Low-field Phase Diagram Research Articles

Low-Field Phase Diagram of the Spin Hall Effect in the Mesoscopic Regime

When a mesoscopic two dimensional four-terminal Hall cross bar with spin-orbit interaction (SOI) is subjected to a perpendicular uniform magnetic field B, both integer quantum Hall effect (IQHE) and mesoscopic spin Hall effect (MSHE) may exist when disorder strength W in the sample is weak. We have calculated the low field "phase diagram" of MSHE in the (B,W) plane for disordered samples in the IQHE regime. For weak disorder, MSHE conductance G(sH) and its fluctuations rms(G(sH)) vanish identically on even numbered IQHE plateaus, they have finite values on those odd numbered plateaus induced by SOI, and they have values G(sH)=1/2 and rms(G(sH))=0 on those odd numbered plateaus induced by Zeeman energy. At larger disorders, the system crosses over into a regime where both G(sH) and rms(G(sH)) are finite, a chaotic regime, and finally a localized regime.

Surface Melting of the Vortex Lattice

We discuss the effect of an (ab) surface on the melting transition of the pancake-vortex lattice in a layered superconductor within a density functional theory approach. Both discontinuous and continuous surface melting are predicted for this system, although the latter scenario occupies the major part of the low-field phase diagram. The formation of a quasiliquid layer below the bulk melting temperature inhibits the appearance of a superheated solid phase, yielding an asymmetric hysteretic behavior which has been seen in experiments.

Finite-size effects and “surface spin-flop” phenomena in antiferromagnetically coupled multilayers

Antiferromagnetic superlattices with low uniaxial anisotropy have been investigated within a phenomenological approach based on a one-dimensional discretized model for the magnetic energy of laterally homogeneous states, which is due to Mills. A transformation of this energy into a system of interacting antiferromagnetic dimers allows to investigate in detail the evolution of the magnetic states in the applied field and elucidates the nature of reorientation transitions. We show that the low anisotropy low field phase diagram is ruled by a first-order transition into an inhomogeneous spin-flop phase that keeps mirror symmetry for ideal multilayer stacks. The continuum limit for Mills models allows to formulate this mechanism in simple terms by the formation and internal transformation of an antiferromagnetic domain wall in the center of the finite superlattices. Within the continuum model, generalizations of Mills model and connections with other antiferromagnetic systems in confined geometries are discussed.

Current-induced switching in single ferromagenetic layer nanopillar junctions

Current-induced magnetization dynamics in asymmetric Cu∕Co∕Cu single magnetic layer nanopillars has been studied experimentally at room temperature and in low magnetic fields applied perpendicular to the thin film plane. In sub-100nm junctions produced using a nanostencil process a bistable state with two distinct resistance values is observed. Current sweeps at fixed applied fields reveal hysteretic and abrupt transitions between these two resistance states. The current induced resistance change is 0.5%, five times greater than the anisotropic magnetoresistance effect. We present an experimentally obtained low field phase diagram of current-induced magnetization dynamics in single ferromagnetic layer pillar junctions.

Insulator-quantum Hall conductor transitions at low magnetic field

Insulator-quantum Hall conductor transitions at low magnetic field B were studied with a gated GaAs-AlGaAs heterostructure. A low field disorder-magnetic field phase diagram was constructed based on the experimental results. This phase diagram shows no floating up of the extended state and allows transitions from the insulating state directly to any Landau level states. The critical filling factor can change from 16 to 6 as the disorder in the sample increases. By inspecting the raw data from this and the other samples and analyzing the scaling behaviors near the transition points, we found that the observed transition has the properties of a genuine phase transition.

