150 research outputs found

### Extracting the Mott gap from energy measurements in trapped atomic gases

We show that the measure of the so-called {\it release-energy}, which is an
experimentally accessible quantity, makes it possible to assess the value of
the Mott gap in the presence of the confinement potential that is unavoidable
in the actual experimental setup. Indeed, the curve of the release-energy as a
function of the total number of particles shows kinks that are directly related
to the existence of excitation gaps. Calculations are presented within the
Gutzwiller approach, but the final results go beyond this simple approximation
and represent a genuine feature of the real system. In the case of harmonic
confinement, the Mott gaps may be renormalized with respect to the uniform
case. On the other hand, in the case of the recently proposed off-diagonal
confinement, our results show an almost perfect agreement with the homogeneous
case.Comment: 4 pages and 5 figure

### Nagaoka ferromagnetism in the two-dimensional infinite-U Hubbard model

We present different numerical calculations based on variational quantum
Monte Carlo simulations supporting a ferromagnetic ground-state for finite and
small hole densities in the two-dimensional infinite-$U$ Hubbard model.
Moreover, by studying the energies of different total spin sectors, these
calculations strongly suggest that the paramagnetic phase is unstable against a
phase with a partial polarization for large hole densities $\delta \sim 0.40$
with evidence for a second-order transition to the paramagnetic large doping
phase.Comment: 4 page

### Quantum Phase Transition in Coupled Spin Ladders

The ground state of an array of coupled, spin-half, antiferromagnetic ladders
is studied using spin-wave theory, exact diagonalization (up to 36 sites) and
quantum Monte Carlo techniques (up to 256 sites). Our results clearly indicate
the occurrence of a zero-temperature phase transition between a N\'eel ordered
and a non-magnetic phase at a finite value of the inter-ladder coupling
($\alpha_c\simeq0.3$). This transition is marked by remarkable changes in the
structure of the excitation spectrum.Comment: 4 pages, 6 postscript figures, to appear in Physical Review

### Ground-state properties of the disordered Hubbard model in two dimensions

We study the interplay between electron correlation and disorder in the
two-dimensional Hubbard model at half-filling by means of a variational wave
function that can interpolate between Anderson and Mott insulators. We give a
detailed description of our improved variational state and explain how the
physics of the Anderson-Mott transition can be inferred from equal-time
correlations functions, which can be easily computed within the variational
Monte Carlo scheme. The ground-state phase diagram is worked out in both the
paramagnetic and the magnetic sector. Whereas in the former a direct
second-order Anderson-Mott transition is obtained, when magnetism is allowed
variationally, we find evidence for the formation of local magnetic moments
that order before the Mott transition. Although the localization length
increases before the Mott transition, we have no evidence for the stabilization
of a true metallic phase. The effect of a frustrating next-nearest-neighbor
hopping $t^\prime$ is also studied in some detail. In particular, we show that
$t^\prime$ has two primary effects. The first one is the narrowing of the
stability region of the magnetic Anderson insulator, also leading to a
first-order magnetic transition. The second and most important effect of a
frustrating hopping term is the development of a ``glassy'' phase at strong
couplings, where many paramagnetic states, with disordered local moments, may
be stabilized.Comment: 13 pages and 16 figure

### Vanishing spin gap in a competing spin-liquid phase in the kagome Heisenberg antiferromagnet

We provide strong numerical evidence, using improved variational wave
functions, for a ground state with vanishing spin gap in the spin-$1/2$ quantum
Heisenberg model on the kagome lattice. Starting from the algebraic $U(1)$
Dirac spin liquid state proposed by Y. Ran $et al.$ [Phys. Rev. Lett. $98$,
$117205$ ($2007$)] and iteratively applying a few Lanczos steps, we compute the
lowest $S=2$ excitation constructed by exciting spinons close to the Dirac
nodes. Our results are compatible with a vanishing spin gap in the
thermodynamic limit and in consonance with a power-law decay of long distance
spin-spin correlations in real space. The competition with a gapped
(topological) spin liquid is discussed.Comment: 5 pages, 3 figures, 2 tables. Published versio

### Projected wave function study of Z2 spin liquids on the kagome lattice for the spin-1/2 quantum Heisenberg antiferromagnet

