Hasil untuk "cond-mat"

K. Motegi, Ta-Sheng Tai, R. Yoshioka

A bstractWe study the link between WZW model and the spin-1/2 XYZ chain. This is achieved by comparing the second-order differential equations from them. In the former case, the equation is the Ward-Takahashi identity satisfied by one-point toric conformal blocks. In the latter case, it arises from Baxter’s TQ relation. We find that the dimension of the representation space w.r.t. the V-valued primary field in these conformal blocks gets mapped to the total number of chain sites. By doing so, Stroganov’s “The importance of being odd” (cond-mat/0012035) can be consistently understood in terms of WZW model language. We fisrt confirm this correspondence by taking a trigonometric limit of the XYZ chain. That eigenstates of the resultant two-body Sutherland model from Baxter’s TQ relation can be obtained by deforming toric conformal blocks supports our proposal.

M. Dayan

Here I extend my last work about the origin of the pseudo-gaps in underdoped cuprates (arXiv: cond-mat. 1011.3206), to include the mechanism of superconductivity. This is done by adapting the formalism of the double correlations in systems with nested Fermi surfaces to the semi one dimensional system of strings of holes. It is proposed that magnetic interaction is crucial for the establishment of the pseudogap and the high temperature superconductivity. It is shown that superconductivity disturbs the completeness of the strings of holes, and creates fluctuations in them. This, in turn, reduces the magnetic interaction and the pseudogap order.

L. Prejbeanu

B. Valenzuela, E. Bas

Talya Zimbovskaya, N. Zimbovskaya

·. Ben, J. Cooper

Koehler, Mark D. Lee, T. Koehler

L. Fidkowski

We compute the dependence of the tunneling current in a double point contact in the k=3 Read-Rezayi state (which is conjectured to describe an incompressible quantum hall fluid at filling fraction nu=12/5) on voltage, separation between the two contacts, and temperature. Using the tunneling hamiltonian of cond-mat/0607431, we show that the effect of quasiholes in the bulk region between the two contacts is simply an overall constant multiplying the interference term. This is the same effect as found for the differential conductivity in cond-mat/0601242; the difference is that we do an actual edge theory calculation and compute the full current-voltage curve at weak tunneling.

A. Brataas, A. Mal'shukov, Y. Tserkovnyak

We consider Rashba spin–orbit effects on spin transport driven by an electric field in semiconductor quantum wells. We derive spin diffusion equations that are valid when the mean free path and the Rashba spin–orbit interaction vary on length scales larger than the mean free path in the weak spin–orbit coupling limit. From these general diffusion equations, we derive boundary conditions between regions of different spin–orbit couplings. We show that spin injection is feasible when the electric field is perpendicular to the boundary between two regions. When the electric field is parallel to the boundary, spin injection only occurs when the mean free path changes within the boundary, in agreement with the recent work by Tserkovnyak et al (Preprint cond-mat/0610190).

S. Kunikeev, Daniel A. Lidar

This work is a sequel to our work "The Spin Density Matrix I: General Theory and Exact Master Equations" (eprint arXiv:0708.0644 [cond-mat]). Here we compare pure- and pseudo-spin dynamics using as an example a system of two quantum dots, a pair of localized conduction-band electrons in an n-doped GaAs semiconductor. Pure-spin dynamics is obtained by tracing out the orbital degrees of freedom, whereas pseudo-spin dynamics retains (as is conventional) an implicit coordinate dependence. We show that magnetic field inhomogeneity and spin-orbit interaction result in a non-unitary evolution in pure-spin dynamics, whereas these interactions contribute to the effective pseudo-spin Hamiltonian via terms that are asymmetric in spin permutations, in particular, the Dzyaloshinskii-Moriya (DM) spin-orbit interaction. We numerically investigate the non-unitary effects in the dynamics of the triplet states population, purity, and Lamb energy shift, as a function of interdot distance and magnetic field difference. The spin-orbit interaction is found to produce effects of roughly four orders of magnitude smaller than those due to magnetic field difference in the pure-spin model. We estimate the spin-orbit interaction magnitude in the DM-interaction term. Our estimate gives a smaller value than that recently obtained by Kavokin [Phys. Rev. B 64, 075305 (2001)], who did not include double occupancy effects. We show that a necessary and sufficient condition for obtaining a universal set of quantum logic gates, involving only two spins, in both pure- and pseudo-spin models is that the magnetic field inhomogeneity and the Heisenberg interaction are both non-vanishing. We also briefly analyze pure-spin dynamics in the electron on liquid helium system recently proposed by Lyon [Phys. Rev. A 74, 052338 (2006)].

