arXiv subjects

Päivi Törmä

Publications and source records attributed to Päivi Törmä.

At least 73 records · Page 4Linked to original sources

arXiv2019

Lasing in Ferromagnetic Plasmonic Arrays

We report on lasing at visible wavelengths in arrays of ferromagnetic Ni nanodisks overlaid with an organic gain medium. We demonstrate that by placing an organic gain material within the mode volume of the plasmonic nanoparticles both the radiative and, in particular, the high ohmic losses of Ni nanodisk resonances can be compensated. Under increasing pump fluence, the systems exhibit a transition from lattice-modified spontaneous emission to lasing, the latter being characterized by highly directional and sub-nanometer linewidth emission. By breaking the symmetry of the array, we observe tunable multimode lasing at two wavelengths corresponding to the particle periodicity along the two principal directions of the lattice. Our results pave the way for loss-compensated magnetoplasmonic devices and topological photonics.

physics.optics↗

arXiv2018

Bose-Einstein Condensation in a Plasmonic Lattice

Bose-Einstein condensation is a remarkable manifestation of quantum statistics and macroscopic quantum coherence. Superconductivity and superfluidity have their origin in Bose-Einstein condensation. Ultracold quantum gases have provided condensates close to the original ideas of Bose and Einstein, while condensation of polaritons and magnons have introduced novel concepts of non-equilibrium condensation. Here, we demonstrate a Bose-Einstein condensate (BEC) of surface plasmon polaritons in lattice modes of a metal nanoparticle array. Interaction of the nanoscale-confined surface plasmons with a room-temperature bath of dye molecules enables thermalization and condensation in picoseconds. The ultrafast thermalization and condensation dynamics are revealed by an experiment that exploits thermalization under propagation and the open cavity character of the system. A crossover from BEC to usual lasing is realized by tailoring the band structure. This new condensate of surface plasmon lattice excitations has promise for future technologies due to its ultrafast, room-temperature and on-chip nature.

cond-mat.quant-gas↗

arXiv2018

Lasing at the K-points of a honeycomb plasmonic lattice

We study lasing at the high-symmetry points of the Brillouin zone in a honeycomb plasmonic lattice. We use symmetry arguments to define singlet and doublet modes at the $\Kp$-points of the reciprocal space. We experimentally demonstrate lasing at the $\Kp$-points \changed{that is based on plasmonic lattice modes and two-dimensional feedback}. By comparing polarization properties to \changed{$T$-matrix} simulations, we identify the lasing mode as one of the singlets with an energy minimum at the $\Kp$-point enabling feedback. Our results offer prospects for studies of topological lasing in radiatively coupled systems.

cond-mat.mes-hall↗

arXiv2018

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.

cond-mat.quant-gas↗

arXiv2018

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.

cond-mat.quant-gas↗

arXiv2018

Spin-Imbalanced Pairing and Fermi Surface Deformation in Flat Bands

We study the attractive Hubbard model with spin imbalance on two lattices featuring a flat band: the Lieb and kagome lattices. We present mean-field phase diagrams featuring exotic superfluid phases, similar to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, whose stability is confirmed by dynamical mean-field theory (DMFT). The nature of the pairing is found to be richer than just the Fermi surface shift responsible for the usual FFLO state. The presence of a flat band allows for changes in the particle momentum distributions at null energy cost. This facilitates formation of nontrivial superfluid phases via multiband Cooper pair formation: the momentum distribution of the spin component in the flat band deforms to mimic the Fermi surface of the other spin component residing in a dispersive band. The Fermi surface of the unpaired particles that are typical for gapless superfluids becomes deformed as well. The results highlight the profound effect of flat dispersions on Fermi surface instabilities, and provide a potential route for observing spin-imbalanced superfluidity and superconductivity.

cond-mat.quant-gas↗

arXiv2018

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.

cond-mat.str-el↗

arXiv2017

Temperature and doping induced instabilities of the repulsive Hubbard model on Lieb lattice

