Nuclear spin relaxation rates in two-Leg spin ladders

Using the transfer-matrix density-matrix renormalization group method, we study the nuclear spin relaxation rate 1/T(1) in the two-leg s = 1 / 2 ladder as a function of the interchain (J( perpendicular)) and intrachain (J( parallel)) couplings. In particular, we separate the q(y) = 0 and pi contributions and show that the latter contribute significantly to the copper relaxation rate (63)(1/T(1)) in the experimentally relevant coupling and temperature range. We compare our results to both theoretical predictions and experimental measures on ladder materials.     

Nature of perpendicular-to-parallel spin reorientation in a Mn-doped GaAs quantum well: canting or phase separation?

It is well known that the magnetic anisotropy in a compressively strained Mn-doped GaAs film changes from perpendicular to parallel with increasing hole concentration p. We study this reorientation transition at T=0 in a quantum well with delta-doped Mn impurities. With increasing p, the angle theta that minimizes the energy E increases continuously from 0 (perpendicular anisotropy) to pi/2 (parallel anisotropy) within some range of p. The shape of E(min)(p) suggests that the quantum well becomes phase separated with regions containing low hole concentrations and perpendicular moments interspersed with other regions containing high hole concentrations and parallel moments. However, because of the Coulomb energy cost associated with phase separation, the true magnetic state in the transition region is canted with 0相似文献   

Superfluid transition in a rotating fermi gas with resonant interactions

We study a rotating atomic Fermi gas near a narrow s-wave Feshbach resonance in a uniaxial trap with frequencies Omega perpendicular, Omega z. We predict the upper-critical angular velocity, omega c2(delta,T), as a function of temperature T and detuning delta across the BEC-BCS crossover. The suppression of superfluidity at omega c2 is distinct in the BCS and BEC regimes, with the former controlled by depairing and the latter by the dilution of bosonic molecules. At low T and Omega z < Omega perpendicular, in the BCS and crossover regimes of 0 less similar delta less similar delta c, omega c2 is implicitly given by [formula: see text], vanishing as omega c2 approximately Omega perpendicular(1 - delta/delta c)(1/2) near [formula: see text] (with Delta the BCS gap and gamma the resonance width), and extending the bulk result variant Planck's over 2pi omega c2 approximately 2Delta2/epsilonF to a trap. In the BEC regime of delta < 0 we find omega c2-->Omega perpendicular-, where molecular superfluidity is destroyed only by large quantum fluctuations associated with comparable boson and vortex densities.     

Persistent spin currents in mesoscopic Heisenberg rings

We show that at low temperatures T an inhomogeneous radial magnetic field with magnitude B gives rise to a persistent magnetization current around a mesoscopic ferromagnetic Heisenberg ring. Under optimal conditions, this spin current can be as large as gmicro(B)(T/ variant Planck's over 2pi )exp([-2pi(gmicro(B)B/delta)(1/2)], as obtained from leading-order spin-wave theory. Here g is the gyromagnetic factor, micro(B) is the Bohr magneton, and delta is the energy gap between the ground-state and the first spin-wave excitation. The magnetization current endows the ring with an electric dipole moment.     

Dimerized phase and transitions in a spatially anisotropic square lattice antiferromagnet

We investigate the spatially anisotropic square lattice quantum antiferromagnet. The model describes isotropic spin-1/2 Heisenberg chains (exchange constant J) coupled antiferromagnetically in the transverse (J( perpendicular )) and diagonal (J(x)), with respect to the chain, directions. Classically, the model admits two ordered ground states-with antiferromagnetic and ferromagnetic interchain spin correlations-separated by a first-order phase transition at J( perpendicular )=2J(x). We show that in the quantum model this transition splits into two, revealing an intermediate quantum-disordered columnar dimer phase, both in two dimensions and in a simpler two-leg ladder version. We describe quantum-critical points separating this spontaneously dimerized phase from classical ones.     

Magnetic spin ladder (C5H12N)2CuBr4: high-field magnetization and scaling near quantum criticality

The magnetization, M(H< or =30 T,0.7< or =T< or =300 K), of (C5H12N)2CuBr4 has been used to identify this system as an S = 1/2 Heisenberg two-leg ladder in the strong-coupling limit, J( perpendicular) = 13.3 K and J( parallel) = 3.8 K, with H(c1) = 6.6 T and H(c2) = 14.6 T. An inflection point in M(H,T = 0.7 K) at half saturation, M(s)/2, is described by an effective XXZ chain. The data exhibit universal scaling behavior in the vicinity of H(c1) and H(c2), indicating that the system is near a quantum critical point.     

