Quantum phase transitions in bilayer quantum hall systems at a total filling factor nuT=1

We construct a quantum Ginsburg-Landau theory to study the quantum phase transition from the excitonic superfluid to a possible pseudospin density wave (PSDW) at some intermediate distances driven by the magnetoroton minimum collapsing at a finite wave vector. We explicitly show that the PSDW takes a square lattice structure. We suggest the existence of zero-point quantum fluctuation generated vacancies in the PSDW and that correlated hopping of vacancies in the active and passive layers in the PSDW state leads to very large and temperature dependent drag consistent with the experimental data. Comparisons with previous numerical calculations are made. Further experimental implications are given.     

Vortex phase diagram of F=1 spinor Bose-Einstein condensates

We have calculated the F=1 ground state of a spinor Bose-Einstein condensate trapped harmonic potential with an applied Ioffe-Pitchard magnetic field. The vortex phase diagram is found in the plane spanned by perpendicular and longitudinal magnetic fields. The ferromagnetic condensate has two vortex phases which differ by winding number in the spinor components. The two vortices for the F(z)=-1 antiferromagnetic condensate are separated in space. Moreover, we considered an average local spin || to testify to what extent it is parallel to magnetic field (the nonadiabatic effects). We have shown that the effects are important at vortex cores.     

Spin degree of freedom in the nu=1 bilayer electron system investigated by nuclear spin relaxation

The nuclear-spin-relaxation rate 1/T(1) has been measured in a bilayer electron system at and around total Landau level filling factor nu=1. The measured 1/T(1), which probes electron spin fluctuations, is found to increase gradually from the quantum Hall (QH) state at low fields through a phase transition to the compressible state at high fields. Furthermore, 1/T(1) in the QH state shows a noticeable increase away from nu=1. These results demonstrate that, as opposed to common assumption, the electron spin degree of freedom is not completely frozen either in the QH or the compressible states.     

Magnetic phase diagram of the dimerized spin S = 1/2 ladder

The ground-state magnetic phase diagram of a spin S=1/2 two-leg ladder with alternating rung exchange J_⊥(n)=J_⊥[1 + (-1)ⁿ δ] is studied using the analytical and numerical approaches. In the limit where the rung exchange is dominant, we have mapped the model onto the effective quantum sine-Gordon model with topological term and identified two quantum phase transitions at magnetization equal to the half of saturation value from a gapped to the gapless regime. These quantum transitions belong to the universality class of the commensurate-incommensurate phase transition. We have also shown that the magnetization curve of the system exhibits a plateau at magnetization equal to the half of the saturation value. We also present a detailed numerical analysis of the low energy excitation spectrum and the ground state magnetic phase diagram of the ladder with rung-exchange alternation using Lanczos method of numerical diagonalizations for ladders with number of sites up to N = 28. We have calculated numerically the magnetic field dependence of the low-energy excitation spectrum, magnetization and the on-rung spin-spin correlation function. We have also calculated the width of the magnetization plateau and show that it scales as δ^ν, where critical exponent varies from ν = 0.87±0.01 in the case of a ladder with isotropic antiferromagnetic legs to ν = 1.82±0.01 in the case of ladder with ferromagnetic legs. Obtained numerical results are in an complete agreement with estimations made within the continuum-limit approach.     

Magnetic phase diagram of a ferromagnet-antiferromagnet bilayer with a rough interface

The thickness-roughness phase diagram of a thin ferromagnetic film on an antiferromagnetic substrate is studied in the case where the roughness of the interface between the layers causes frustration of the exchange interaction between them. It is shown that the inclusion of single-ion anisotropy makes the phase diagram significantly more complicated in comparison with that calculated within the exchange approximation. The evolution of a new type of domain walls caused by frustrations is traced with an increase in the film thickness and the width of the atomic steps on the film-substrate interface.     

The phase diagram of the system lithium-cadmium

Liquidus and solidus temperatures of a large number of Li-Cd alloys with compositions between 35 and 100 at % Cd have been determined by means of resistivity measurements and differential scanning calorimetry. The results are partly in disagreement with old literature data. According to our measurements the liquidus, which is bell-shaped, attains a maximum of 544°C for 59 at% Cd.     

The phase diagram of the system lithium-silicon

A large number of LiSi alloys with compositions between 5 and 50 at.% Si have been investigated by means of the Differential Scanning Calorimetry (DSC) technique. The results are in good agreement with a number of independently determined liquidus temperatures, and compatible with the compositions of the intermetallic compounds in this system. The phase diagram obtained is slightly at variance with earlier reported ones. It is shown that the latter probably are inaccurate as a consequence of corrosion of the used sample holders.     

Magnetic phase diagram in a quasi-two-dimensional electron gas

Using spin density functional theory within the framework of the local spin density approximation with Perdew-Zunger type exchange-correlation energy, ferromagnetism in a quasi-two-dimensional electron gas (Q-2DEG) is studied. The electronic and magnetic structures of a thin film are calculated as a function of film thickness and electron density. Ferromagnetism in the Q-2DEG is found to appear at a higher electron density than in the three-dimensional electron gas. Unless a film is very thin, with decreasing electron density, a magnetic phase transition occurs from a spin-unpolarized fluid to a Wigner film with surface magnetism, in which the spin polarization localizes only in the neighborhood of surfaces. Further decreasing density induces another transition to a fully spin-polarized ferromagnetic Wigner film.     

D=1+2 phase diagram forZ N with a variational method

Trial ground state are constructed which forZ _N gauge theories inD=1+2 dimensions lead to phase diagrams which are in agreement both with theoretical expectations and Monte Carlo results. The nature of the phase transition is correctly predicted for eachN.     