Van der Waals interaction between flux lines in high- superconductors

In anisotropic or layered superconductors thermal fluctuations as well as impurities induce a van der Waals (vdW) attraction between flux lines, as has recently been shown by Blatter and Geshkenbein in the thermal case [Phys. Rev. Lett. 77, 4958 (1996)] and by Mukherji and Nattermann in the disorder dominated case [Phys. Rev. Lett. 79, 139 (1997)]. This attraction together with the entropic or disorder induced repulsion has interesting consequences for the low field phase diagram. We present two derivations of the vdW attraction, one of which is based on an intuitive picture, the other one following from a systematic expansion of the free energy of two interacting flux lines. Both the thermal and the disorder dominated case are considered. In the thermal case in the absence of disorder, we use scaling arguments as well as a functional renormalization of the vortex-vortex interaction energy to calculate the effective Gibbs free energy on the scale of the mean flux line distance. We discuss the resulting low field phase diagram and make quantitative predictions for pure BiSCCO (Bi_2-Sr_2-CaCu_2-O_8). In the case with impurities, the Gibbs free energy is calculated on the basis of scaling arguments, allowing for a semi-quantitative discussion of the low-field, low-temperature phase diagram in the presence of impurities.

Van der Waals interaction between flux lines in high- superconductors: A variational approach

In pure anisotropic or layered superconductors thermal fluctuations induce a van der Waals attraction between flux lines. This attraction together with the entropic repulsion has interesting consequences for the low field phase diagram; in particular, a first order transition from the Meissner phase to the mixed state is induced. We introduce a new variational approach that allows for the calculation of the effective free energy of the flux line lattice on the scale of the mean flux line distance, which is based on an expansion of the free energy around the regular triangular Abrikosov lattice. Using this technique, the low field phase diagram of these materials may be explored. The results of this technique are compared with a recent functional RG treatment of the same system.

Van der Waals attraction of vortices in superconductors

We show that in anisotropic and layered superconductors the fluctuations of vortex lines produces an attractive long-range vortex — vortex interaction of the van der Waals type. This attraction follows from the anisotropic screening properties of the material and has profound consequences for the low-field phase diagram of these materials.

Role of electromagnetic coupling in the low-field phase diagram ofBi2.15Sr1.85CaCu2O8+δ

A detailed study has been made of a transition in the flux lattice of the superconductor ${\mathrm{Bi}}_{2.15}$${\mathrm{Sr}}_{1.85}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ using the technique of muon-spin rotation. The results are in excellent agreement with a recent theoretical analysis on the low-field phase diagram of highly anisotropic superconductors. In particular, the important role of electromagnetic interactions between vortex segments is demonstrated. The form of the phase boundary suggests that the transition may be identified with a simultaneous melting and decoupling of the vortices in adjacent layers.

Van der Waals Attraction of Vortices in Anisotropic and Layered Superconductors.

We show that in anisotropic and layered superconductors the fluctuations of vortex lines produce an attractive long-range vortex-vortex interaction of the van der Waals type. This attraction follows from the anisotropic screening properties of the material and has profound consequences for the low-field phase diagram of these materials.

Low-field phase diagram of layered superconductors: The role of electromagnetic coupling.

We determine the position and shape of the melting line in a layered superconductor taking the electromagnetic coupling between layers into account. In the limit of vanishing Josephson coupling we obtain a new generic reentrant low-field melting line. Finite Josephson coupling pushes the melting line to higher temperatures and fields and a new line shape $B_{{\rm m}} \propto (1-T/T_c)^{3/2}$ is found. We construct the low-field phase diagram including melting and decoupling lines and discuss various experiments in the light of our new results.