Nachi Gupta, Raphael Andreas Hauser, N. Johnson

We discuss the theoretical machinery involved in predicting financial market movements using an artificial market model which has been trained on real financial data. This approach to market prediction - in particular, forecasting financial time-series by training a third-party or 'black box' game on the financial data itself -- was discussed by Johnson et al. in cond-mat/0105303 and cond-mat/0105258 and was based on some encouraging preliminary investigations of the dollar-yen exchange rate, various individual stocks, and stock market indices. However, the initial attempts lacked a clear formal methodology. Here we present a detailed methodology, using optimization techniques to build an estimate of the strategy distribution across the multi-trader population. In contrast to earlier attempts, we are able to present a systematic method for identifying 'pockets of predictability' in real-world markets. We find that as each pocket closes up, the black-box system needs to be 'reset' - which is equivalent to saying that the current probability estimates of the strategy allocation across the multi-trader population are no longer accurate. Instead, new probability estimates need to be obtained by iterative updating, until a new 'pocket of predictability' emerges and reliable prediction can resume.

R. Mélin

Zhenyu Zhu, Qing-feng Sun, Bin Chen et al.

We numerically investigate the transport behavior of a quasi-one-dimensional (1D) square loop device containing the Rashba spin-orbital interaction in the presence of a magnetic flux. The conductance versus the magnetic field shows the Al'tshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB) oscillations. We focus on the oscillatory amplitudes, and find that both of them are strongly dependent on the spin precession angle (i.e., the strength of the spin-orbit interaction) and exhibit no periodic oscillations, in good agreement with a recent experiment by Koga [cond-mat/0504743 (unpublished)]. However, our numerical results for the ideal 1D square loop device for the node positions of the amplitudes of the AB and AAS oscillations are found to show some discrepancies with the results for quasi-1D square loops with a finite width. In the presence of disorder and taking the disorder ensemble average, the AB oscillation in the conductance disappears, while the time-reversal symmetric AAS oscillation still remains. Furthermore, the node positions of the AAS oscillatory amplitude remain the same.

D. Benedetto, E. Caglioti, V. Loreto

Motivated by the recent submission to cond-mat archives by J. Goodman (cond-mat/0202383) whose results apparently discredit the approach we have proposed in a recent paper (Phys. Rev. Lett., 88, 048702 (2002), cond-mat/0108530), we report the results of the same experiment performed by Goodman using three different data compression schemes. As a matter of fact the three zippers display the same efficiency Goodman obtained using Naive Bayesian Methods and not, as Goodman claimed, an efficiency three times smaller. We point out the question of the extreme generality of approaches based on data compression techniques and we list a large range of potential applications, including those of interest for the physics community.

G. M. Shim, B. Park, Hoyun Lee

We present an analytic study of the urn model for separation of sand recently introduced by Lipowski and Droz [Phys. Rev. E 65, 031307 (2002)]. We solve analytically the master equation and the first-passage problem. The analytic results confirm the numerical results obtained by Lipowski and Droz. We find that the stationary probability distribution and the shortest one among the characteristic times are governed by the same free energy. We also analytically derive the form of the critical probability distribution on the critical line, which supports their results obtained by numerically calculating Binder cumulants (A. Lipowski and M. Droz, e-print cond-mat/0201472).

D. Helbing, I. Farkas, T. Vicsek

Simulation software used to produce results in cond-mat/0009448 -- published as Helbing et.al, Simulating Dynamical Features of Escape Panic, Nature 407, 487-490 (2000) -- has been made available via the website of the publication at this http URL

D. Sornette, D. Sornette

We introduce a simple generalization of rational bubble models which removes the fundamental problem discovered by Lux and Sornette (J. Money, Credit and Banking, preprint at http://xxx.lanl.gov/abs/cond-mat/9910141) that the distribution of returns is a power law with exponent rδ corresponds to a generalization of the rational bubble model in which the fundamental price is no more given by the discounted value of future dividends. We explain how this is possible. Our model predicts that, the higher is the market remuneration r above the discount rate, the larger is the power-law exponent and thus the thinner is the tail of the distribution of price returns.

H. Tasaki

Recently Allahverdyan and Nieuwenhuizen (cond-mat/0006404) argued that the second law of thermodynamics may be violated in a quantum system as a "consequence of quantum coherence in the presence of the slightly off-equilibrium nature of the bath." By using a standard result about relative entropy, we prove rigorously that the second law is never violated (and, in particular, a perpetual motion of the second kind can never be realized) in quantum systems no matter how strong ``quantum coherence'' is or no matter how far one goes from equilibrium.