A dynamical mean-field theory study of stripe order and d-wave superconductivity in the two-dimensional Hubbard model

We use cellular dynamical mean-field theory with extended unit cells to study the ground state of the two-dimensional repulsive Hubbard model at finite doping. We calculate the energy of states with d-wave superconductivity coexisting with spatially uniform magnetic order and find that they are energetically favoured in a large doping region as compared to the uniform solution. We study the spatial form of the superconducting and magnetic order parameters at different doping values.Comment: 11 pages, 6 figure

Fulde-Ferrell--Larkin-Ovchinnikov state in the dimensional crossover between one- and three-dimensional lattices

We present a full phase diagram for the one-dimensional (1D) to three-dimensional (3D) crossover of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in an attractive Hubbard model of 3D-coupled chains in a har- monic trap. We employ real-space dynamical mean-field theory which describes full local quantum fluctuations beyond the usual mean-field and local density approximation. We find strong dimensionality effects on the shell structure undergoing a crossover between distinctive quasi-1D and quasi-3D regimes. We predict an optimal regime for the FFLO state that is considerably extended to intermediate interchain couplings and polarizations, directly realizable with ultracold atomic gases. We find that the 1D-like FFLO feature is vulnerable to thermal fluctuations, while the FFLO state of mixed 1D-3D character can be stabilized at a higher temperature

Spin-Imbalanced Fermi Superfluidity in a Lieb Lattice Hubbard Model

We obtain a phase diagram of the spin imbalanced Hubbard model on the Lieb lattice, which is known to feature a flat band in its single-particle spectrum. Using the BCS mean-field theory for multiband systems, we find a variety of superfluid phases with imbalance. In particular, we find four different types FFLO phases, i.e. superfluid phases with periodic spatial modulation. They differ by the magnitude and direction of the centre-of-mass momentum of Cooper pairs. We also see a large region of stable Sarma phase, where the density imbalance is associated with zero Cooper pair momentum. In the mechanism responsible for the formation of those phases, the crucial role is played by the flat band, wherein particles can readjust their density at zero energy cost. The multiorbital structure of the unit cell is found to stabilize the Sarma phase by allowing for a modulation of the order parameter within a unit cell. We also study the effect of finite temperature and a lattice with staggered hopping parameters on the behaviour of these phases.Comment: 10 pages, 10 figure

Probing the FFLO phase by double occupancy modulation spectroscopy

We propose here that for a spin-imbalanced two-component attractive Fermi gas loaded in a 1D optical lattice in presence of an harmonic confining potential, the observation of the change in the double occupancy after a lattice depth modulation can provide clear evidence of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. Simulating the time evolution of the system, we can characterize the double occupancy spectrum for different initial conditions, relating its features to the FFLO wavevector $q$. In particular, the narrowing of the width of the spectrum can be related, through Bethe-ansatz equations in the strongly interacting limit, to the FFLO wavevector $q$.Comment: 4 pages, 6 figure

Pienuus, kylmyys ja nopeus tieteessä - kvanttitietokoneista atomilasereihin

Mihin tiede on menossa? Tämä kuulostaa vaikeasti vastattavalta kysymykseltä. Mutta itse asiassa tieteen ja teknologian kehityksessä on hyvin helposti nähtävissä tiettyjä suuntia: ollaan menossa kohti pienuutta, kylmyyttä ja nopeutta. Pienuuden saavutukset ovat tuttuja jokaiselle: esimerkiksi yhä pienemmät integroidut piirit, jotka antavat meille yhä nopeampia tietokoneita. Nopeus ja pienuus liittyvätkin usein yhteen. Sen sijaan kylmyyden saavutukset ovat vähemmän jokapäiväisiä: jäähdyttämällä ainetta muutamien asteiden tai asteiden osien päähän absoluuttisesta nollapisteestä on päästy näkemään kvanttimekaniikan ilmiöitä, joita on ennustettu jo miltei sata vuotta sitten, mutta joiden havaitseminen on mahdotonta korkeammissa lämpötiloissa

Flat band induced non-Fermi liquid behavior of multicomponent fermions

We investigate multicomponent fermions in a flat band and predict experimental signatures of non-Fermi liquid behavior. We use dynamical mean-field theory to obtain the density, double occupancy and entropy in a Lieb lattice for $\mathcal{N} = 2$ and $\mathcal{N} = 4$ components. We derive a mean-field scaling relation between the results for different values of $\mathcal{N}$, and study its breakdown due to beyond-mean field effects. The predicted signatures occur at temperatures above the N\'eel temperature and persist in presence of a harmonic trapping potential, thus they are observable with current ultracold gas experiments.Comment: 6 pages, 5 figures and and a supplementary materia

Selkeä teos kvanttitietokoneista

Julian Brown: Kvanttitietokone. Suomentanut Kimmo Pietiläinen. Terra Cognita2001

Superfluid weight and Berezinskii-Kosterlitz-Thouless temperature of spin-imbalanced and spin-orbit-coupled Fulde-Ferrell phases in lattice systems

We study the superfluid weight $D^s$ and Berezinskii-Kosterlitz-Thouless (BKT) transition temperatures $T_{BKT}$ in case of exotic Fulde-Ferrell (FF) superfluid states in lattice systems. We consider spin-imbalanced systems with and without spin-orbit coupling (SOC) accompanied with in-plane Zeeman field. By applying mean-field theory, we derive general equations for $D^s$ and $T_{BKT}$ in the presence of SOC and the Zeeman fields for 2D Fermi-Hubbard lattice models, and apply our results to a 2D square lattice. We show that conventional spin-imbalanced FF states without SOC can be observed at finite temperatures and that FF phases are further stabilized against thermal fluctuations by introducing SOC. We also propose how topologically non-trivial SOC-induced FF phases could be identified experimentally by studying the total density profiles. Furthermore, the relative behavior of transverse and longitudinal superfluid weight components and the role of the geometric superfluid contribution are discussed.Comment: 35 pages, 7 figure