8,235 research outputs found
Symmetry Breaking and Finite Size Effects in Quantum Many-Body Systems
We consider a quantum many-body system on a lattice with a continuous
symmetry which exhibits a spontaneous symmetry breaking in its infinite volume
ground states, but in which the order operator does not commute with the
Hamiltonian. A typical example is the Heisenberg antiferromagnet with a Neel
order. In the corresponding finite system, the symmetry breaking is usually
"obscured" by "quantum fluctuation" and one gets a symmetric ground state with
a long range order. In such a situation, we prove that there exist ever
increasing numbers of low-lying eigenstates whose excitation energies are
bounded by a constant times 1/N, where N denotes the number of sites. By
forming linear combinations of these low-lying states and the (finite-volume)
ground state, and by taking infinite volume limits, we construct infinite
volume ground states with explicit symmetry breaking. Our general theorems do
not only shed light on the nature ofsymmetry breaking in quantum many-body
systems, but provide indispensable information for numerical approaches to
these systems. We also discuss applications of our general results to a variety
of examples. The present paper is intended to be accessible to the readers
without background in mathematical approaches to quantum many-body systems.Comment: LaTeX, 58 pages, no figures. Notes about Bose-Einstein condenstaion
are added after the publicatio
Hyponatremia and hypercalcemia: a study of a large cohort of patients with lung cancer
Background: Hyponatremia and hypercalcemia are reported to be associated with poorer prognosis in lung cancer. Our study assessed the incidence of hyponatremia and hypercalcemia in a recent large cohort of patients diagnosed with lung cancer in an academic institution and correlated incidence with patient and tumour parameters. Methods: All patients presented at our regional lung cancer multidisciplinary team meeting between January 2011 and December 2016 were included. The incidence of hyponatremia (serum sodium ≤135 mEq/L) and hypercalcemia (serum calcium >2.62 mmol/L), including severity (mild, moderate or severe) was evaluated and stratified by tumour subtype and stage, and correlated with patient parameters. Results: A total of 624 patients (mean age, 67.4 years; 59.3% male) diagnosed with tissue-proven lung cancer were included. Hyponatremia and hypercalcemia were present in 31.6% (n=197) and 7.1% (n=44) at time of diagnosis. Hyponatremia occurred most commonly in patients with small cell lung carcinoma (SCLC) (n=42; 41.2%; P=0.001). Hypercalcemia occurred most commonly in patients with non-small cell lung carcinoma (NSCLC) squamous subtype (n=27; 12.2%; P=0.003). The incidence of hyponatremia and hypercalcemia were significantly higher in the advanced stages (P<0.041), except in SCLC where no difference in hypercalcemia incidence across the stages was observed (P=0.573). The Eastern Cooperative Oncology Group Performance Status (ECOG-PS) score was positively correlated with severity of hyponatremia at the early stage of NSCLC (Spearman correlation coefficient =0.325; P=0.003). Conclusions: Hyponatremia is a common association in lung cancer, especially in SCLC. Hypercalcemia is an uncommon but significant association in the NSCLC squamous subtype. Hyponatremia might contribute to poorer ECOG-PS scores at the early stage of NSCLC
Influence of Hybridization on the Properties of the Spinless Falicov-Kimball Model
Without a hybridization between the localized f- and the conduction (c-)
electron states the spinless Falicov-Kimball model (FKM) is exactly solvable in
the limit of high spatial dimension, as first shown by Brandt and Mielsch. Here
I show that at least for sufficiently small c-f-interaction this exact
inhomogeneous ground state is also obtained in Hartree-Fock approximation. With
hybridization the model is no longer exactly solvable, but the approximation
yields that the inhomogeneous charge-density wave (CDW) ground state remains
stable also for finite hybridization V smaller than a critical hybridization
V_c, above which no inhomogeneous CDW solution but only a homogeneous solution
is obtained. The spinless FKM does not allow for a ''ferroelectric'' ground
state with a spontaneous polarization, i.e. there is no nonvanishing
-expectation value in the limit of vanishing hybridization.Comment: 7 pages, 6 figure
Hybrid Monte Carlo with Fat Link Fermion Actions
The use of APE smearing or other blocking techniques in lattice fermion
actions can provide many advantages. There are many variants of these fat link
actions in lattice QCD currently, such as FLIC fermions. The FLIC fermion
formalism makes use of the APE blocking technique in combination with a
projection of the blocked links back into the special unitary group. This
reunitarisation is often performed using an iterative maximisation of a gauge
invariant measure. This technique is not differentiable with respect to the
gauge field and thus prevents the use of standard Hybrid Monte Carlo simulation
algorithms. The use of an alternative projection technique circumvents this
difficulty and allows the simulation of dynamical fat link fermions with
standard HMC and its variants. The necessary equations of motion for FLIC
fermions are derived, and some initial simulation results are presented. The
technique is more general however, and is straightforwardly applicable to other
smearing techniques or fat link actions
Quantum fluctuations in quantum lattice-systems with continuous symmetry
We discuss conditions for the absence of spontaneous breakdown of continuous
symmetries in quantum lattice systems at . Our analysis is based on
Pitaevskii and Stringari's idea that the uncertainty relation can be employed
to show quantum fluctuations. For the one-dimensional systems, it is shown that
the ground state is invariant under the continuous transformation if a certain
uniform susceptibility is finite. For the two- and three-dimensional systems,
it is shown that truncated correlation functions cannot decay any more rapidly
than whenever the continuous symmetry is spontaneously broken.
