581 research outputs found
Entanglement scaling of excited states in large one-dimensional many-body localized systems
We study the properties of excited states in one-dimensional many-body
localized (MBL) systems using a matrix product state algorithm. First, the
method is tested for a large disordered non-interacting system, where for
comparison we compute a quasi-exact reference solution via a Monte Carlo
sampling of the single-particle levels. Thereafter, we present extensive data
obtained for large interacting systems of L~100 sites and large bond dimensions
chi~1700, which allows us to quantitatively analyze the scaling behavior of the
entanglement S in the system. The MBL phase is characterized by a logarithmic
growth (L)~log(L) over a large scale separating the regimes where volume and
area laws hold. We check the validity of the eigenstate thermalization
hypothesis. Our results are consistent with the existence of a mobility edge
Thermal conductivity of the one-dimensional Fermi-Hubbard model
We study the thermal conductivity of the one-dimensional Fermi-Hubbard model
at finite temperature using a density matrix renormalization group approach.
The integrability of this model gives rise to ballistic thermal transport. We
calculate the temperature dependence of the thermal Drude weight at half
filling for various interactions and moreover, we compute its filling
dependence at infinite temperature. The finite-frequency contributions
originating from the fact that the energy current is not a conserved quantity
are investigated as well. We report evidence that breaking the integrability
through a nearest-neighbor interaction leads to vanishing Drude weights and
diffusive energy transport. Moreover, we demonstrate that energy spreads
ballistically in local quenches with initially inhomogeneous energy density
profiles in the integrable case. We discuss the relevance of our results for
thermalization in ultra-cold quantum gas experiments and for transport
measurements with quasi-one dimensional materials
Transport properties of the one-dimensional Hubbard model at finite temperature
We study finite-temperature transport properties of the one-dimensional
Hubbard model using the density matrix renormalization group. Our aim is
two-fold: First, we compute both the charge and the spin current correlation
function of the integrable model at half filling. The former decays rapidly,
implying that the corresponding Drude weight is either zero or very small.
Second, we calculate the optical charge conductivity sigma(omega) in presence
of small integrability-breaking next-nearest neighbor interactions (the
extended Hubbard model). The DC conductivity is finite and diverges as the
temperature is decreased below the gap. Our results thus suggest that the
half-filled, gapped Hubbard model is a normal charge conductor at finite
temperatures. As a testbed for our numerics, we compute sigma(omega) for the
integrable XXZ spin chain in its gapped phase
Probing electron-electron interaction in quantum Hall systems with scanning tunneling spectroscopy
Using low-temperature scanning tunneling spectroscopy applied to the
Cs-induced two-dimensional electron system (2DES) on p-type InSb(110), we probe
electron-electron interaction effects in the quantum Hall regime. The 2DES is
decoupled from p-doped bulk states and exhibits spreading resistance within the
insulating quantum Hall phases. In quantitative agreement with calculations we
find an exchange enhancement of the spin splitting. Moreover, we observe that
both the spatially averaged as well as the local density of states feature a
characteristic Coulomb gap at the Fermi level. These results show that
electron-electron interaction effects can be probed down to a resolution below
all relevant length scales.Comment: supplementary movie in ancillary file
Transport in quasiperiodic interacting systems: from superdiffusion to subdiffusion
Using a combination of numerically exact and renormalization-group techniques
we study the nonequilibrium transport of electrons in an one-dimensional
interacting system subject to a quasiperiodic potential. For this purpose we
calculate the growth of the mean-square displacement as well as the melting of
domain walls. While the system is nonintegrable for all studied parameters,
there is no on finite region default of parameters for which we observe
diffusive transport. In particular, our model shows a rich dynamical behavior
crossing over from superdiffusion to subdiffusion. We discuss the implications
