66 research outputs found
Giant mass and anomalous mobility of particles in fermionic systems
We calculate the mobility of a heavy particle coupled to a Fermi sea within a
non-perturbative approach valid at all temperatures. The interplay of particle
recoil and of strong coupling effects, leading to the orthogonality catastrophe
for an infinitely heavy particle, is carefully taken into account. We find two
novel types of strong coupling effects: a new low energy scale and
a giant mass renormalization in the case of either near-resonant scattering or
a large transport cross section . The mobility is shown to obey two
different power laws below and above . For ,
where is the Fermi wave length, an exponentially large effective
mass suppresses the mobility.Comment: 4 pages, 4 figure
Mechanisms of decoherence in weakly anisotropic molecular magnets
Decoherence mechanisms in crystals of weakly anisotropic magnetic molecules,
such as V15, are studied. We show that an important decohering factor is the
rapid thermal fluctuation of dipolar interactions between magnetic molecules. A
model is proposed to describe the influence of this source of decoherence.
Based on the exact solution of this model, we show that at relatively high
temperatures, about 0.5 K, the quantum coherence in a V15 molecule is not
suppressed, and, in principle, can be detected experimentally. Therefore, these
molecules may be suitable prototype systems for study of physical processes
taking place in quantum computers.Comment: 4 pages RevTeX, 1 figure (PostScript
Quantum Relaxation of Magnetisation in Magnetic Particles
At temperatures below the magnetic anisotropy energy, monodomain magnetic
systems (small particles, nanomagnetic devices, etc.) must relax quantum
mechanically. This quantum relaxation must be mediated by the coupling to both
nuclear spins and phonons (and electrons if either particle or substrate is
conducting. We analyze the effect of each of these couplings, and then combine
them. Conducting systems can be modelled by a "giant Kondo" Hamiltonian, with
nuclear spins added in as well. At low temperatures, even microscopic particles
on a conducting substrate (containing only spins) will have their
magnetisation frozen over millenia by a combination of electronic dissipation
and the "degeneracy blocking" caused by nuclear spins. Raising the temperature
leads to a sudden unblocking of the spin dynamics at a well defined
temperature. Insulating systems are quite different. The relaxation is strongly
enhanced by the coupling to nuclear spins. At short times the magnetisation of
an ensemble of particles relaxes logarithmically in time, after an initial very
fast decay; this relaxation proceeds entirely via the nuclear spins. At longer
times phonons take over, but the decay rate is still governed by the
temperature-dependent nuclear bias field acting on the particles - decay may be
exponential or power-law depending on the temperature. The most surprising
feature of the results is the pivotal role played by the nuclear spins. The
results are relevant to any experiments on magnetic particles in which
interparticle dipolar interactions are unimportant. They are also relevant to
future magnetic device technology.Comment: 30 pages, RevTex, e:mail , Submitted to J.Low
Temp.Phys. on 1 Nov. 199
Friedel oscillations for interacting fermions in one dimension
We study Friedel oscillations in one-dimensional electron liquid for
arbitrary electron-electron interaction and arbitrary impurity strength. For
Luttinger liquid leads, the Friedel oscillations decay as x^-g far away from
the impurity, where g is the interaction constant. For a weak scatterer, a
slower decay is found at intermediate distances from the impurity, with a
crossover to the asymptotic x^-g law.Comment: 4 pages REVTeX, includes two figure
Bosons in optical lattices - from the Mott transition to the Tonks-Girardeau gas
We present results from quantum Monte Carlo simulations of trapped bosons in
optical lattices, focusing on the crossover from a gas of softcore bosons to a
Tonks-Girardeau gas in a one-dimensional optical lattice. We find that
depending on the quantity being measured, the behavior found in the
Tonks-Girardeau regime is observed already at relatively small values of the
interaction strength. A finite critical value for entering the Tonks-Girardeau
regime does not exist. Furthermore, we discuss the computational efficiency of
two quantum Monte Carlo methods to simulate large scale trapped bosonic
systems: directed loops in stochastic series expansions and the worm algorithm.Comment: 7 pages with 9 figures;v2: improved discussion on Tonks-Girardeau ga
Macroscopic Quantum Coherence in a Magnetic Nanoparticle Above the Surface of a Superconductor
We study macroscopic quantum tunneling of the magnetic moment in a
single-domain particle placed above the surface of a superconductor. Such a
setup allows one to manipulate the height of the energy barrier, preserving the
degeneracy of the ground state. The tunneling amplitude and the effect of the
dissipation in the superconductor are computed.Comment: RevTeX, 4 pages, 1 figure. Submitted to Phys. Rev. Let
Exact Critical Properties of the Multi-Component Interacting Fermion Model with Boundaries
Exact critical properties of the one-dimensional SU() interacting fermion
model with open boundaries are studied by using the Bethe ansatz method. We
derive the surface critical exponents of various correlation functions using
boundary conformal field theory. They are classified into two types, i.e. the
exponents for the chiral SU() Tomonaga-Luttinger liquid and those related to
the orthogonality catastrophe. We discuss a possible application of the results
to the photoemission (absorption) in the edge state of the fractional quantum
Hall effect.Comment: 17 pages, RevTe
Kondo Problems in Tomonaga-Luttinger liquids
Quantum impurity problems in Tomonaga-Luttinger liquids (TLLs) are reviewed
with emphasis on their analogy to the Kondo problem in Fermi liquids. First,
the problem of a static impurity in a spinless TLL is considered, which is
related to the model studied in the context of the macroscopic quantum
coherence. In the low-energy limit the TLL is essentially cut into two pieces
when interaction is repulsive. The orthogonality catastrophe in a TLL is then
discussed. Finally, the Kondo effect of a spin-1/2 impurity in a
one-dimensional repulsively interacting electron liquids (a spinful TLL) is
reviewed. Regardless of the sign of the exchange coupling, the impury spin is
completely screened in the ground state. The leading low-temperature
contributions to thermodynamic quantities come from boundary contributions of a
bulk leading irrelevant operator.Comment: 7 pages, submitted to a special edition of JPSJ "Kondo Effect -- 40
Years after the Discovery"; corrected typos, added reference
Suppression of decoherence via strong intra-environmental coupling
We examine the effects of intra-environmental coupling on decoherence by
constructing a low temperature spin--spin-bath model of an atomic impurity in a
Debye crystal. The impurity interacts with phonons of the crystal through
anti-ferromagnetic spin-spin interactions. The reduced density matrix of the
central spin representing the impurity is calculated by dynamically integrating
the full Schroedinger equation for the spin--spin-bath model for different
thermally weighted eigenstates of the spin-bath. Exact numerical results show
that increasing the intra-environmental coupling results in suppression of
decoherence. This effect could play an important role in the construction of
solid state quantum devices such as quantum computers.Comment: 4 pages, 3 figures, Revtex fil
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