Transverse voltage in superconductors at zero applied magnetic field

A systematic study of the transverse voltage at zero magnetic field in the superconducting state is reported. The effects of warming rate, temperature, applied magnetic field, and electrical current on the transversal resistance (R_XY) of polycrystalline superconducting sample are taken into account. At zero magnetic field two peaks are observed in R_XY(T) curves which are related to the double superconducting transition in the R_XX(T) component. In the superconducting (R_XX = zero) and normal states no transverse voltage has been detected at zero magnetic field as expected. The results are discussed within the framework of the motion of Abrikosov and Josephson vortices and anti-vortices. A new scaling relation between transverse and longitudinal components given by R_XY ～ dR_XX/dT has been confirmed.     

Excess resistance in the superconducting transition of a mesoscopic Al disk

We report resistance measurements on a mesoscopic Al disk whose size is comparable to the superconducting coherence length. As the magnetic field increases, resistance peaks successively appear and some of the peak resistances are larger than the normal state value R_N. These peaks are ascribed to the transitions between different vortex states in the superconducting Al disk. The experimental results suggest that some anomalous energy dissipation is caused by the dynamics of the vortices in the confined geometry.     

Collective excitation modes in the intermediate and superconducting states of doped and undoped indium and lead

Ultrasonic attenuation was studied in pure In, In + 0.003 at.% Pb, pure Pb and Pb + 0.003 at.% In in the intermediate states (for the magnetic fields 0.7H _c and 0.9H _c ) and superconducting states, for frequencies varying from 9.9 to 29.7 MHz, in the temperature range 4.2 to 1.4K. Collective excitation modes were observed in both the states for all the samples. There exist two distinct phases in the intermediate state but only one phase in the superconducting state in all the samples. The first phase was dependent on the magnetic field and independent of the concentration and nature of the dopant. The second phase was independent of the magnetic field and dependent essentially on the concentration of vacancies and marginally on the concentration of the dopant. The origin of the two phases has been discussed.     

Static and dynamic behaviours of multivortex states in a superconducting sample with mesoscopic pinning sites

This preliminary work has focused on the static transitions between the multivortex states interacting with square arrays of the mesoscopic pinning sites in superconducting samples. Our results were obtained from an extensive series of numerical simulations as functions of the magnetic field, pinning radius, and sample size. We have presented a wide range of multivortex configurations from commensurate dimer states to more concentric vortex shells at the matching fields. The stability of these states was also studied by means of the current-voltage V(I) curves which illustrate dynamic phase transitions as a function of applied driving force. These transitions manifested themselves as either a sudden jump in velocity or a nonlinear increase with velocity fluctuations in V(I) curves. We have investigated whether that the phase transitions between the pinned regime and the elastic flow regime are indicative of the stability of the initial vortex states. The variety of intermediate flow phases is attributed to large pinning size (reentrant behavior), strong commensurability and caging effects. In particular, three-shell vortex structures were obtained in the presence of larger pinning sites at adequate matching magnetic fields.     

Antiferromagnetism and superconductivity in a model with extended pairing interactions

The competition between antiferromagnetism and the d + id superconducting state is studied in a model with near and next near neighbour interactions in the absence of any on-site repulsion. A mean field study shows that it is possible to have simultaneous occurrence of an antiferromagnetic and a singlet d + id superconducting state in this model. In addition, such a coexistence generates a triplet d + id superconducting order parameter with centre of mass momentum Q = (π,π) dynamically having the same orbital symmetry as the singlet superconductor. Inclusion of next nearest neighbour hopping in the band stabilises the d_xy superconducting state away from half filling, the topology of the phase diagram, though, remains similar to the near neighbour model. In view of the very recent observation of a broad region of coexistence of antiferromagnetic and unconventional superconducting states in organic superconductors, the possibility of observation of the triplet state has been outlined. Received 30 November 2000 and Received in final form 27 March 2001     

