284 research outputs found
Fermion pairing in mixed-dimensional atomic mixtures
We investigate the quantum phases of mixed-dimensional cold atom mixtures. In
particular, we consider a mixture of a Fermi gas in a two-dimensional lattice,
interacting with a bulk Fermi gas or a Bose-Einstein condensate in a
three-dimensional lattice. The effective interaction of the two-dimensional
system mediated by the bulk system is determined. We perform a functional
renormalization group analysis, and demonstrate that by tuning the properties
of the bulk system, a subtle competition of several superconducting orders can
be controlled among -wave, -wave, -wave, and
-wave pairing symmetries. Other instabilities such as a
charge-density wave order are also demonstrated to occur. In particular, we
find that the critical temperature of the -wave pairing induced by the
next-nearest-neighbor interactions can be an order of magnitude larger than
that of the same pairing induced by doping in the simple Hubbard model. We
expect that by combining the nearest-neighbor interaction with the
next-nearest-neighbor hopping (known to enhance -wave pairing), an even
higher critical temperature may be achieved.Comment: 10 pages, 10 figure
Localization and spectrum of quasiparticles in a disordered fermionic Dicke model
We study a fermionic two-band model with the interband transition resonantly
coupled to a cavity. This model was recently proposed to explain
cavity-enhanced charge transport, but a thorough characterization of the closed
system, in particular localization of various excitations, is lacking. In this
work, using exact diagonalization, we characterize the system by its spectrum
under various filling factors and variable disorder. As in the Dicke model, the
effective light-matter coupling scales with the square root of the system size.
However, there is an additional factor that decreases with increasing doping
density. The transition from the weak-coupling regime to the strong-coupling
regime occurs when the effective light-matter coupling is larger than the
electronic bandwidth. Here, the formation of exciton-polaritons is accompanied
by the formation of bound excitons. Photon spectral functions exhibit
significant weights on the in-gap states between the polaritons, even without
disorder. Finally, while the localization of electron-hole excitations in a
disordered system is lifted by strong coupling, the same is not true for free
charges, which remain localized at strong and even ultrastrong coupling. Based
on this finding, we discuss scenarios for charge transport
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Theoretical study of charge density waves in transition metal materials
In this thesis we theoretically study new aspects of charge density waves in transition metal materials recently revealed by scanning tunneling microscopy measurements. The two important problems that we have investigated are the effects of orbital degeneracy on the formation of the charge-density waves in cobalt nanowires, and the effects of dilute but strongly pinning impurities on the charge-density wave in niobium diselenide.
We first present an overview on charge-density waves, and then introduce a general theoretical model describing charge-density waves. We also explain several known results about disorder effects on charge-density waves. We briefly touch on the principle of scanning tunneling microscopy and its advantages compared to other experimental tools.
Second, we discuss the physics of one-dimensional cobalt nanowires along with experimental results. We propose a theoretical model that is relevant to cobalt nanowires, and then analyze the model by two theoretical tools: mean-field theory and bosonization. Our results show that the multi-orbitals allow a spin-triplet interaction among electrons leading to different phase diagrams from the ones considered previously for similar models. Numerical results obtained by first-principles calculations are also briefly explained.
Third, we consider the effects of dilute strong impurities on the charge-density wave in niobium diselenide, a transition metal dichalcogenide. We first explain the material and properties of its charge-density wave phase. Then, detailed analysis of a scanning tunneling microscopy measurement is presented. Next, we analytically and numerically study a phenomenological model relevant to the experiment. We show that the dilute strong impurities have little effects at large length scales compared to the average inter-impurity distance, leading to a topologically ordered phase with a (quasi-)long-range autocorrelation; this result is quite different from conventional pictures predicting short-range order with the proliferation of topological defects
Interaction-driven dynamical quantum phase transitions in a strongly correlated bosonic system
We study dynamical quantum phase transitions (DQPTs) in the extended
Bose-Hubbard model after a sudden quench of the nearest-neighbor interaction
strength. Using the time-dependent density matrix renormalization group, we
demonstrate that interaction-driven DQPTs can appear after quenches between two
topologically trivial insulating phases -- a phenomenon that has so far only
been studied between gapped and gapless phases. These DQPTs occur when the
interaction strength crosses a certain threshold value that does not coincide
with the equilibrium phase boundaries, which is in contrast to quenches that
involve a change of topology. In order to elucidate the nonequilibrium
excitations during the time evolution, we define a new set of string and parity
order parameters. We find a close connection between DQPTs and these newly
defined order parameters for both types of quenches. In the interaction-driven
case, the order parameter exhibits a singularity at the time of the DQPT only
when the quench parameter is close to the threshold value. Finally, the
timescales of DQPTs are scrutinized and different kinds of power laws are
revealed for the topological and interaction-driven cases.Comment: 6 pages, 4 figures, and supplemental materia
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