of our results for the general problem of many-body localization, with a
particular emphasis on the rare region Griffiths picture of subdiffusion.Comment: 6 pages, 5 figures. A more detailed analysis of the dynamical
exponents extraction and discussion of the relevant times. Adds a
log-derivative for the FRG sectio
Tuning the Josephson current in carbon nanotubes with the Kondo effect
We investigate the Josephson current in a single wall carbon nanotube
connected to superconducting electrodes. We focus on the parameter regime in
which transport is dominated by Kondo physics. A sizeable supercurrent is
observed for odd number of electrons on the nanotube when the Kondo temperature
Tk is sufficiently large compared to the superconducting gap. On the other hand
when, in the center of the Kondo ridge, Tk is slightly smaller than the
superconducting gap, the supercurrent is found to be extremely sensitive to the
gate voltage Vbg. Whereas it is largely suppressed at the center of the ridge,
it shows a sharp increase at a finite value of Vbg. This increase can be
attributed to a doublet-singlet transition of the spin state of the nanotube
island leading to a pi shift in the current phase relation. This transition is
very sensitive to the asymmetry of the contacts and is in good agreement with
theoretical predictions.Comment: 5 pages, 4 figure
Magnetic properties of a capped kagome molecule with 60 quantum spins
We compute ground-state properties of the isotropic, antiferromagnetic
Heisenberg model on the sodalite cage geometry. This is a 60-spin spherical
molecule with 24 vertex-sharing tetrahedra which can be regarded as a molecular
analogue of a capped kagome lattice and which has been synthesized with
high-spin rare-earth atoms. Here, we focus on the case where quantum
effects are strongest. We employ the SU(2)-symmetric density-matrix
renormalization group (DMRG).
We find a threefold degenerate ground state that breaks the spatial symmetry
and that splits up the molecule into three large parts which are almost
decoupled from each other. This stands in sharp contrast to the behaviour of
most known spherical molecules. On a methodological level, the disconnection
leads to "glassy dynamics" within the DMRG that cannot be targeted via standard
techniques.
In the presence of finite magnetic fields, we find broad magnetization
plateaus at 4/5, 3/5, and 1/5 of the saturation, which one can understand in
terms of localized magnons, singlets, and doublets which are again nearly
decoupled from each other. At the saturation field, the zero-point entropy is
in units of the Boltzmann constant
Spin and thermal conductivity of quantum spin ladders
We study the spin and thermal conductivity of spin-1/2 ladders at finite
temperature. This is relevant for experiments with quantum magnets. Using a
state-of-the-art density matrix renormalization group algorithm, we compute the
current autocorrelation functions on the real-time axis and then carry out a
Fourier integral to extract the frequency dependence of the corresponding
conductivities. The finite-time error is analyzed carefully. We first
investigate the limiting case of spin-1/2 XXZ chains, for which our analysis
suggests non-zero dc-conductivities in all interacting cases irrespective of
the presence or absence of spin Drude weights. For ladders, we observe that all
models studied are normal conductors with no ballistic contribution.
Nonetheless, only the high-temperature spin conductivity of XX ladders has a
simple diffusive, Drude-like form, while Heisenberg ladders exhibit a more
complicated low-frequency behavior. We compute the dc spin conductivity down to
temperatures of the order of T~0.5J, where J is the exchange coupling along the
legs of the ladder. We further extract mean-free paths and discuss our results
in relation to thermal conductivity measurements on quantum magnets
Many-body localization and the area law in two dimensions
We study the high-energy phase diagram of a two-dimensional
spin- Heisenberg model on a square lattice in the presence of
disorder. The use of large-scale tensor network numerics allows us to compute
the bi-partite entanglement entropy for systems of up to lattice
sites. We demonstrate the existence of a finite many-body localized phase for
large disorder strength for which the eigenstate thermalization hypothesis
is violated. Moreover, we show explicitly that the area law holds for excited
states in this phase and determine an estimate for the critical
where the transition to the ergodic phase occurs.Comment: 5 pages, 5 figure
- …