Triangular antiferromagnets with a layered structure in a uniform field

We study the magnetic states and phase transitions in layered triangular antiferromagnets and show that in compounds of the VBr₂ (or VCl₂) type the quantum effects alter the structure of the ground state and initiate a series of transitions as the magnetic field strength is increased. We establish that planar structures with different spin configurations are realized when the magnetic field strength is far from the saturation value, while a nonplanar structure of the umbrella type is realized in fields close to the saturation value. Finally, we build the phase diagram of the ground state and indicate a finite range of field strengths where a collinear phase is possible, too. Zh. éksp. Teor. Fiz. 111, 627–643 (February 1997)     

Phase transition in antiferromagnets with anisotropic exchange interactions and uniaxial anisotropy

Abstract

Following the Bogoliubov variational principle, the equilibrium and stability equations of the free energy for the two sublattice antiferromagnetic system with inter- and intrasublattice exchange interactions and with an external magnetic field are investigated. For the Ising spin system with uniaxial anisotropy, the phase diagrams have been calculated for various values of anisotropy constant d and the ratio of intra- to intersublattice interaction constants γ. It is shown that first-order, as well as second-order transitions, occur for γ > 0, whereas only a second-order transition occurs for γ ≦ 0, irrespective of the sign of d. Furthermore, similar calculations are extended for the anisotropic Heisenberg spin system and quite interesting phase diagrams have been obtained. Next, the effects of the anisotropic exchange interactions on the magnetic ordered states and the magnetizations of the singlet ground state system of spin one and with a uniaxial anisotropy term are investigated in the vicinity of the level crossing field H ? D/gμ _B . A field-induced ordered state without the transverse component of magnetization is shown to appear in a certain range of magnetic field as the spin dimensionality decreases. It has also turned out that the phase transition between this ordered state and the canted antiferromagnetic state ordinarily found for the isotropic singlet ground state system is of first order. Lastly, the stable spin configurations at a temperature of absolute zero for a two-sublattice uniaxial antiferromagnet under an external magnetic field of arbitrary direction are studied. In particular, the effects of a single ionic anisotropy D-term and anisotropy in the exchange interactions on the magnetic phases are investigated. The antiferromagnetic state has turned out to appear only for the external magnetic field along the easy axis of sublattice magnetization, and makes a first-order phase transition to the canted-spin state or the ferromagnetic state. For other field directions, no antiferromagnetic state appears and only a second-order phase transition between the canted-spin and the ferromagnetic states occurs. The critical field as a function of external field direction has been calculated for several D-values.     

Dynamic phase transition in the kinetic Blume--Emery--Griffiths model: Phase diagrams in the temperature and interaction parameters planes

As a continuation of our previously published work, the dynamic phase transitions are studied further, within a mean-field approach, in the kinetic Blume--Emery--Griffiths model in the presence of a time varying (sinusoidal) magnetic field by using the Glauber-type stochastic dynamics. The dynamic phase transitions are obtained and the phase diagrams are constructed in two different planes, namely in the reduced temperature (T) and biquadratic interaction (k) plane and found eight fundamental types of phase diagrams for various values of reduced crystal-field interaction (d) and magnetic field amplitude (h), and in the (T,?d) plane and obtained six distinct topologies for different values of k and h. Phase diagrams exhibit one or two dynamic tricritical points and a dynamic double critical end point, dynamic triple and quadruple points, and besides disordered and ordered phases, three coexistence phase regions exist in which occurring of these strongly depend on the values of d, k and h.     

