### Browsing by Author "Tylutki, Marek"

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Item Coherent oscillations in small Fermi-polaron systems(2017-12-01) Tylutki, Marek; Astrakharchik, G. E.; Recati, Alessio; Department of Applied Physics; Quantum Dynamics; Polytechnic University of Catalonia; University of TrentoWe study the ground state and excitations of a one-dimensional trapped polarized Fermi gas interacting with a single impurity. First, we study the tunneling dynamics of the impurity through a potential barrier, such as one effectively created by a double-well trap. To this end, we perform an exact diagonalization of the full few-body Hamiltonian and analyze the results in a local-density approximation. Off-diagonal and one-particle correlation matrices are studied and are shown to be useful for discerning between different symmetries of the states. Second, we consider a radio-frequency spectroscopy of our system and the resulting spectral function. These calculations can motivate future experiments, which can provide further insight into the physics of a Fermi polaron.Item Collective excitations of a one-dimensional quantum droplet(American Physical Society, 2020-05-21) Tylutki, Marek; Astrakharchik, Grigori E.; Malomed, Boris A.; Petrov, Dmitry S.; Department of Applied Physics; Polytechnic University of Catalonia; Tel Aviv University; University of Paris-SaclayWe calculate the excitation spectrum of a one-dimensional self-bound quantum droplet in a two-component bosonic mixture described by the Gross-Pitaevskii equation (GPE) with cubic and quadratic nonlinearities. The cubic term originates from the mean-field energy of the mixture proportional to the effective coupling constant δg, whereas the quadratic nonlinearity corresponds to the attractive beyond-mean-field contribution. The droplet properties are governed by a control parameter γ∞δgN2/3, where N is the particle number. For large γ>0, the droplet features the flat-top shape with the discrete part of its spectrum consisting of plane-wave Bogoliubov phonons propagating through the flat-density bulk and reflected by edges of the droplet. With decreasing γ, these modes cross into the continuum, sequentially crossing the particle-emission threshold at specific critical values. A notable exception is the breathing mode, which we find to be always bound. The balance point γ=0 provides implementation of a system governed by the GPE with an unusual quadratic nonlinearity. This case is characterized by the ratio of the breathing-mode frequency to the particle-emission threshold equal to 0.8904. As γ tends to -∞, this ratio tends to 1 and the droplet transforms into the soliton solution of the integrable cubic GPE.Item Scaling and Diabatic Effects in Quantum Annealing with a D-Wave Device(American Physical Society, 2020-03-06) Weinberg, Phillip; Tylutki, Marek; Ronkko, Jami M.; Westerholm, Jan; Astrom, Jan A.; Manninen, Pekka; Torma, Paivi; Sandvik, Anders W.; Department of Applied Physics; Quantum Dynamics; Boston University; CSC - IT Center for Science Ltd.; Åbo Akademi University; Chinese Academy of SciencesWe discuss quantum annealing of the two-dimensional transverse-field Ising model on a D-Wave device, encoded on L×L lattices with L≤32. Analyzing the residual energy and deviation from maximal magnetization in the final classical state, we find an optimal L dependent annealing rate v for which the two quantities are minimized. The results are well described by a phenomenological model with two powers of v and L-dependent prefactors to describe the competing effects of reduced quantum fluctuations (for which we see evidence of the Kibble-Zurek mechanism) and increasing noise impact when v is lowered. The same scaling form also describes results of numerical solutions of a transverse-field Ising model with the spins coupled to noise sources. We explain why the optimal annealing time is much longer than the coherence time of the individual qubits.Item Spin-imbalanced Fermi superfluidity in a Hubbard model on a Lieb lattice(2018-09-17) Tylutki, Marek; Törmä, Päivi; Department of Applied Physics; Quantum DynamicsWe 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 of FFLO phases, i.e., superfluid phases with periodic spatial modulation. They differ by the magnitude and direction of the center-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 behavior of these phases.Item Spin-Imbalanced Pairing and Fermi Surface Deformation in Flat Bands(2018) Huhtinen, Kukka-Emilia; Tylutki, Marek; Kumar, Pramod; Vanhala, Tuomas I.; Peotta, Sebastiano; Törmä, Päivi; Department of Applied Physics; Quantum DynamicsWe 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 qualitatively different from 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 supercondutivity.