The properties of a phase at finite interactions can be significantly influenced by the underlying dispersion of the non-interacting Hamiltonian. We demonstrate this by studying the repulsive Hubbard model on the $2$D Lieb lattice, which has a flat band for vanishing interaction $U$. We perform real-space dynamical mean-field theory calculations at different temperatures and dopings using a continuous time quantum Monte Carlo impurity solver. Studying the frequency dependence of the self-energy, we show that a finite temperature non-magnetic non-Fermi liquid behavior is a concomitant of the flat band singularity. At half-filling we also find a magnetically ordered region, where the order parameter varies linearly with the interaction strength, and a strongly correlated Mott insulating phase. The double occupancy decreases sharply for small $U$, highlighting the flat band contribution. Away from half-filling, we observe the stripe order, i.e. an inhomogeneous spin and charge density wave of finite wavelength which turns into a sub-lattice ordering at higher temperatures.

cond-mat.str-el↗

arXiv2017

Plasmon-induced demagnetization and magnetic switching in nickel nanoparticle arrays

We report on the manipulation of magnetization by femtosecond laser pulses in a periodic array of cylindrical nickel nanoparticles. By performing experiments at different wavelength, we show that the excitation of collective surface plasmon resonances triggers demagnetization in zero field or magnetic switching in a small perpendicular field. Both magnetic effects are explained by plasmon-induced heating of the nickel nanoparticles to their Curie temperature. Model calculations confirm the strong correlation between the excitation of surface plasmon modes and laser-induced changes in magnetization.

cond-mat.mtrl-sci↗

arXiv2017

Wave packet dynamics of Bogoliubov quasiparticles: quantum metric effects

We study the dynamics of the Bogoliubov wave packet in superconductors and calculate the supercurrent carried by the wave packet. We discover an anomalous contribution to the supercurrent, related to the quantum metric of the Bloch wave function. This anomalous contribution is most important for flat or quasiflat bands, as exemplified by the attractive Hubbard models on the Creutz ladder and sawtooth lattice. Our theoretical framework is general and can be used to study a wide variety of phenomena, such as spin transport and exciton transport.

cond-mat.supr-con↗

arXiv2017

Band geometry, Berry curvature and superfluid weight

We present a theory of the superfluid weight in multiband attractive Hubbard models within the Bardeen-Cooper-Schrieffer (BCS) mean field framework. We show how to separate the geometric contribution to the superfluid weight from the conventional one, and that the geometric contribution is associated with the interband matrix elements of the current operator. Our theory can be applied to systems with or without time reversal symmetry. In both cases the geometric superfluid weight can be related to the quantum metric of the corresponding noninteracting systems. This leads to a lower bound on the superfluid weight given by the absolute value of the Berry curvature. We apply our theory to the attractive Kane-Mele-Hubbard and Haldane-Hubbard models, which can be realized in ultracold atom gases. Quantitative comparisons are made to state of the art dynamical mean-field theory and exact diagonalization results.

cond-mat.supr-con↗

arXiv2016

Quantum emitter dipole-dipole interactions in nanoplasmonic systems

We introduce a generalized Dicke-like model to describe two-level systems coupled with a single bosonic mode. In addition, the two-level systems mutually interact via direct dipole-dipole interaction. We apply the model to an ensemble of dye molecules coupled to a plasmonic excitation in a metallic nanoparticle and study how the dipole-dipole interaction and configurational randomness introduced to the system affect the energy spectra. Comparing the system eigenenergies obtained by our model with the light spectra from a multiple-scattering simulation, we suggest a way to identify dark modes in our model. Finally, we perform a parameter sweep in order to determine the scaling properties of the system and to classify the regions of the parameter space where the dipole-dipole interactions can have significant effects.

cond-mat.mes-hall↗

arXiv2016

Effective theory and emergent $SU(2)$ symmetry in the flat bands of attractive Hubbard models

