93 research outputs found
Multiparticle interference in electronic Mach-Zehnder interferometers
We study theoretically electronic Mach-Zehnder interferometers built from
integer quantum Hall edge states, showing that the results of recent
experiments can be understood in terms of multiparticle interference effects.
These experiments probe the visibility of Aharonov-Bohm (AB) oscillations in
differential conductance as an interferometer is driven out of equilibrium by
an applied bias, finding a lobe pattern in visibility as a function of voltage.
We calculate the dependence on voltage of the visibility and the phase of AB
oscillations at zero temperature, taking into account long range interactions
between electrons in the same edge for interferometers operating at a filling
fraction . We obtain an exact solution via bosonization for models in
which electrons interact only when they are inside the interferometer. This
solution is non-perturbative in the tunneling probabilities at quantum point
contacts. The results match observations in considerable detail provided the
transparency of the incoming contact is close to one-half: the variation in
visibility with bias voltage consists of a series of lobes of decreasing
amplitude, and the phase of the AB-fringes is practically constant inside the
lobes but jumps by at the minima of the visibility. We discuss in
addition the consequences of approximations made in other recent treatments of
this problem. We also formulate perturbation theory in the interaction strength
and use this to study the importance of interactions that are not internal to
the interferometer.Comment: 20 pages, 15 figures, final version as publishe
Density matrix renormalization group for bosonic quantum Hall effect
We developed a density matrix renormalization-group technique to study
quantum Hall fractions of fast rotating bosons. In this paper we present a
discussion of the method together with the results which we obtain in three
distinct cases of the narrow channel, cylinder and spherical geometries. In the
narrow channel case, which is relevant to anisotropic confining traps in the
limit of extremely fast rotation, we find a series of zero-temperature phase
transitions in the strongly interacting regime as a function of the interaction
strength between bosons. We compute energies and density profiles for different
filling fractions on a cylinder and compare the convergence rates of the method
in the cylinder and a sphere geometries.Comment: 8 pages, 7 figures, final version as publishe
Density wave and supersolid phases of correlated bosons in an optical lattice
Motivated by the recent experiment on the Bose-Einstein condensation of
Cr atoms with long-range dipolar interactions (Werner J. et al., Phys.
Rev. Lett., 94 (2005) 183201), we consider a system of bosons with repulsive
nearest and next-nearest neighbor interactions in an optical lattice. The
ground state phase diagram, calculated using the Gutzwiller ansatz, shows,
apart from the superfluid (SF) and the Mott insulator (MI), two modulated
phases, \textit{i.e.}, the charge density wave (CDW) and the supersolid (SS).
Excitation spectra are also calculated which show a gap in the insulators,
gapless, phonon mode in the superfluid and the supersolid, and a mode softening
of superfluid excitations in the vicinity of the modulated phases. We discuss
the possibility of observing these phases in cold dipolar atoms and propose
experiments to detect them
Solution of a model for the two-channel electronic Mach-Zehnder interferometer
We develop the theory of electronic Mach-Zehnder interferometers built from
quantum Hall edge states at Landau level filling factor \nu = 2, which have
been investigated in a series of recent experiments and theoretical studies. We
show that a detailed treatment of dephasing and non-equlibrium transport is
made possible by using bosonization combined with refermionization to study a
model in which interactions between electrons are short-range. In particular,
this approach allows a non-perturbative treatment of electron tunneling at the
quantum point contacts that act as beam-splitters. We find an exact analytic
expression at arbitrary tunneling strength for the differential conductance of
an interferometer with arms of equal length, and obtain numerically exact
results for an interferometer with unequal arms. We compare these results with
previous perturbative and approximate ones, and with observations.Comment: 13 pages, 9 figures, final version as publishe
Dynamics of a two-dimensional quantum spin liquid: signatures of emergent Majorana fermions and fluxes
Topological states of matter present a wide variety of striking new
phenomena. Prominent among these is the fractionalisation of electrons into
unusual particles: Majorana fermions [1], Laughlin quasiparticles [2] or
magnetic monopoles [3]. Their detection, however, is fundamentally complicated
by the lack of any local order, such as, for example, the magnetisation in a
ferromagnet. While there are now several instances of candidate topological
spin liquids [4], their identification remains challenging [5]. Here, we
provide a complete and exact theoretical study of the dynamical structure
factor of a two-dimensional quantum spin liquid in gapless and gapped phases.
We show that there are direct signatures - qualitative and quantitative - of
the Majorana fermions and gauge fluxes emerging in Kitaev's honeycomb model.
These include counterintuitive manifestations of quantum number
fractionalisation, such as a neutron scattering response with a gap even in the
presence of gapless excitations, and a sharp component despite the
fractionalisation of electron spin. Our analysis identifies new varieties of
the venerable X-ray edge problem and explores connections to the physics of
quantum quenches.Comment: 7 pages, 3 figure
Dynamics of Fractionalization in Quantum Spin Liquids
We present the theory of dynamical spin-response for the Kitaev honeycomb
model, obtaining exact results for the structure factor (SF) in gapped and
gapless, Abelian and non-Abelian quantum spin-liquid (QSL) phases. We also
describe the advances in methodology necessary to compute these results. The
structure factor shows signatures of spin-fractionalization into emergent
quasiparticles -- Majorana fermions and fluxes of gauge field. In
addition to a broad continuum from spin-fractionalization, we find sharp
(-function) features in the response. These arise in two distinct ways:
from excited states containing only (static) fluxes and no (mobile) fermions;
and from excited states in which fermions are bound to fluxes. The SF is
markedly different in Abelian and non-Abelian QSLs, and bound fermion-flux
composites appear only in the non-Abelian phase.Comment: 21 pages, 14 figure
Neutron scattering signatures of the 3D hyper-honeycomb Kitaev quantum spin-liquid
Motivated by recent synthesis of the hyper-honeycomb material
-, we study the dynamical structure factor (DSF)
of the corresponding 3D Kitaev quantum spin-liquid (QSL), whose fractionalised
degrees of freedom are Majorana fermions and emergent flux-loops. Properties of
this 3D model are known to differ in important ways from those of its 2D
counterpart -- it has finite-temperature phase transition, as well as distinct
features in Raman response. We show, however, that the qualitative behaviour of
the DSF is broadly dimension-independent. Characteristics of the 3D DSF include
a response gap even in the gapless QSL phase and an energy dependence deriving
from the Majorana fermion density of states. Since the majority of the response
is from states containing a single Majorana excitation, our results suggest
inelastic neutron scattering as the spectroscopy of choice to illuminate the
physics of Majorana fermions in Kitaev QSLs.Comment: 5 pages, 5 figure
Bose-Einstein condensation of magnons in CsCuCl: a dilute gas limit near the saturation magnetic field
Based on a realistic spin Hamiltonian for a frustrated quasi-two dimensional
spin-1/2 antiferromagnet CsCuCl, a three-dimensional spin ordering
in the applied magnetic field near the saturation value is studied
within the magnon Bose-Einstein condensation (BEC) scenario. With the use of a
hard-core boson formulation of the spin model, a strongly anysotropic magnon
dispersion in CsCuCl is calculated. In the dilute magnon limit near
, the hard-core boson constraint is resulted in an effective magnon
interaction which is treated in the Hartree-Fock approximation. The critical
temperature is calculated as a function of a magnetic field and
compared with the phase boundary experimentally determined in
CsCuCl [Phys. Rev. Lett. \textbf{95}, 127202 (2005)]
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