Analytical ground state for the three-band Hubbard model

For the calculation of charge excitations as those observed in, e.g., photo-emission spectroscopy or in electron-energy loss spectroscopy, a correct description of ground-state charge properties is essential. In strongly correlated systems like the undoped cuprates this is a highly non-trivial problem. In this paper we derive a non-perturbative analytical approximation for the ground state of the three-band Hubbard model on an infinite, half filled CuO_2 plane. By comparison with Projector Quantum Monte Carlo calculations it is shown that the resulting expressions correctly describe the charge properties of the ground state. Relations to other approaches are discussed. The analytical ground state preserves size consistency and can be generalized for other geometries, while still being both easy to interpret and to evaluate.Comment: REVTeX, 8 pages, 6 figures, to appear in Phys. Rev.

Combining 10Be surface exposure and OSL dating to reconstruct Holocene lake-level fluctuations: a case study at Tangra Yumco (southern Tibet)

Abstract HKT-ISTP 2013 B

Phase Transition in the Three-Dimensional ±J\pm J Ising Spin Glass

We have studied the three-dimensional Ising spin glass with a $\pm J$ distribution by Monte Carlo simulations. Using larger sizes and much better statistics than in earlier work, a finite size scaling analysis shows quite strong evidence for a finite transition temperature, $T_c$, with ordering below $T_c$. Our estimate of the transition temperature is rather lower than in earlier work, and the value of the correlation length exponent, $\nu$, is somewhat higher. Because there may be (unknown) corrections to finite size scaling, we do not completely rule out the possibility that $T_c = 0$ or that $T_c$ is finite but with no order below $T_c$. However, from our data, these possibilities seem less likely.Comment: Postscript file compressed using uufiles. The postscript file is also available by anonymous ftp at ftp://chopin.ucsc.edu/pub/sg3d.p

First-Order Vortex Lattice Melting and Magnetization of YBa2_2Cu3_3O$_{7-\delta}

We present the first non-mean-field calculation of the magnetization $M(T)$ of YBa$_2$Cu$_3$O$_{7-\delta}$ both above and below the flux-lattice melting temperature $T_m(H)$. The results are in good agreement with experiment as a function of transverse applied field $H$. The effects of fluctuations in both order parameter $\psi({\bf r})$ and magnetic induction $B$ are included in the Ginzburg-Landau free energy functional: $\psi({\bf r})$ fluctuates within the lowest Landau level in each layer, while $B$ fluctuates uniformly according to the appropriate Boltzmann factor. The second derivative $(\partial^2 M/\partial T^2)_H$ is predicted to be negative throughout the vortex liquid state and positive in the solid state. The discontinuities in entropy and magnetization at melting are calculated to be $\sim 0.034\, k_B$ per flux line per layer and $\sim 0.0014$~emu~cm$^{-3}$ at a field of 50 kOe.Comment: 11 pages, 4 PostScript figures in one uuencoded fil

Kinetic Inductance of Josephson Junction Arrays: Dynamic and Equilibrium Calculations

We show analytically that the inverse kinetic inductance $L^{-1}$ of an overdamped junction array at low frequencies is proportional to the admittance of an inhomogeneous equivalent impedance network. The $ij^{th}$ bond in this equivalent network has an inverse inductance $J_{ij}\cos(\theta_i^0-\theta_j^0-A_{ij})$, where $J_{ij}$ is the Josephson coupling energy of the $ij^{th}$ bond, $\theta_i^0$ is the ground-state phase of the grain $i$, and $A_{ij}$ is the usual magnetic phase factor. We use this theorem to calculate $L^{-1}$ for square arrays as large as $180\times 180$. The calculated $L^{-1}$ is in very good agreement with the low-temperature limit of the helicity modulus $\gamma$ calculated by conventional equilibrium Monte Carlo techniques. However, the finite temperature structure of $\gamma$, as a function of magnetic field, is \underline{sharper} than the zero-temperature $L^{-1}$, which shows surprisingly weak structure. In triangular arrays, the equilibrium calculation of $\gamma$ yields a series of peaks at frustrations $f = \frac{1}{2}(1-1/N)$, where $N$ is an integer $\geq 2$, consistent with experiment.Comment: 14 pages + 6 postscript figures, 3.0 REVTe

Metamagnetism in the 2D Hubbard Model with easy axis

Although the Hubbard model is widely investigated, there are surprisingly few attempts to study the behavior of such a model in an external magnetic field. Using the Projector Quantum Monte Carlo technique, we show that the Hubbard model with an easy axis exhibits metamagnetic behavior if an external field is turned on. For the case of intermediate correlations strength $U$, we observe a smooth transition from an antiferromagnetic regime to a paramagnetic phase. While the staggered magnetization will decrease linearly up to a critical field $B_c$, uniform magnetization develops only for fields higher than $B_c$.Comment: RevTeX 5 pages + 2 postscript figures (included), accepted for PRB Rapid Communication

Stability of the vortex lattice in a rotating superfluid

We analyze the stability of the vortex lattice in a rotating superfluid against thermal fluctuations associated with the long-wavelength Tkachenko modes of the lattice. Inclusion of only the two-dimensional modes leads formally to instability in infinite lattices; however, when the full three-dimensional spectrum of modes is taken into account, the thermally-induced lattice displacements are indeed finite.Comment: 16 page

Effect of disorder on the vortex-lattice melting transition

We use a three dimensional stacked triangular network of Josephson junctions as a model for the study of vortex structure in the mixed state of high Tc superconductors. We show that the addition of disorder destroys the first order melting transition occurring for clean samples. The melting transition splits in two different (continuous) transitions, ocurring at temperatures Ti and Tp (>Ti). At Ti the perpendicular-to-field superconductivity is lost, and at Tp the parallel-to-field superconductivity is lost. These results agree well with recent experiments in YBaCuO.Comment: 4 pages + 2 figure

Magnetization Jump in a Model for Flux Lattice Melting at Low Magnetic Fields

Using a frustrated XY model on a lattice with open boundary conditions, we numerically study the magnetization change near a flux lattice melting transition at low fields. In both two and three dimensions, we find that the melting transition is followed at a higher temperature by the onset of large dissipation associated with the zero-field XY transition. It is characterized by the proliferation of vortex-antivortex pairs (in 2D) or vortex loops (in 3D). At the upper transition, there is a sharp increase in magnetization, in qualitative agreement with recent local Hall probe experiments.Comment: updated figures and texts. new movies available at http://www.physics.ohio-state.edu:80/~ryu/jj.html. Accepted for publication in Physical Review Letter

Charge-order transition in the extended Hubbard model on a two-leg ladder

We investigate the charge-order transition at zero temperature in a two-leg Hubbard ladder with additional nearest-neighbor Coulomb repulsion V using the Density Matrix Renormalization Group technique. We consider electron densities between quarter and half filling. For quarter filling and U=8t, we find evidence for a continuous phase transition between a homogeneous state at small V and a broken-symmetry state with "checkerboard" [wavevector Q=(pi,pi)] charge order at large V. This transition to a checkerboard charge-ordered state remains present at all larger fillings, but becomes discontinuous at sufficiently large filling. We discuss the influence of U/t on the transition and estimate the position of the tricritical points.Comment: 4 pages, 5 figs, minor changes, accepted for publication in PRB R