10 research outputs found
Tensor renormalization group approach to 2D classical lattice models
We describe a simple real space renormalization group technique for two
dimensional classical lattice models. The approach is similar in spirit to
block spin methods, but at the same time it is fundamentally based on the
theory of quantum entanglement. In this sense, the technique can be thought of
as a classical analogue of DMRG. We demonstrate the method - which we call the
tensor renormalization group method - by computing the magnetization of the
triangular lattice Ising model.Comment: 4 pages, 7 figure
Susceptibility of a spinon Fermi surface coupled to a U(1) gauge field
We study the theory of a U(1) gauge field coupled to a spinon Fermi surface.
Recently this model has been proposed as a possible description of the organic
compound . We calculate the susceptibility of
this system and in particular examine the effect of pairing of the underlying
spin liquid. We show that this proposed theory is consistent with the observed
susceptibility measurements.Comment: 5 pages, 4 figure
Transport Properties of a spinon Fermi surface coupled to a U(1) gauge field
With the organic compound -(BEDT-TTF)-Cu(CN) in mind, we
consider a spin liquid system where a spinon Fermi surface is coupled to a U(1)
gauge field. Using the non-equilibrium Green's function formalism, we derive
the Quantum Boltzmann Equation (QBE) for this system. In this system, however,
one cannot a priori assume the existence of Landau quasiparticles. We show that
even without this assumption one can still derive a linearized equation for a
generalized distribution function. We show that the divergence of the effective
mass and of the finite temperature self-energy do not enter these transport
coefficients and thus they are well-defined. Moreover, using a variational
method, we calculate the temperature dependence of the spin resistivity and
thermal conductivity of this system.Comment: 12 page
A Variational Monte Carlo Study of the Current Carried by a Quasiparticle
With the use of Gutzwiller-projected variational states, we study the
renormalization of the current carried by the quasiparticles in
high-temperature superconductors and of the quasiparticle spectral weight. The
renormalization coefficients are computed by the variational Monte Carlo
technique, under the assumption that quasiparticle excitations may be described
by Gutzwiller-projected BCS quasiparticles. We find that the current
renormalization coefficient decreases with decreasing doping and tends to zero
at zero doping. The quasiparticle spectral weight Z_+ for adding an electron
shows an interesting structure in k space, which corresponds to a depression of
the occupation number k just outside the Fermi surface. The perturbative
corrections to those quantities in the Hubbard model are also discussed.Comment: 9 pages, 9 figure
Quantum decoherence of a charge qubit in a spin-fermion model
We consider quantum decoherence in solid-state systems by studying the
transverse dynamics of a single qubit interacting with a fermionic bath and
driven by external pulses. Our interest is in investigating the extent to which
the lost coherence can be restored by the application of external pulses to the
qubit. We show that the qubit evolution under various pulse sequences can be
mapped onto Keldysh path integrals. This approach allows a simple diagrammatic
treatment of different bath excitation processes contributing to qubit
decoherence. We apply this theory to the evolution of the qubit coupled to the
Andreev fluctuator bath in the context of widely studied superconducting
qubits. We show that charge fluctuations within the Andreev-fluctuator model
lead to a 1/f noise spectrum with a characteristic temperature depedence. We
discuss the strategy for suppression of decoherence by the application of
higher-order (beyond spin echo) pulse sequences.Comment: 7 pages, 4 figures; extended version (accepted to Phys. Rev. B
Monte Carlo Simulations of Globular Cluster Evolution - II. Mass Spectra, Stellar Evolution and Lifetimes in the Galaxy
We study the dynamical evolution of globular clusters using our new 2-D Monte
Carlo code, and we calculate the lifetimes of clusters in the Galactic
environment. We include the effects of a mass spectrum, mass loss in the
Galactic tidal field, and stellar evolution. We consider initial King models
containing N = 10^5 - 3x10^5 stars, and follow the evolution up to core
collapse, or disruption, whichever occurs first. We find that the lifetimes of
our models are significantly longer than those obtained using 1-D Fokker-Planck
(F-P) methods. We also find that our results are in very good agreement with
recent 2-D F-P calculations, for a wide range of initial conditions. Our
results show that the direct mass loss due to stellar evolution can
significantly accelerate the mass loss through the tidal boundary, causing most
clusters with a low initial central concentration (Wo <~ 3) to disrupt quickly
in the Galactic tidal field. Only clusters born with high initial central
concentrations (Wo >~ 7) or steep initial mass functions are likely to survive
to the present and undergo core collapse. We also study the orbital
characteristics of escaping stars, and find that the velocity distribution of
escaping stars in collapsing clusters looks significantly different from the
distribution in disrupting clusters. We calculate the lifetime of a cluster on
an eccentric orbit in the Galaxy, such that it fills its Roche lobe only at
perigalacticon. We find that such an orbit can extend the lifetime by at most a
factor of a few compared to a circular orbit in which the cluster fills its
Roche lobe at all times.Comment: 32 pages, including 10 figures, to appear in ApJ, minor corrections
onl
How to Enhance Dephasing Time in Superconducting Qubits
We theoretically investigate the influence of designed pulse sequences in
restoring quantum coherence lost due to background noise in superconducting
qubits. We consider both 1/f noise and Random Telegraph Noise, and show that
the qubit coherence time can be substantially enhanced by carefully engineered
pulse sequences. Conversely, the time dependence of qubit coherence under
external pulse sequences could be used as a spectroscopic tool for extracting
the noise mechanisms in superconducting qubits, i.e. by using Uhrig's pulse
sequence one can obtain information about moments of the spectral density of
noise. We also study the effect of pulse sequences on the evolution of the
qubit affected by a strongly coupled fluctuator, and show that the non-Gaussian
features in decoherence are suppressed by the application of pulses.Comment: 12 pages, 5 figures, extended version accepted for publication in
Phys. Rev.