Anisotropy of the Sommerfeld coefficient in magnesium diboride single crystals

The anisotropic field dependence of the Sommerfeld coefficient gamma has been measured down to B-->0 by combining specific heat and Hall probe magnetization measurements in MgB2 single crystals. We find that gamma(B,theta) is the sum of two contributions arising from the sigma and pi band, respectively. We show that gammasigma(B,theta)=B/Bc2(theta) where Bc2(theta)=Bc2ab/sqrt[sin2theta+Gamma2cos2theta] with Gamma approximately 5.4 (theta being the angle between the applied field and the c axis) and gammapi(B,theta)=gammapi(B)=B/Bpi(B). The "critical field" of the pi band Bpi is fully isotropic but field dependent increasing from approximately 0.25 T for B< or =0.1 T up to 3 T approximately Bc2c for B-->3 T. Because of the coupling of the two bands, superconductivity survives in the pi band up to 3 T but is totally destroyed above for any orientation of the field.     

Ehrenfest-time-dependent excitation gap in a chaotic Andreev billiard

A semiclassical theory is developed for the appearance of an excitation gap in a ballistic chaotic cavity connected by a point contact to a superconductor. Diffraction at the point contact is a singular perturbation in the limit variant Planck's over 2pi -->0, which opens up a gap E(gap) in the excitation spectrum. The time scale variant Planck's over 2pi /E(gap) proportional, variant alpha(-1)ln( variant Planck's over 2pi (with alpha the Lyapunov exponent) is the Ehrenfest time, the characteristic time scale of quantum chaos.     

Conformally invariant fractals and potential theory

The multifractal (MF) distribution of the electrostatic potential near any conformally invariant fractal boundary, like a critical O(N) loop or a Q-state Potts cluster, is solved in two dimensions. The dimension &fcirc;(straight theta) of the boundary set with local wedge angle straight theta is &fcirc;(straight theta) = pi / straight theta-25-c / 12 (pi-straight theta)(2) / straight theta(2pi-straight theta), with c the central charge of the model. As a corollary, the dimensions D(EP) of the external perimeter and D(H) of the hull of a Potts cluster obey the duality equation (D(EP)-1) (D(H)-1) = 1 / 4. A related covariant MF spectrum is obtained for self-avoiding walks anchored at cluster boundaries.     

207Pb NMR of minium, Pb3O4: evidence for the [Pb2]4+ ion and possible relativistic effects in the Pb-Pb bond

Solid Pb3O4 has been studied with 207Pb nuclear magnetic resonance (NMR) spectroscopy. The 207Pb NMR chemical-shift tensor of the Pb2+ site has principal values of delta11 = 1980 +/- 5 ppm, delta22 = 1540 +/- 5 ppm, and delta33 = -1108 +/- 10 ppm; delta(iso) = 804 +/- 10 ppm. The chemical-shift tensor of the Pb4+ site is axial, with principal values delta(parallel) = -1009 +/- 3 ppm and delta(perpendicular) = 1132 +/- 3 ppm; delta(iso) = -1091 +/- 3 ppm. The Pb4+-Pb2+ scalar coupling constant J(Pb-Pb) = 2.3 +/- 0.1 kHz. The main contribution to the Pb2- chemical-shift anisotropy is proposed to arise from an exchange interaction in the Pb2+-Pb2+ pairs, conventionally regarded as molecular [Pb2]4+ ions.     

Effects of a parallel magnetic field on the metal-insulator transition in a dilute two-dimensional electron system

The temperature dependence of conductivity sigma(T) of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B = 0, the system displays a metal-insulator transition at a critical electron density n(c)(0), and dsigma/dT>0 in the metallic phase. At low fields ( B < or approximately equal to 2 T), n(c) increases as n(c)(B)-n(c)(0) proportional, variant Bbeta ( beta approximately 1), and the zero-temperature conductivity scales as sigma(n(s),B,T = 0)/sigma(n(s),0,0) = f(B(beta)/delta(n)), where delta(n) = [n(s)-n(c)(0)]/n(c)(0) and n(s) is electron density, as expected for a quantum phase transition. The metallic phase persists in fields of up to 18 T, consistent with the saturation of n(c) at high fields.     

Transport in almost integrable models: perturbed Heisenberg chains

The heat conductivity kappa(T) of integrable models, like the one-dimensional spin-1/2 nearest-neighbor Heisenberg model, is infinite even at finite temperatures as a consequence of the conservation laws associated with integrability. Small perturbations lead to finite but large transport coefficients which we calculate perturbatively using exact diagonalization and moment expansions. We show that there are two different classes of perturbations. While an interchain coupling of strength J(perpendicular) leads to kappa(T) proportional to 1/J(perpendicular)2 as expected from simple golden-rule arguments, we obtain a much larger kappa(T) proportional to 1/J'4 for a weak next-nearest-neighbor interaction J'. This can be explained by a new approximate conservation law of the J-J' Heisenberg chain.     