Self-consistent theory of pairbreaking in kondo superconductors

A theory is presented for the calculation of equilibrium properties of superconductors containing magnetic impuritiesof finite concentration. The theory is based on the Nagaoka-Suhl approach to thes — d model and treats finite impurity concentrations within a self-consistentT-matrix-approximation. An integral equation is derived for the pairbreaking parameter which describes de-pairing of Cooper pairs by spin flip scattering. Numerical solutions of this equation are used to compute the phase diagram in the temperature-concentration-plane and the transition-temperature-dependence of the upper critical field. These calculations are in good agreement with experimental data on LaCeAl₂-samples. In addition, the recent discovery of three normal superconducting transitions in LaCeY is discussed quantitatively.Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Jülich/Köln     

无序二维约瑟夫森结阵列中磁场引起的涡旋玻璃态相变

采用电阻阻错结的无序二维约瑟夫森结阵列模型,数值研究超导薄膜中垂直磁场引起的涡旋运动.通过分析磁场激发产生的涡旋度Ne及低频电压噪声S₀的变化特性,得到如下结论：在无序超导体中固定温度不变,随着磁场的减弱涡旋液态经过准有序的布拉格相,涡旋玻璃相重新进入到低磁场下的钉扎稀磁液相. 由于在涡旋玻璃相中,电流驱动下的噪声值表现出一个峰,表明系统处于无序与有序相互竞争的亚稳态,并且临界电流应有峰值效应. 计算得到噪声值的变化与Okuma等得到的无序超导Mo_xSi_1-x膜实验现象一致,并能解释磁场降低引起的重新进入钉扎的稀磁液相行为. 关键词： 约瑟夫森结阵列 磁通玻璃 重新进入 峰值效应     

Magnetic flux structures of composite superconducting structures with d- and s-waves superconductors (d-dots)

Composite superconducting structures with d- and s-wave superconductors, d-dots, can be used as two state devices. Their functions depend on structures of the spontaneous magnetic field, which appears because of the anisotropy of d-wave superconductivity. Solving two-components Ginzburg–Landau equation, we have investigated magnetic field structures for d-dots with smaller and larger holes around the corners of d-wave superconducting region. And we argued the effect of holes on the magnetic structures.     

Magnetic and superconducting phase diagrams in ErNi2B2C

We present measurements of the superconducting upper critical field H_c2(T) and the magnetic phase diagram of the superconductor ErNi₂B₂C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi₂B₂C, extending between different magnetic transitions. A close correlation between magnetic transitions and H_c2(T) is found, showing that superconductivity is strongly linked to magnetism.     

Magnetic system under a fluctuating field

We study nonequilibrium steady states, phase transitions and critical phenomena in a d-dimensional lattice model which represents a magnetic system under the action of a field fluctuating very rapidly with time. This induces competing kinetics which produces a sort of (dynamical) frustration which might occur also in some natural disordered systems. The exact solution for d = 1, partial exact results for d ≥ 2, and a comparison with some related models are reported.     

The quantum compass chain in a transverse magnetic field

We study the magnetic behaviors of a spin-1/2 quantum compass chain (QCC) in a transverse magnetic field, by means of the analytical spinless fermion approach and numerical Lanczos method. In the absence of the magnetic field, the phase diagram is divided into four gapped regions. To determine what happens by applying a transverse magnetic field, using the spinless fermion approach, critical fields are obtained as a function of exchanges. Our analytical results show, the field-induced effects depend on in which one of the four regions the system is. In two regions of the phase diagram, the Ising-type phase transition happens in a finite field. In another region, we have identified two quantum phase transitions (QPT)s in the ground state magnetic phase diagram. These quantum phase transitions belong to the universality class of the commensurate-incommensurate phase transition. We also present a detailed numerical analysis of the low energy spectrum and the ground state magnetic phase diagram. In particular, we show that the intermediate state (h _c1 < h < h _c2) is gapful, describing the spin-flop phase.     

