63 research outputs found
Thermodynamics of the Quantum Critical Point at Finite Doping in the 2D Hubbard Model: A Dynamical Cluster Approximation Study
We study the thermodynamics of the two-dimensional Hubbard model within the
dynamical cluster approximation. We use continuous time quantum Monte Carlo as
a cluster solver to avoid the systematic error which complicates the
calculation of the entropy and potential energy (double occupancy). We find
that at a critical filling, there is a pronounced peak in the entropy divided
by temperature, S/T, and in the normalized double occupancy as a function of
doping. At this filling, we find that specific heat divided by temperature,
C/T, increases strongly with decreasing temperature and kinetic and potential
energies vary like T^2 ln(T). These are all characteristics of quantum critical
behavior.Comment: 4 pages, 4 figures. Submitted to Phys. Rev. B Rapid Communications on
June 27, 200
Quantum Criticality and Incipient Phase Separation in the Thermodynamic Properties of the Hubbard Model
Transport measurements on the cuprates suggest the presence of a quantum
critical point hiding underneath the superconducting dome near optimal hole
doping. We provide numerical evidence in support of this scenario via a
dynamical cluster quantum Monte Carlo study of the extended two-dimensional
Hubbard model. Single particle quantities, such as the spectral function, the
quasiparticle weight and the entropy, display a crossover between two distinct
ground states: a Fermi liquid at low filling and a non-Fermi liquid with a
pseudogap at high filling. Both states are found to cross over to a marginal
Fermi-liquid state at higher temperatures. For finite next-nearest-neighbor
hopping t' we find a classical critical point at temperature T_c. This
classical critical point is found to be associated with a phase separation
transition between a compressible Mott gas and an incompressible Mott liquid
corresponding to the Fermi liquid and the pseudogap state, respectively. Since
the critical temperature T_c extrapolates to zero as t' vanishes, we conclude
that a quantum critical point connects the Fermi-liquid to the pseudogap
region, and that the marginal-Fermi-liquid behavior in its vicinity is the
analogous of the supercritical region in the liquid-gas transition.Comment: 18 pages, 9 figure
Role of the van Hove Singularity in the Quantum Criticality of the Hubbard Model
A quantum critical point (QCP), separating the non-Fermi liquid region from
the Fermi liquid, exists in the phase diagram of the 2D Hubbard model
[Vidhyadhiraja et. al, Phys. Rev. Lett. 102, 206407 (2009)]. Due to the
vanishing of the critical temperature associated with a phase separation
transition, the QCP is characterized by a vanishing quasiparticle weight. Near
the QCP, the pairing is enhanced since the real part of the bare d-wave p-p
susceptibility exhibits algebraic divergence with decreasing temperature,
replacing the logarithmic divergence found in a Fermi liquid [Yang et. al,
Phys. Rev. Lett. 106, 047004 (2011)]. In this paper we explore the
single-particle and transport properties near the QCP. We focus mainly on a van
Hove singularity (vHS) coming from the relatively flat dispersion that crosses
the Fermi level near the quantum critical filling. The flat part of the
dispersion orthogonal to the antinodal direction remains pinned near the Fermi
level for a range of doping that increases when we include a negative
next-near-neighbor hopping t' in the model. For comparison, we calculate the
bare d-wave pairing susceptibility for non-interacting models with the usual
two-dimensional tight binding dispersion and a hypothetical quartic dispersion.
We find that neither model yields a vHS that completely describes the critical
algebraic behavior of the bare d-wave pairing susceptibility. The resistivity,
thermal conductivity, thermopower, and the Wiedemann-Franz Law are examined in
the Fermi liquid, marginal Fermi liquid, and pseudo-gap doping regions. A
negative next-near-neighbor hopping t' increases the doping region with
marginal Fermi liquid character. Both T and negative t' are relevant variables
for the QCP, and both the transport and the motion of the vHS with filling
suggest that they are qualitatively similar in their effect.Comment: 15 pages, 17 figure
Multi-qubit gate with trapped ions for microwave and laser-based implementation
A proposal for a phase gate and a Mølmer–Sørensen gate in the dressed state basis is presented. In order to perform the multi-qubit interaction, a strong magnetic field gradient is required to couple the phonon-bus to the qubit states. The gate is performed using resonant microwave driving fields together with either a radio-frequency (RF) driving field, or additional detuned microwave driving fields. The gate is robust to ambient magnetic field fluctuations due to an applied resonant microwave driving field. Furthermore, the gate is robust to fluctuations in the microwave Rabi frequency and is decoupled from phonon dephasing due to a resonant RF or a detuned microwave driving field. This makes this new gate an attractive candidate for the implementation of high-fidelity microwave based multi-qubit gates. The proposal can also be realized in laser-based set-ups
Search for Light Gluinos via the Spontaneous Appearance of pi+pi- Pairs with an 800 GeV/c Proton Beam at Fermilab
We searched for the appearance of pi+pi- pairs with invariant mass greater
than 648 MeV in a neutral beam. Such an observation could signify the decay of
a long-lived light neutral particle. We find no evidence for this decay. Our
null result severely constrains the existence of an R0 hadron, which is the
lightest bound state of a gluon and a light gluino, and thereby also the
possibility of a light gluino. Depending on the photino mass, we exclude the R0
in the mass and lifetime ranges of 1.2 -- 4.6 GeV and 2E-10 -- 7E-4 seconds,
respectively. (To Appear in Phys. Rev. Lett.)Comment: Documentstyle aps,epsfig,prl (revtex), 6 pages, 7 figure
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