Phase diagram of a two-dimensional electron system

The melting curve of the two-dimensional electron system is interpolated between the known classical and ground state limits. The coexistence curve encloses a finite solid-phase domain, as in the three-dimensional case.     

The magnetic phase diagram of the up-USe system

The magnetic phase diagram of the UP-USe solid solution was determined using neutron diffraction and magnetic susceptibility data obtained from polycrystalline samples. The lattice constants obey Vegard's Law. As the Se content increases magnetic phases with long periodicity gradually appear. The characteristic for UP magnetic structure (type I) consisting of ferromagnetic sheets stacked in the sequence +-+- is replaced by the type IA structure, ++--, which is stable over a fairly large composition range. A new phase transition called “step-like” is observed in this composition range at about 0.5T_N in both the neutron diffraction and the susceptibility data. In the UP_0.76Se_0.24 sample the type IA phase transforms to a new magnetic phase consisting of ferromagnetic sheets stacked nearly in the sequence +++--- or (3+, 3-). The repeat distance in real space is almost 3 chemical unit cells in one direction and the neutron diffraction data suggests that the square wave modulation is not fully developed. For compositions close to UP_0.70Se_0.30 the (3+,3-) type magnetic ordering coexists with ferromagnetism at low temperatures. The latter is the only magnetic phase obseved in compositions with more than 30 mol% of USe. Magnetization measurements performed at field strengths up to 5 T show that the samples with Se content around 30 mol% of USe exhibit typical metamagnetic behavior.     

Manifestation of the bulk phase transition in the edge energy spectrum in a two-dimensional bilayer electron system

We use a quasi-Corbino sample geometry with independent contacts to different edge states in the quantum Hall effect regime to investigate the edge energy spectrum of a bilayer electron system at a total filling factor of ν=2. By analyzing nonlinear I–V curves in normal and tilted magnetic fields, we conclude that the edge energy spectrum is in a close connection with the bulk one. At the bulk phase transition spin-singlet-canted antiferromagnetic phase, the I–V curve becomes linear, indicating the disappearance or strong narrowing of the ν=1 incompressible strip at the edge of the sample.     

自旋相关的电子隧穿和自旋极化

研究了电子的自旋相关的隧穿和极化。在外加磁场的作用下，自旋向上的电子与自旋向下的电子具有不同的隧穿系数。当电子的自旋方向与磁场方向相反时，其隧穿概率受到磁场的抑制而变小；反之，当两平行时，电子的了隧穿系数增大。这种差异可以用本中定义的自旋极化率来表示。本对不同磁场下的自旋极化率进行了计算，结果也表明当电子的动能较小，这种自旋极化的效应越显。     

Thermopower of interacting GaAs bilayer hole systems in the reentrant insulating phase near nu=1

We report thermopower measurements of interacting GaAs bilayer hole systems. When the carrier densities in the two layers are equal, these systems exhibit a reentrant insulating phase near the quantum Hall state at total filling factor nu=1. Our data show that, as the temperature is decreased, the thermopower diverges in the insulating phase. This behavior indicates the opening of an energy gap at low temperature, consistent with the formation of a pinned Wigner solid. We extract an energy gap and a Wigner solid melting phase diagram.     

Magnetic phase diagram of the frustrated two-layer system ferromagnet-antiferromagnet

The thickness-roughness phase diagram of a thin ferromagnetic film on an antiferromagnetic substrate is studied in the case where the roughness of the interface between the layers causes frustration of the exchange interaction between them. It is shown that the account of single-ion anisotropy makes the phase diagram significantly more complicated in comparison with that calculated within the exchange approximation. The evolution of a new type of domain walls caused by frustrations is traced with an increase in the film thickness and the width of the atomic steps on the film-substrate interface.     

Thermodynamic phase diagram of the quantum hall skyrmion system

We numerically study the interacting quantum Hall Skyrmion system based on the Chern-Simons action. By noticing that the action is invariant under global spin rotations in the spin space with respect to the magnetic field direction, we obtain the low-energy effective action for a many-Skyrmion system. Performing extensive molecular dynamics simulations, we establish the thermodynamic phase diagram for a many-Skyrmion system.     

Phase diagram of interacting composite fermions in the bilayer nu=2/3 quantum hall effect

We study the phase diagram of composite fermions (CFs) in the presence of spin and pseudospin degrees of freedom in the bilayer nu=2/3 quantum Hall (QH) state. Activation studies elucidate the existence of three different QH states with two different types of hysteresis in the magnetotransport. While a noninteracting CF model provides a qualitative account of the phase diagram, the observed renormalization of tunneling gap and a non-QH state at high densities are not explained in the noninteracting CF model, and are suggested to be manifestations of interactions between CFs.     

Magnetic phase diagram of perovskite Mn oxides at T = 0

Minimizing total free energy by numerical calculations, we obtain the magnetic phase diagram of perovskite Mn oxides, such as with , Ca, Sr, etc. in the whole doping region from x =0 to x =1 at temperature T =0. It is discovered that a spiral state is stable in a low concentration of X ions while a canted state is stable in a high concentration of X ions, and a ferromagnetic phase can exist in the intermediate concentrations when the antiferromagnetic interaction is weak. The energy difference between spiral and canted states is found to be small when the Hund coupling is large. Magnetic field induced spiral/canted phase transition is considered as a possible mechanism of the colossal magnetoresistance (CMR) in the Mn oxides. Received: 11 July 1996 / Revised: 7 December 1996 / Accepted: 24 July 1997