In a partially filled flat Bloch band electrons do not have a well defined Fermi surface and hence the low-energy theory is not a Fermi liquid. Neverethless, under the influence of an attractive interaction, a superconductor well described by the Bardeen-Cooper-Schrieffer (BCS) wave function can arise. Here we study the low-energy effective Hamiltonian of a generic Hubbard model with a flat band. We obtain an effective Hamiltonian for the flat band physics by eliminating higher lying bands via perturbative Schrieffer-Wolff transformation. At first order in the interaction energy we recover the usual procedure of projecting the interaction term onto the flat band Wannier functions. We show that the BCS wave function is the exact ground state of the projected interaction Hamiltonian and that the compressibility is diverging as a consequence of an emergent $SU(2)$ symmetry. This symmetry is broken by second order interband transitions resulting in a finite compressibility, which we illustrate for a one-dimensional ladder with two perfectly flat bands. These results motivate a further approximation leading to an effective ferromagnetic Heisenberg model. The gauge-invariant result for the superfluid weight of a flat band can be obtained from the ferromagnetic Heisenberg model only if the maximally localized Wannier functions in the Marzari-Vanderbilt sense are used. Finally, we prove an important inequality $D \geq \mathcal{W}^2$ between the Drude weight $D$ and the winding number $\mathcal{W}$, which guarantees ballistic transport for topologically nontrivial flat bands in one dimension.

cond-mat.str-el↗

arXiv2016

Geometric origin of superfluidity in the Lieb lattice flat band

The ground state and transport properties of the Lieb lattice flat band in the presence of an attractive Hubbard interaction are considered. It is shown that the superfluid weight can be large even for an isolated and strictly flat band. Moreover the superfluid weight is proportional to the interaction strength and to the quantum metric, a band structure invariant obtained from the flat-band Bloch functions. These predictions are amenable to verification with ultracold gases and may explain the anomalous behaviour of the superfluid weight of high-Tc superconductors.

cond-mat.quant-gas↗

arXiv2016

Topological phase transitions in the repulsively interacting Haldane-Hubbard model

Using dynamical mean-field theory and exact diagonalization we study the phase diagram of the repulsive Haldane-Hubbard model, varying the interaction strength and the sublattice potential difference. In addition to the quantum Hall phase with Chern number $C=2$ and the band insulator with $C=0$ present already in the noninteracting model, the system also exhibits a $C=0$ Mott insulating phase, and a $C=1$ quantum Hall phase. We explain the latter phase by a spontaneous symmetry breaking where one of the spin-components is in the Hall state and the other in the band insulating state.

cond-mat.quant-gas↗

arXiv2016

Superfluidity and Density Order in a Bilayer Extended Hubbard Model

We use cluster dynamical mean field theory to study the phase diagram of the square lattice bilayer Hubbard model with an interlayer interaction. The layers are populated by two-component fermions, and the densities in both layers and the strength of the interactions are varied. We find that an attractive interlayer interaction can induce a checkerboard density ordered phase and superfluid phases, with either interlayer or intralayer pairing. Remarkably, the latter phase does not require an intralayer interaction to be present: it can be attributed to an induced attractive interaction caused by density fluctuations in the other layer.

cond-mat.quant-gas↗

arXiv2015

Superfluidity in topologically nontrivial flat bands

Topological invariants built from the periodic Bloch functions characterize new phases of matter, such as topological insulators and topological superconductors. The most important topological invariant is the Chern number that explains the quantized conductance of the quantum Hall effect. Here, we provide a general result for the superfluid weight $D_{\rm s}$ of a multiband superconductor that is applicable to topologically nontrivial bands with nonzero Chern number $C$. We find that the integral over the Brillouin zone of the quantum metric, an invariant calculated from the Bloch functions, gives the superfluid weight in a flat band, with the bound $D_{\rm s} \geq |C|$. Thus, even a flat band can carry finite superfluid current, provided the Chern number is nonzero. As an example, we provide $D_{\rm s}$ for the time-reversal invariant attractive Harper-Hubbard model that can be experimentally tested in ultracold gases. In general, our results establish that a topologically nontrivial flat band is a promising concept for increasing the critical temperature of the superconducting transition.

cond-mat.supr-con↗

arXiv2015

Dynamical symmetry and pair tunneling in a one-dimensional Bose gas colliding with a mobile impurity

Using the time-dependent density matrix renormalization group (TDMRG) we theoretically study the collision of a one-dimensional gas of interacting bosons with an impurity trapped in a shallow box potential. We study the dynamic response of both the impurity and the bosonic gas and find an approximate independence of the sign of the interaction (attractive or repulsive) between the impurity and the bosons. This sign-independence breaks down when the interaction energy is of the same order as the box potential depth, leading to resonant pair tunneling. Our predictions can be tested using contemporary techniques with ultracold atoms.

cond-mat.quant-gas↗