Low-field vortex dynamics over seven time decades in aBi2Sr2CaCu2O8+δsingle crystal for temperatures13<~T<~83K

Using a custom-made dc superconducting quantum interference device (dc-SQUID) magnetometer, we have measured the time relaxation of the remanent magnetization ${M}_{\mathrm{rem}}$ of a ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystal from the fully critical state for temperatures $13 \mathrm{K}<~T<~83 \mathrm{K}$. The measurements cover a time window of seven decades ${10}^{\ensuremath{-}2} \mathrm{s}\ensuremath{\lesssim}t\ensuremath{\lesssim}{10}^{5} \mathrm{s}$, so that the current density $j$ can be studied from values very close to ${j}_{c}$ down to values considerably smaller than ${j}_{c}$. From the data we have obtained (i) the flux-creep activation barriers $U$ as a function of current density $j$, (ii) the current-voltage characteristics $E(j)$ in a typical range of ${10}^{\ensuremath{-}7}$-${10}^{\ensuremath{-}15}$ V/cm, and (iii) the critical current density ${j}_{c}(0)$ at $T=0$. Three different regimes of vortex dynamics are observed: For temperatures $T\ensuremath{\lesssim}20$ K the activation barrier $U(j)$ is logarithmic, no unique functional dependence $U(j)$ could be found for the intermediate-temperature interval $20 \mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}40 \mathrm{K}$, and finally for $T\ensuremath{\gtrsim}40$ K the activation barrier $U(j)$ follows a power-law behavior with an exponent $\ensuremath{\mu}\ensuremath{\simeq}0.6$. From the analysis of the data within the weak collective pinning theory for strongly layered superconductors, it is argued that for temperatures $T\ensuremath{\lesssim}20$ K pancake vortices are pinned individually, while for temperatures $T\ensuremath{\gtrsim}40$ K pinning involves large collectively pinned vortex bundles. A description of the vortex dynamics in the intermediate-temperature interval $20 \mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}40 \mathrm{K}$ is given on the basis of a qualitative low-field phase diagram of the vortex state in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. Within this description a second peak in the magnetization loop should occur for temperatures between 20 and 40 K, as has been observed in several magnetization measurements in the literature.

Influence of processing on the low-field magnetic phase diagram of high-temperature superconductors

Low AC field magnetic phase diagrams in the (hAC,T) plane have been deduced from AC susceptibility measurements for YBa2Cu3O7- delta sintered polycrystals, magnetically melt-textured polycrystals and single crystals. It is shown that dissipative regimes appear in all cases above a critical line located well below the irreversibility line, and the strongest shifts of the critical line against the irreversibility line are observed in magnetically melt-textured polycrystals. The relative positions of the irreversibility lines show that the increase of jc in magnetically melt-textured polycrystals is relevant to the creation of pinning sites with high activation energies.

Microwave absorption in high-Tc superconductors studied by the EPR method

Dependences of the microwave absorption on temperature, magnetic field and microwave power obtained for the high-Tc superconductors are presented. Shape of the magnetically modulated microwave absorption, low-field phase diagram and overheating of the Josephson junction system induced by microwave irradiation are discussed. The model of the Josephson junction system interacting with microwaves has been used to explain the behavior of the high-Tc superconductors in low magnetic field.

Magnetic phase diagram of 32 K superconductor (Bi l.6Pb 0.4)(Sr 1.5La 0.5)Cu0 6+δ

Vortex lattice melting for temperature near transition temperature T C was observed for 32 K superconductor (Bi 1.6 Pb 0.4 )(Sr 1.5 La 0.5 )CuO 6+; through the temperature dependence of magnetization in static applied field B a for both field-cooled (FC) and zero-field-cooled (ZFC) measurements. The irreversibility line (de-Almeida-Thouless line) H * (T) in the low field region (B a c ) 3/2 . Low-field magnetic phase diagram shows very small lower critical field H cl (T) with H cl (0 K) = 26 G. The effects of grain geometry and magnetic relaxation are discussed.

Low magnetic field superconducting phase diagram of the high-T c YBa 2Cu 3O 9-δ

The low magnetic field phase diagram of the high-T c superconductor YBa 2Cu 3O 9-δ with δ∼2.1 has been determined in fields to 5000 0e. The lower critical field is about 300 0e near T=0 and less than 100 0e at liquid nitrogen temperature. The mixed phase shows significant relaxation and field cooling effects.