42,418 research outputs found
Superfluid-Insulator Transitions on the Triangular Lattice
We report on a phenomenological study of superfluid to Mott insulator
transitions of bosons on the triangular lattice, focusing primarily on the
interplay between Mott localization and geometrical charge frustration at
1/2-filling. A general dual vortex field theory is developed for arbitrary
rational filling factors f, based on the appropriate projective symmetry group.
At the simple non-frustrated density f=1/3, we uncover an example of a
deconfined quantum critical point very similar to that found on the half-filled
square lattice. Turning to f=1/2, the behavior is quite different. Here, we
find that the low-energy action describing the Mott transition has an emergent
nonabelian SU(2)\times U(1) symmetry, not present at the microscopic level.
This large nonabelian symmetry is directly related to the frustration-induced
quasi-degeneracy between many charge-ordered states not related by microscopic
symmetries. Through this ``pseudospin'' SU(2)symmetry, the charged excitations
in the insulator close to the Mott transition develop a skyrmion-like
character. This leads to an understanding of the recently discovered supersolid
phase of the triangular lattice XXZ model (cond-mat/0505258, cond-mat/0505257,
cond-mat/0505298) as a ``partially melted'' Mott insulator. The latter picture
naturally explains a number of puzzling numerical observations of the
properties of this supersolid. Moreover, we predict that the nearby quantum
phase transition from this supersolid to the Mott insulator is in the
recently-discovered non-compact CP^1 critical universality class (PRB 70,
075104 (2004)). A description of a broad range of other Mott and supersolid
states, and a diverse set of quantum critical points between them, is also
provided.Comment: 24 pages, 14 figure
Perturbational approach to the quantum capacity of additive Gaussian quantum channel
For a quantum channel with additive Gaussian quantum noise, at the large
input energy side, we prove that the one shot capacity is achieved by the
thermal noise state for all Gaussian state inputs, it is also true for
non-Gaussian input in the sense of first order perturbation. For a general case
of copies input, we show that up to first order perturbation, any
non-Gaussian perturbation to the product thermal state input has a less quantum
information transmission rate when the input energy tend to infinitive.Comment: 5 page
The missing ingredient in effective-medium theories: Standard deviations
Effective-medium theories for electromagnetic constitutive parameters of
particulate composite materials are theories of averages. Standard deviations
are absent because of the lack of rigorous theories. But ensemble averages and
standard deviations can be calculated from a rigorous theory of reflection by
planar multilayers. Average reflectivities at all angles of incidence and two
orthogonal polarization states for a multilayer composed of two kinds of
electrically thin layers agree well with reflectivities for a single layer with
the same overall thickness and a volume-weighted average of the relative
permittivities of these two components. But the relative standard deviation can
be appreciable depending on the angle of incidence and the polarization state
of the incident illumination, and increases with increasing difference between
the constitutive parameters of the two layers. This suggests that average
constitutive parameters obtained from effective-medium theories do not have
uniform validity for all calculations in which they might be used.Comment: 12 pages (accepted for publication in Journal of Modern Optics
Unified Band Theoretic Description of Electronic and Magnetic Properties of Vanadium Dioxide Phases
The debate about whether the insulating phases of vanadium dioxide (VO2) can
be described by band theory or must be described by a theory of strong electron
correlations remains unresolved even after decades of research. Energy-band
calculations using hybrid exchange functionals or including self-energy
corrections account for the insulating or metallic nature of different phases,
but have not yet successfully accounted for the observed magnetic orderings.
Strongly-correlated theories have had limited quantitative success. Here we
report that, by using hard pseudopotentials and an optimized hybrid exchange
functional, the energy gaps and magnetic orderings of both monoclinic VO2
phases and the metallic nature of the high-temperature rutile phase are
consistent with available experimental data, obviating an explicit role for
strong correlations. We also report a potential candidate for the newly-found
metallic monoclinic phase and present a detailed magnetic structure of the M2
monoclinic phase
Determining quantum phase diagrams of topological Kitaev-inspired models on NISQ quantum hardware
Topological protection is employed in fault-tolerant error correction and in
developing quantum algorithms with topological qubits. But, topological
protection intrinsic to models being simulated, also robustly protects
calculations, even on NISQ hardware. We leverage it by simulating
Kitaev-inspired models on IBM quantum computers and accurately determining
their phase diagrams. This requires constructing conventional quantum circuits
for Majorana braiding to prepare the ground states of Kitaev-inspired models.
The entanglement entropy is then measured to calculate the quantum phase
boundaries. We show how maintaining particle-hole symmetry when sampling
through the Brillouin zone is critical to obtaining high accuracy. This work
illustrates how topological protection intrinsic to a quantum model can be
employed to perform robust calculations on NISQ hardware, when one measures the
appropriate protected quantum properties. It opens the door for further
simulation of topological quantum models on quantum hardware available today.Comment: 17 pages and 11 figures final versio
Ordering in weakly coupled random singlet spin chains
The influence of bond randomness on long range magnetic ordering in the
weakly coupled S = 1/2 antiferromagnetic spin chain materials
Cu(py)2(Cl1-xBrx)2 is studied by muon spin rotation and bulk measurements.
Disorder is found to have a strong effect on the ordering temperature TN, and
an even stronger one on the saturation magnetization m0, but considerably more
so in the effectively lower-dimensional Br-rich materials. The observed
behavior is attributed to Random Singlet ground states of individual spin
chains, but remains in contradiction with chain mean field theory predictions.
In this context, we discuss the possibility of a universal distribution of
ordered moments in the weakly coupled Random Singlet chains model
Robust measurement of wave function topology on NISQ quantum computers
Topological quantum phases of quantum materials are defined through their
topological invariants. These topological invariants are quantities that
characterize the global geometrical properties of the quantum wave functions
and thus are immune to local noise. Here, we present a strategy to measure
topological invariants on quantum computers. We show that our strategy can be
easily integrated with the variational quantum eigensolver (VQE) so that the
topological properties of generic quantum many-body states can be characterized
on current quantum hardware. We demonstrate two explicit examples that show how
the Chern number can be measured exactly; that is, it is immune to the noise of
NISQ machines. This work shows that the robust nature of wave function topology
allows NISQ machines to determine topological invariants accurately.Comment: 14 pages, 9 figures, 3 table
Spin Transfer Torque for Continuously Variable Magnetization
We report quantum and semi-classical calculations of spin current and
spin-transfer torque in a free-electron Stoner model for systems where the
magnetization varies continuously in one dimension.Analytic results are
obtained for an infinite spin spiral and numerical results are obtained for
realistic domain wall profiles. The adiabatic limit describes conduction
electron spins that follow the sum of the exchange field and an effective,
velocity-dependent field produced by the gradient of the magnetization in the
wall. Non-adiabatic effects arise for short domain walls but their magnitude
decreases exponentially as the wall width increases. Our results cast doubt on
the existence of a recently proposed non-adiabatic contribution to the
spin-transfer torque due to spin flip scattering.Comment: 11 pages, 9 figure
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