Both of these phenomena occur owing to quantum fluctuations. Our theorems cover
a wide class of quantum lattice-systems having not-too-long-range interactions.Comment: 14 pages. To appear in J.Stat.Phy
The spectral gap for some spin chains with discrete symmetry breaking
We prove that for any finite set of generalized valence bond solid (GVBS)
states of a quantum spin chain there exists a translation invariant
finite-range Hamiltonian for which this set is the set of ground states. This
result implies that there are GVBS models with arbitrary broken discrete
symmetries that are described as combinations of lattice translations, lattice
reflections, and local unitary or anti-unitary transformations. We also show
that all GVBS models that satisfy some natural conditions have a spectral gap.
The existence of a spectral gap is obtained by applying a simple and quite
general strategy for proving lower bounds on the spectral gap of the generator
of a classical or quantum spin dynamics. This general scheme is interesting in
its own right and therefore, although the basic idea is not new, we present it
in a system-independent setting. The results are illustrated with an number of
examples.Comment: 48 pages, Plain TeX, BN26/Oct/9
Optical lattice quantum simulator for QED in strong external fields: spontaneous pair creation and the Sauter-Schwinger effect
Spontaneous creation of electron-positron pairs out of the vacuum due to a
strong electric field is a spectacular manifestation of the relativistic
energy-momentum relation for the Dirac fermions. This fundamental prediction of
Quantum Electrodynamics (QED) has not yet been confirmed experimentally as the
generation of a sufficiently strong electric field extending over a large
enough space-time volume still presents a challenge. Surprisingly, distant
areas of physics may help us to circumvent this difficulty. In condensed matter
and solid state physics (areas commonly considered as low energy physics), one
usually deals with quasi-particles instead of real electrons and positrons.
Since their mass gap can often be freely tuned, it is much easier to create
these light quasi-particles by an analogue of the Sauter-Schwinger effect. This
motivates our proposal of a quantum simulator in which excitations of
ultra-cold atoms moving in a bichromatic optical lattice represent particles
and antiparticles (holes) satisfying a discretized version of the Dirac
equation together with fermionic anti-commutation relations. Using the language
of second quantization, we are able to construct an analogue of the spontaneous
pair creation which can be realized in an (almost) table-top experiment.Comment: 21 pages, 10 figure
Epitaxial growth of visible to infra-red transparent conducting In2O3 nanodot dispersions and reversible charge storage as a Li-ion battery anode
peer-reviewedUnique bimodal distributions of single crystal epitaxially grown In2O3 nanodots on silicon are shown to have excellent IR transparency greater than 87% at IR wavelengths up to 4 mu m without sacrificing transparency in the visible region. These broadband antireflective nanodot dispersions are grown using a two-step metal deposition and oxidation by molecular beam epitaxy, and backscattered diffraction confirms a dominant (111) surface orientation. We detail the growth of a bimodal size distribution that facilitates good surface coverage (80%) while allowing a significant reduction in In2O3 refractive index. This unique dispersion offers excellent surface coverage and three-dimensional volumetric expansion compared to a thin film, and a step reduction in refractive index compared to bulk active materials or randomly porous composites, to more closely match the refractive index of an electrolyte, improving transparency. The (111) surface orientation of the nanodots, when fully ripened, allows minimum lattice mismatch strain between the In2O3 and the Si surface. This helps to circumvent potential interfacial weakening caused by volume contraction due to electrochemical reduction to lithium, or expansion during lithiation. Cycling under potentiodynamic conditions shows that the transparent anode of nanodots reversibly alloys lithium with good Coulombic efficiency, buffered by co-insertion into the silicon substrate. These properties could potentially lead to further development of similarly controlled dispersions of a range of other active materials to give transparent battery electrodes or materials capable of non-destructive in situ spectroscopic characterization during charging and discharging.ACCEPTEDpeer-reviewe
A Characterization of Topological Insulators: Chern Numbers for a Ground State Multiplet
We propose to use generic Chern numbers for a characterization of topological
insulators. It is suitable for a numerical characterization of low dimensional
quantum liquids where strong quantum fluctuations prevent from developing
conventional orders. By twisting parameters of boundary conditions, the
non-Abelian Chern number are defined for a few low lying states near the ground
state in a finite system, which is a ground state multiplet with a possible
(topological) degeneracy. We define the system as a topological insulator when
energies of the multiplet are well separated from the above. Translational
invariant twists up to a unitary equivalence are crutial to pick up only bulk
properties without edge states. As a simple example, the setup is applied for a
two-dimensional -spin system with an ising anisotropy where the ground
state multiplet is composed of doubly almost degenerate states. It gives a
vanishing Chern number due to a symmetry. Also Chern numbers for the generic
fractional quantum Hall states are discussed shortly.Comment: 2 figure
Two-Level Atom in an Optical Parametric Oscillator: Spectra of Transmitted and Fluorescent Fields in the Weak Driving Field Limit
We consider the interaction of a two-level atom inside an optical parametric
oscillator. In the weak driving field limit, we essentially have an atom-cavity
system driven by the occasional pair of correlated photons, or weakly squeezed
light. We find that we may have holes, or dips, in the spectrum of the
fluorescent and transmitted light. This occurs even in the strong-coupling
limit when we find holes in the vacuum-Rabi doublet. Also, spectra with a
sub-natural linewidth may occur. These effects disappear for larger driving
fields, unlike the spectral narrowing obtained in resonance fluorescence in a
squeezed vacuum; here it is important that the squeezing parameter tends to
zero so that the system interacts with only one correlated pair of photons at a
time. We show that a previous explanation for spectral narrowing and spectral
holes for incoherent scattering is not applicable in the present case, and
propose a new explanation. We attribute these anomalous effects to quantum
interference in the two-photon scattering of the system.Comment: 10 pages, 17 figures, submitted to Phys Rev
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