Critical fields of a superconducting cylinder

The self-consistent solutions of the nonlinear Ginzburg-Landau equations, which describe the behavior of a superconducting mesoscopic cylinder in an axial magnetic field H (provided there are no vortices inside the cylinder), are studied. Different, vortex-free states (M-, e-, d-, p-), which exist in a superconducting cylinder, are described. The critical fields (H ₁, H ₂, H _p , H _i , H _r ), at which the first or second order phase transitions between different states of the cylinder occur, are found as functions of the cylinder radius R and the GL-parameter . The boundary , which divides the regions of the first and second order (s, n)-transitions in the icreasing field, is found. It is found that at R→∞ the critical value, is . The hysteresis phenomena, which appear when the cylinder passes from the normal to superconducting state in the decreasing field, are described. The connection between the self-consistent results and the linearized theory is discussed. It is shown that in the limiting case and R ≫ λ (λ is the London penetration length) the self-consistent solution (which correponds to the socalled metastable p-state) coincides with the analitic solution found from the degenerate Bogomolnyi equations. The reason for the existence of two critical GL-parameters and in, bulk superconductors is discussed. An erratum to this article is available at.     

The changes in electronic structure and optical properties of magnetic semiconductors at magnetic phase transitions

The influence of magnetic phase transitions on electronic structure and optical properties of magnetic semiconductors is discussed. Europium chalcogenides and chromium chalcogenide spinels are the main subjects of the investigation. It is shown, that many-body effects are responsible for the changes of optical properties and non-rigid band behavior of electronic structure. Magnetic phase transition leads to energy shift of wide bands and change in density of states of “magnetic” d(f)-electrons without any significant shift of their energies. The influence of fluctuations at T ～ T _c and antiferromagnetic semiconductors are also considered.     

Mixed state and critical current in narrow semiconducting films

The mixed state of thin narrow superconducting films with an edge barrier placed in a transverse magnetic field is considered. The boundaries of the region for the existence of metastable mixed states with an assigned number of vortices N [H _min(N)⩽H⩽H _max(N)] are found. The magnetic-field dependence of the critical field is found for the films. The transition from the Meissner state to the static mixed state is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1773–1777 (October 1998)     

Resistance and magnetic susceptibility of superconducting lead embedded in nanopores of glass

This paper reports on a study of the resistance and differential magnetic susceptibility χ _ac of lead embedded in nanosized glass pores with a diameter of ∼7 mm, which was performed at temperatures of 6–300 K and magnetic fields of up to 6 T. The field dependence of the resistance R(H) and the temperature dependences of the real, χ″(T), and imaginary, χ″(T), parts of magnetic susceptibility reveal indications of superconducting phase transitions associated with the volume and surface superconductivity of Pb nanopar ticles. The measurements of the field dependence of resistance have been used to set up the H _c -T _c phase diagram and to carry out a comparison with the study of the heat capacity performed on the same samples.     

Superconducting properties of tin embedded in nanometer-sized pores of glass

The electrical resistance of tin embedded from a melt in porous glasses with an average pore diameter of ??7 nm has been investigated at low temperatures in magnetic fields up to 2 T. The temperatures of the transition to the superconducting state for nanocrystalline tin have been determined in magnetic fields of 0, 0.3, 0.5, 1.0, 1.5, and 2.0 T. It has been found that the temperature and magnetic-field dependences of the electrical resistance of the nanocomposite under investigation exhibit two transitions to the superconducting state. The nature of the double superconducting transitions has been discussed. The H _c -T _c phase diagram has been constructed using the entire set of data on the magnetic-field and temperature dependences of the electrical resistance of nanostructured tin. This phase diagram indicates that the upper critical magnetic field H _c2(0) for nanostructured tin is almost two orders of magnitude higher than the corresponding field for bulk tin.     

Excess current in point contacts on two-band superconductor MgB2 in magnetic field

A series of I(V) characteristics and bias-dependent differential resistance dV/dI(V) curves of point contacts made between a single crystal of two-band superconductor MgB₂ and Cu were measured in magnetic fields up to 9 T. The magnetic field dependences of the excess current in the I(V) curves were obtained and analyzed using Koshelev and Golubov's [Phys. Rev. Lett. 90, 177002 (2003)] theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state, our data can be qualitatively described using the theoretical magnetic field dependence of the superconducting order parameter of the σ and π-bands and the averaged electronic density of states in MgB₂.