromagnet DyCrO3

The magnetostriction of the Ising antiferromagnet DyCrO₃ during metamagnetic transitions induced by a magnetic field applied along the a and b axes of the crystal is investigated experimentally and theoretically. The magnetostriction measurements performed in the temperature range 1.5–4.2 K in magnetic fields up to 3 T clearly reveal the two-step character of the transition from the original antiferromagnetic structure to the ferromagnetic structure with the intermediate formation of a low-symmetry structure (when H is parallel to the a axis). The unusual behavior of the magnetostriction discovered during the magnetizing process provides information on the nature of magnetostriction in rare-earth Ising metamagnets. Fiz. Tverd. Tela (St. Petersburg) 39, 668–670 (April 1997)     

Effects of dimensional crossover on the resistivity in Bi2Sr2CaCu2O8+x crystal

We report a study of the resistive transitions in Bi₂Sr₂CaCu₂O_8+x in the presence of an applied magnetic field (up to 12 T) at various field orientations. We focus our attention on the results obtained with orientations close to the parallelism to the (a,b) planes. We show that scaling arguments, typical of dissipative regimes independent of oriented defects, can explain qualitatively and semi-quantitatively the experimental findings, provided a 3D–2D crossover is assumed as the temperature is lowered, due to the variation of the out-of-plane coherence length. An estimate of the anisotropy in the 3D and 2D regions is given.     

Size effect of the longitudinal magnetoresistance of thin films of n- type Ge

In a study of the longitudinal magnetoresistance as a function of the magnetic field in samples of n-type Ge of various thicknesses at 300°K and 77°K, the surface of the samples was oriented parallel to the (001) plane. The directions of the current and the magnetic field were along either the <100> direction or the <110> direction. As the thickness of the samples was reduced, regardless of the orientation of the current and the magnetic field with respect to the crystallographic directions, a decrease was observed in the magnetoresistance over the entire range of magnetic fields studied, 1–280 kOe. The experimental results are attributed to a screening-length size effect and to “sliding” orbits in the enriched surface layers, which lead to an increase in the electron mobility in a strong magnetic field.     

Anisotropy of the vortex structure and resistivity in the basal plane of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript> single crystals

The vortex lattice of the YBa₂Cu₄O₈ high-temperature superconductor is studied in the basal plane of monocrystalline samples using the decoration technique in a field interval of 40–600 Oe. Vortex lattice anisotropy (field-independent “compression” of a regular hexagonal vortex cell in the poorly conducting direction by a factor of about 1.3) is detected. Resistivity anisotropy ρ _a /ρ _b measured at temperatures from T _c to room temperature is 16–9. Possible reasons for the discrepancy between our results and the available data are discussed.     

NMR study of the electric field gradient in the paramagnetic phase of M<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> (M = Co,Ni) compounds

The NMR spectra and the decay of a spin echo signal from ⁵¹V nuclei in Kagome-staircase Co₃V₂O₈ (CVO) and Ni₃V₂O₈ (NVO) single crystals are measured in the temperature range 30–300 K and a magnetic field H ₀ = 20 kOe. The orientation dependences of the ⁵¹V NMR line shape are used to determine the electric field gradient (EFG) parameters, namely, quadrupole frequency ν _Q and asymmetry parameter η. These parameters for NVO and CVO are ν _Q = 180(10) kHz, η = 0.5(1) and ν _Q = 130(10) kHz, η = 0.6(1), respectively. A comparison of the results of calculating EFG tensors with a point charge model and the NMR data indicates that the crystallographically equivalent vanadium atoms in the Ni₃V₂O₈ and Co₃V₂O₈ compounds differ in the EFG axis orientation. M₃V₂O₈ crystals are found to have vanadium positions (V1, V2) with different orientations of the z axis, which specifies the direction of the principal value of EFG (V _zz ): these orientations lie in the bc plane and make an angle of either +51(5)° (V1) or −51(5)° (V2) with axis c. In the temperature range 30–300 K, the EFG tensor components and the local symmetry of the charge surrounding of the vanadium positions in NVO and CVO oxides are found to change insignificantly.