12 research outputs found

### Non-k-diagonality in the interlayer pair-tunneling model of high-temperature superconductivity

We investigate the effect of k-space broadening of the interlayer pairing
kernel on the critical temperature T_c and the k-dependence of the gap function
in a one-dimensional version of the interlayer pair-tunneling model of high-T_c
superconductivity. We consider constant as well as k-dependent intralayer
pairing kernels. We find that the sensitivity to k-space broadening is larger
the smaller the width of the peak of the Fermi-level gap calculated for zero
broadening. This width increases with the overall magnitude of the interlayer
tunneling matrix element, and decreases with the bandwidth of the
single-electron intralayer excitation spectrum. The width also increases as the
Fermi level is moved towards regions where the excitation spectrum flattens
out. We argue that our qualitative conclusions are valid also for a
two-dimensional model. This indicates that at or close to half-filling in two
dimensions, when the Fermi-surface gap for zero broadening attains its peaks at
$(\pm \pi/a,0)$ and $(0,\pm\pi/a)$ where the excitation spectrum is flat, these
peaks should be fairly robust to moderate momentum broadening.Comment: 10 pages including 4 figures, to be published in Journal of Low
Temperature Physic

### Unified explanation of the Kadowaki-Woods ratio in strongly correlated materials

Discoveries of ratios whose values are constant within broad classes of
materials have led to many deep physical insights. The Kadowaki-Woods ratio
(KWR) compares the temperature dependence of a metal's resistivity to that of
its heat capacity; thereby probing the relationship between the
electron-electron scattering rate and the renormalisation of the electron mass.
However, the KWR takes very different values in different materials. Here we
introduce a ratio, closely related to the KWR, that includes the effects of
carrier density and spatial dimensionality and takes the same (predicted) value
in organic charge transfer salts, transition metal oxides, heavy fermions and
transition metals - despite the numerator and denominator varying by ten orders
of magnitude. Hence, in these materials, the same emergent physics is
responsible for the mass enhancement and the quadratic temperature dependence
of the resistivity and no exotic explanations of their KWRs are required.Comment: Final version accepted by Nature Phy

### Field-theoretical renormalization group for a flat two-dimensional Fermi surface

We implement an explicit two-loop calculation of the coupling functions and
the self-energy of interacting fermions with a two-dimensional flat Fermi
surface in the framework of the field theoretical renormalization group (RG)
approach. Throughout the calculation both the Fermi surface and the Fermi
velocity are assumed to be fixed and unaffected by interactions. We show that
in two dimensions, in a weak coupling regime, there is no significant change in
the RG flow compared to the well-known one-loop results available in the
literature. However, if we extrapolate the flow to a moderate coupling regime
there are interesting new features associated with an anisotropic suppression
of the quasiparticle weight Z along the Fermi surface, and the vanishing of the
renormalized coupling functions for several choices of the external momenta.Comment: 16 pages and 22 figure

### The Entanglement Entropy of Solvable Lattice Models

We consider the spin k/2 analogue of the XXZ quantum spin chain. We compute
the entanglement entropy S associated with splitting the infinite chain into
two semi-infinite pieces. In the scaling limit, we find S ~ c_k/6
(ln(xi))+ln(g)+... . Here xi is the correlation length and c_k=3k/(k+2) is the
central charge associated with the sl_2 WZW model at level k. ln(g) is the
boundary entropy of the WZW model. Our result extends previous observations and
suggests that this is a simple and perhaps rather general way both of
extracting the central charge of the ultraviolet CFT associated with the
scaling limit of a solvable lattice model, and of matching lattice and CFT
boundary conditions.Comment: 6 pages; connection with boundary entropy of Affleck and Ludwig added
in revised version and notation slightly change

### Bound states in d-density-wave phases

We investigate the quasiparticle spectrum near surfaces in a two-dimensional
system with d-density-wave order within a mean-field theory. For Fermi surfaces
with perfect nesting for the ordering wave vector of the d-density-wave, a zero
energy bound state occurs at [110] surfaces, in close analogy with the known
effect in d-wave superconducting states or graphite. When the shape of the
Fermi surface is changed by doping, the bound state energy moves away from the
Fermi level. Furthermore, away from half-filling we find inhomogeneous phases
with domain walls of the d-density-wave order parameter. The domain walls also
support low energy bound states. These phenomena might provide an experimental
test for hidden d-density-wave order in the high-Tc cuprates.Comment: 6 pages, 5 figure

### Itinerancy and Hidden Order in $URu_2Si_2$

We argue that key characteristics of the enigmatic transition at $T_0= 17.5K$ in $URu_2Si_2$ indicate that the hidden order is a density wave formed within
a band of composite quasiparticles, whose detailed structure is determined by
local physics. We expand on our proposal (with J.A. Mydosh) of the hidden order
as incommnesurate orbital antiferromagnetism and present experimental
predictions to test our ideas. We then turn towards a microscopic description
of orbital antiferromagnetism, exploring possible particle-hole pairings within
the context of a simple one-band model. We end with a discussion of recent
high-field and thermal transport experiment, and discuss their implications for
the nature of the hidden order.Comment: 18 pages, 7 figures. v2 contains added referenc

### Duality relations and exotic orders in electronic ladder systems

We discuss duality relations in correlated electronic ladder systems to
clarify mutual relations between various conventional and unconventional
phases. For the generalized two-leg Hubbard ladder, we find two exact duality
relations, and also one asymptotic relation which holds in the low-energy
regime. These duality relations show that unconventional (exotic) density-wave
orders such as staggered flux or circulating spin-current are directly mapped
to conventional density-wave orders, which establishes the appearance of
various exotic states with time-reversal and/or spin symmetry breaking. We also
study duality relations in the SO(5) symmetry that was proposed to unify
antiferromagnetism and d-wave superconductivity. We show that the same SO(5)
symmetry also unifies circulating spin current order and s-wave
superconductivity.Comment: 9 pages, 2 figures; Proceedings of SPQS2004 (Sendai

### Entanglement Entropy dynamics in Heisenberg chains

By means of the time-dependent density matrix renormalization group algorithm
we study the zero-temperature dynamics of the Von Neumann entropy of a block of
spins in a Heisenberg chain after a sudden quench in the anisotropy parameter.
In the absence of any disorder the block entropy increases linearly with time
and then saturates. We analyze the velocity of propagation of the entanglement
as a function of the initial and final anisotropies and compare, wherever
possible, our results with those obtained by means of Conformal Field Theory.
In the disordered case we find a slower (logarithmic) evolution which may
signals the onset of entanglement localization.Comment: 15 pages, 9 figure

### Effect of Ring Exchange on Orbital Antiferromagnet

We study the effect of four-particle ring exchange process on orbital
antiferromagnetic state that occurs in some correlated electron systems in two
dimensions. The primary question is whether the ring exchange process enhances
or suppresses the orbital antiferromagnetic ordering. Using the fact that the
orbital antiferromagnetic state arises in the large-N limit of the SU(N)
generalization of the t-J model, we consider the large-N limit of the t-J-$K$
model where $K$ represents the four-particle ring exchange term. The phase
diagrams in the large-N mean field theory are obtained for the half-filling and
finite hole concentrations at zero temperature. It is found that the ring
exchange in general favors dimerized states or the inhomogeneous orbital
antiferromagnetic state, and suppresses the homogeneous orbital
antiferromagnetic state. We compare our results with other related models of
strongly correlated systems with ring exchange processes.Comment: 14 pages, 17 figure