A study of the effective magnetic anisotropy and the corresponding spin configurations in two dimensional ferromagnetic/antiferromagnetic system

The spin configurations of two dimensional ferromagnetic/antiferromagnetic system were investigated using model calculations and Monte-Carlo simulation methods. The lowest energy state was obtained under various coupling conditions to investigate the role of interfacial interaction on anisotropy. We found that the total ferromagnetic layer anisotropy is contributed not only from its own crystalline anisotropy but also from the antiferromagnetic layer spin flop effect. The overall ferromagnetic layer effective anisotropy is calculated as a function of the exchange energy of antiferromagnetic layer and the interfacial interaction energy. If the effective anisotropy from the spin flop effect is comparable with the crystalline anisotropy, the asymmetric spin configuration is generated. In this configuration, the magnetization direction of the ferromagnetic layer is neither perpendicular nor parallel to the antiferromagnetic spin direction. Temperature effect on the perpendicular-to-collinear coupling transition was also investigated using Monte-Carlo simulation, and the relationship between the effective anisotropy and the temperature was obtained.     

Critical behavior of CP(1) at theta=pi, Haldane's conjecture, and the relevant universality class

Using an approach to analyze the theta dependence of systems with a theta term we recently proposed, the critical behavior of CP(10 at theta=pi is studied. We find a region outside the strong coupling regime where Haldane's conjecture is verified. The critical line, however, does not belong to the universality class of the Wess-Zumino-Novikov-Witten model at topological coupling k=1 since it shows continuously varying critical exponents.     

Variation of the electronic structure in systematically synthesized Sr2MO4 (M = Ti, V, Cr, Mn, and Co)

We have systematically synthesized single-crystalline thin films of layered perovskites Sr2MO4 (M = Ti, V, Cr, Mn, and Co) which cannot be obtained in a form of bulk crystal apart from M = Mn. The two-dimensional electronic structure of these M4+ oxides, ranging from a correlated insulator to a ferromagnetic metal, has been investigated by using their optical conductivity spectra with polarizations E is perpendicular to c and E is parallel to c, which reveal systematic variation of the correlated charge gap, Mott-Hubbard gap, or charge-transfer gap. Temperature dependence of the gap-transition spectra is argued in the light of possible spin and/or orbital ordering.     

Stripes induced by orbital ordering in layered manganites

Spin-charge-orbital ordered structures in doped layered manganites are investigated using an orbital-degenerate double-exchange model tightly coupled to Jahn-Teller distortions. In the ferromagnetic phase, unexpected diagonal stripes at x = 1/m ( m = integer) are observed, as in recent experiments. These stripes are induced by the orbital degree of freedom, which forms a staggered pattern in the background. A pi shift in the orbital order across stripes is identified, analogous to the pi shift in spin order across stripes in cuprates. At x = 1/4 and 1/3, another nonmagnetic phase with diagonal static charge stripes is stabilized at intermediate values of the t(2g)-spins exchange coupling.     

Phase diagram of the frustrated spin ladder with the next nearest intrachain couplings

By the density matrix renormalization group method, the phase diagram of the frustrated spin-1/2 ladder with the next nearest intrachain and ferromagnetic interchain couplings is obtained. According to our results, the two-rung entanglement is shown to be more sensitive than order parameters for determining phase boundary in such a frustrated spin ladder model. Furthermore, the additional next nearest intrachain coupling is found to enhance the intermediate columnar dimerized phase evidently.     

Mass spectrum of the two-dimensional O(3) sigma model with a theta term

Form factor perturbation theory is applied to study the spectrum of the O(3) nonlinear sigma model with the topological term in the vicinity of theta=pi. Its effective action near this value is given by the nonintegrable double sine-Gordon model. Using previous results by Affleck and the explicit expressions of the form factors of the exponential operators e(+/-isqrt[8pi]phi(x)), we show that the spectrum consists of a stable triplet of massive particles for all values of theta and a singlet state of higher mass. The singlet is a stable particle only in an interval of values of theta close to pi, whereas it becomes a resonance below a critical value theta(c).     

Spin nematics and magnetization plateau transition in anisotropic kagome magnets

We study S=1 kagome antiferromagnets with an isotropic Heisenberg exchange J and strong easy-axis single-ion anisotropy D. For D>J, the low-energy physics can be described by an effective S=1/2 XXZ model with antiferromagnetic Jz approximately J and ferromagnetic J perpendicular approximately J2/D. Exploiting this connection, we argue that nontrivial ordering into a "spin-nematic" occurs whenever D dominates over J, and discuss its experimental signatures. We also study a magnetic field induced transition to a magnetization plateau state at magnetization 1/3 which breaks lattice translation symmetry due to ordering of the Sz and occupies a lobe in the B/Jz-Jz/J perpendicular phase diagram.     

Free energy and torque for superconductors with different anisotropies of H(c2) and lambda

The free energy is evaluated for a uniaxial superconductor with the anisotropy of the upper critical field, gamma(H)=H(c2,a)/H(c2,c), different from the anisotropy of the penetration depth gamma(lambda)=lambda(c)/lambda(a). With increasing difference between gamma(H) and gamma(lambda), the equilibrium orientation of the crystal relative to the applied field may shift from theta=pi/2 (theta is the angle between the field and the c axis) to lower angles and reach theta=0 for large enough gamma(H). These effects are expected to take place in MgB2.