Motivated by recent density-matrix renormalization group (DMRG) calculations
[Yan, Huse, and White, Science 332, 1173 (2011)], which claimed that the ground
state of the nearest-neighbor spin-1/2 Heisenberg antiferromagnet on the kagome
lattice geometry is a fully gapped spin liquid with numerical signatures of Z2
gauge structure, and a further theoretical work [Lu, Ran, and Lee, Phys. Rev. B
83, 224413 (2011)], which gave a classification of all Schwinger-fermion
mean-field fully symmetric Z2 spin liquids on the kagome lattice, we have
thoroughly studied Gutzwiller-projected fermionic wave functions by using
quantum variational Monte Carlo techniques, hence implementing exactly the
constraint of one fermion per site. In particular, we investigated the
energetics of all Z2 candidates (gapped and gapless) that lie in the
neighborhood of the energetically competitive U(1) gapless spin liquids. By
using a state-of-the-art optimization method, we were able to conclusively show
that the U(1) Dirac state is remarkably stable with respect to all Z2 spin
liquids in its neighborhood, and in particular for opening a gap toward the
so-called Z2[0,{\pi}]{\beta} state, which was conjectured to describe the
ground state obtained by the DMRG method. Finally, we also considered the
addition of a small second nearest-neighbor exchange coupling of both
antiferromagnetic and ferromagnetic type, and obtained similar results, namely,
a U(1) Dirac spin-liquid ground state.Comment: 5 pages + supplementary material (2 pages), 3 figures, 1 Table: Final
published version, selected as an Editor's suggestio

### Electronic properties driven by strong correlation

The fascinating subject of superconductivity was opened over a century ago by Onnes [1], but until quite recently it was strictly a low-temperature phenomenon. The discovery of the cuprate superconductors [2] in a family of transition metal oxides, with transition temperatures up to Tc ~ 100K, has generated tremendous excitement for two main reasons. First, from a practical point of view, these compounds open a new temperature realm for superconducting devices which may have interesting commercial applications, and these potential benefits have attracted extraordinary attention from the whole scientific community. The second reason, relevant to those in a more abstract field, is the interest in the microscopic mechanism driving superconductivity

### Valence-bond crystal in the extended kagome spin-1/2 quantum Heisenberg antiferromagnet: A variational Monte Carlo approach

The highly-frustrated spin-1/2 quantum Heisenberg model with both nearest
($J_1$) and next-nearest ($J_2$) neighbor exchange interactions is revisited by
using an extended variational space of projected wave functions that are
optimized with state-of-the-art methods. Competition between modulated
valence-bond crystals (VBCs) proposed in the literature and the Dirac spin
liquid (DSL) is investigated. We find that the addition of a {\it small}
ferromagnetic next-nearest-neighbor exchange coupling $|J_2|>0.09 J_1$ leads to
stabilization of a 36-site unit cell VBC, although the DSL remains a local
minimum of the variational parameter landscape. This implies that the VBC is
not trivially connected to the DSL: instead it possesses a non-trivial flux
pattern and large dimerization.Comment: 5 pages, 4 figure

### From magnetism to one-dimensional spin liquid in the anisotropic triangular lattice

We investigate the anisotropic triangular lattice that interpolates from
decoupled one-dimensional chains to the isotropic triangular lattice and has
been suggested to be relevant for various quasi-two-dimensional materials, such
as Cs$_2$CuCl$_4$ or $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$, an organic material that
shows intriguing magnetic properties. We obtain an excellent accuracy by means
of a novel representation for the resonating valence bond wave function with
both singlet and triplet pairing. This approach allows us to establish that the
magnetic order is rapidly destroyed away from the pure triangular lattice and
incommensurate spin correlations are short range. A non-magnetic spin liquid
naturally emerges in a wide range of the phase diagram, with strong
one-dimensional character. The relevance of the triplet pairing for
$\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ is also discussed.Comment: 4+epsilon pages, 6 figure

### Spin-$\frac{1}{2}$ Heisenberg $J_1$-$J_2$ antiferromagnet on the kagome lattice

We report variational Monte Carlo calculations for the spin-$\frac{1}{2}$
Heisenberg model on the kagome lattice in the presence of both nearest-neighbor
$J_1$ and next-nearest-neighbor $J_2$ antiferromagnetic superexchange
couplings. Our approach is based upon Gutzwiller projected fermionic states
that represent a flexible tool to describe quantum spin liquids with different
properties (e.g., gapless and gapped). We show that, on finite clusters, a
gapped $\mathbb{Z}_{2}$ spin liquid can be stabilized in the presence of a
finite $J_2$ superexchange, with a substantial energy gain with respect to the
gapless $U(1)$ Dirac spin liquid. However, this energy gain vanishes in the
thermodynamic limit, implying that, at least within this approach, the $U(1)$
Dirac spin liquid remains stable in a relatively large region of the phase
diagram. For $J_2/J_1 \gtrsim 0.3$, we find that a magnetically ordered state
with ${\bf q}={\bf 0}$ overcomes the magnetically disordered wave functions,
suggesting the end of the putative gapless spin-liquid phase.Comment: 6 pages, 4 figures. Published versio

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