1,423 research outputs found
On The Violation Of Marshall-Peierls Sign Rule In The Frustrated Heisenberg Antiferromagnet
We present a number of arguments in favor of the suggestion that the
Marshall-Peierls sign rule survives the frustration in the square-lattice
Heisenberg antiferromagnet with frustrating next-nearest-neighbor (diagonal)
bonds ( model) for relatively large values of the parameter
. Both the spin-wave analysis and the exact-diagonalization data
concerning the weight of Marshall states support the above suggestion.Comment: 8 pages, LaTex, 2 figurs on reques
Cooling a micro-mechanical resonator by quantum back-action from a noisy qubit
We study the role of qubit dephasing in cooling a mechanical resonator by
quantum back-action. With a superconducting flux qubit as a specific example,
we show that ground-state cooling of a mechanical resonator can only be
realized if the qubit dephasing rate is sufficiently low.Comment: 5 pages, 3 figure
Improved position measurement of nano electromechanical systems using cross correlations
We consider position measurements using the cross-correlated output of two
tunnel junction position detectors. Using a fully quantum treatment, we
calculate the equation of motion for the density matrix of the coupled
detector-detector-mechanical oscillator system. After discussing the presence
of a bound on the peak-to-background ratio in a position measurement using a
single detector, we show how one can use detector cross correlations to
overcome this bound. We analyze two different possible experimental
realizations of the cross correlation measurement and show that in both cases
the maximum cross-correlated output is obtained when using twin detectors and
applying equal bias to each tunnel junction. Furthermore, we show how the
double-detector setup can be exploited to drastically reduce the added
displacement noise of the oscillator.Comment: 9 pages, 1 figure; v2: new Sec.
Electrical transport through a single-electron transistor strongly coupled to an oscillator
We investigate electrical transport through a single-electron transistor
coupled to a nanomechanical oscillator. Using a combination of a
master-equation approach and a numerical Monte Carlo method, we calculate the
average current and the current noise in the strong-coupling regime, studying
deviations from previously derived analytic results valid in the limit of
weak-coupling. After generalizing the weak-coupling theory to enable the
calculation of higher cumulants of the current, we use our numerical approach
to study how the third cumulant is affected in the strong-coupling regime. In
this case, we find an interesting crossover between a weak-coupling transport
regime where the third cumulant heavily depends on the frequency of the
oscillator to one where it becomes practically independent of this parameter.
Finally, we study the spectrum of the transport noise and show that the two
peaks found in the weak-coupling limit merge on increasing the coupling
strength. Our calculation of the frequency-dependence of the noise also allows
to describe how transport-induced damping of the mechanical oscillations is
affected in the strong-coupling regime.Comment: 11 pages, 9 figure
Bose-Einstein condensation in inhomogeneous Josephson arrays
We show that spatial Bose-Einstein condensation of non-interacting bosons
occurs in dimension d < 2 over discrete structures with inhomogeneous topology
and with no need of external confining potentials. Josephson junction arrays
provide a physical realization of this mechanism. The topological origin of the
phenomenon may open the way to the engineering of quantum devices based on
Bose-Einstein condensation. The comb array, which embodies all the relevant
features of this effect, is studied in detail.Comment: 4 pages, 5 figure
Paramagnetic reentrant effect in high purity mesoscopic AgNb proximity structures
We discuss the magnetic response of clean Ag coated Nb proximity cylinders in
the temperature range 150 \mu K < T < 9 K. In the mesoscopic temperature
regime, the normal metal-superconductor system shows the yet unexplained
paramagnetic reentrant effect, discovered some years ago [P. Visani, A. C.
Mota, and A. Pollini, Phys. Rev. Lett. 65, 1514 (1990)], superimposing on full
Meissner screening. The logarithmic slope of the reentrant paramagnetic
susceptibility chi_para(T) \propto \exp(-L/\xi_N) is limited by the condition
\xi_N=n L, with \xi_N=\hbar v_F/2 \pi k_B T, the thermal coherence length and
n=1,2,4. In wires with perimeters L=72 \mu m and L=130 \mu m, we observe
integer multiples n=1,2,4. At the lowest temperatures, \chi_para compensates
the diamagnetic susceptibility of the \textit{whole} AgNb structure.Comment: 4 pages, 4 figures (color
Magnetization process of the spin-1/2 XXZ models on square and cubic lattices
The magnetization process of the spin-1/2 antiferromagnetic XXZ model with
Ising-like anisotropy in the ground state is investigated. We show numerically
that the Ising-like XXZ models on square and cubic lattices show a first-order
phase transition at some critical magnetic field. We estimate the value of the
critical field and the magnetization jump on the basis of the Maxwell
construction. The magnetization jump in the Ising-limit is investigated by
means of perturbation theory. Based on our numerical results, we briefly
discuss the phase diagram of the extended Bose-Hubbard model in the hard-core
limit.Comment: 13 pages, RevTex, 7 PostScript figures, to appear in Phys.Rev.
Topology, Hidden Spectra and Bose Einstein Condensation on low dimensional complex networks
Topological inhomogeneity gives rise to spectral anomalies that can induce
Bose-Einstein Condensation (BEC) in low dimensional systems. These anomalies
consist in energy regions composed of an infinite number of states with
vanishing weight in the thermodynamic limit (hidden states). Here we present a
rigorous result giving the most general conditions for BEC on complex networks.
We prove that the presence of hidden states in the lowest region of the
spectrum is the necessary and sufficient condition for condensation in low
dimension (spectral dimension ), while it is shown that BEC
always occurs for .Comment: 4 pages, 10 figure
On the Coexistence of Diagonal and off-Diagonal Long-Range Order, a Monte Carlo Study
The zero temperature properties of interacting 2 dimensional lattice bosons
are investigated. We present Monte Carlo data for soft-core bosons that
demonstrate the existence of a phase in which crystalline long-range order and
off-diagonal long-range order (superfluidity) coexist. We comment on the
difference between hard and soft-core bosons and compare our data to mean-field
results that predict a larger coexistence region. Furthermore, we determine the
critical exponents for the various phase transitions.Comment: 7 pages and 8 figures appended in postscript, KA-TFP-93-0
- …