arXiv subjects

T. Domanski

Publications and source records attributed to T. Domanski.

48 records · Page 3Linked to original sources

arXiv2004

Ultracold atom superfluidity induced by the Feshbach resonance

We discuss the possible signatures of superfluidity induced by the Feshbach resonance in ultracold gas of fermion atoms. Approaching the phase transition from above there appear various manifestations of the gradually emerging order parameter, but yet the long range coherence is not established due to strong quantum fluctuations. The single particle excitation spectrum becomes gapped while at the same time the pair excitations are characterized by the narrow quasiparticle peak surrounded by the incoherent background. This quasiparticle shows up certain collective features such as the remnant of the "first sound" which at Tc spreads down to low momenta. Presence of this Goldstone mode is the most unambiguous proof for appearance of the superfluid state. We discuss how such mode can be detected experimentally.

cond-mat.supr-con↗

arXiv2004

Interplay between the single particle and collective features in the boson fermion model

We study the interplay between the single particle and fermion pair features in the boson fermion model, both above and below the transition temperature $T_{c}$, using the flow equation method. Upon lowering the temperature the single particle fermionic spectral function: (a) gradually develops a depletion of the low energy states (pseudogap) for $T^{*}>T>T_{c}$ and a true superconducting gap for $T<T_{c}$, (b) exhibits a considerable transfer of spectral weight between the incoherent background and the narrow coherent peak(s) signifying long-lived quasi-particle features. The cooperon spectral function consists of a delta function peak, centered at the renormalized boson energy $ω=\tilde{E}_{\bf q}$, and a surrounding incoherent background which is spread over a wide energy range. When the temperature approaches $T_{c}$ from above this peak for ${\bf q}={\bf 0}$ moves to $ω=0$, so that the static pair susceptibility diverges (Thouless criterion for the broken symmetry phase transition). Upon decreasing the temperature below $T_{c}$ the cooperon peak becomes the collective (Goldstone) mode $E_{\bf q} \propto |{\bf q}|$ in the small momentum region and simultaneously splits off from the incoherent background states which are expelled to the high energy sector $|ω| \geq 2 Δ_{sc}(T)$. We discuss the smooth evolution of these features upon approaching $T_{c}$ from above and consider its feedback on the single particle spectrum where a gradual formation of damped Bogoliubov modes (above $T_{c}$) is observed.

cond-mat.supr-con↗

arXiv2003

Bogoliubov shadow bands in the normal state of superconducting systems with strong pair fluctuations

On the basis of a scenario where electron pairing is induced by resonant two-particle scattering (the Boson Fermion model), we show how precursors of the superconducting state - in form of overdamped Bogoliubov modes - emerge in the normal state upon approaching the transition temperature from above. This result is obtained by a renormalization technique based on continuous unitary transformations (the flow equations), projecting out the coherent contributions in the electron spectral function from an incoherent background.

cond-mat.supr-con↗

arXiv2003

Nature of correlations in the atomic limit of the boson fermion model

Using the equation of motion technique for Green's functions we derive the exact solution of the boson fermion model in the atomic limit. Both (fermion and boson) subsystems are characterised by the effective three level excitation spectra. We compute the spectral weights of these states and analyse them in detail with respect to all possible parameters. Although in the atomic limit there is no true phase transition, we notice that upon decreasing temperature some pairing correlations start to appear. Their intensity is found to be proportional to the depleted amount of the fermion nonbonding state. We notice that pairing correlations behave in a fashion observed for the optimally doped and underdoped high $T_{c}$ superconductors. We try to identify which parameter of the boson fermion model can possibly correspond to the actual doping level. This study c larifies the origin of pairing correlations within the boson fermion model and may elucidate how to apply it for interpretation of experimental data.

cond-mat.supr-con↗

arXiv2003

Feshbach resonance described by the boson fermion coupling

We consider a possibility to describe the Feshbach resonance in terms of the Boson Fermion (BF) model. Using such model we show that after a gradual disentangling of the boson from fermion subsystem the resonant type scattering between fermions is indeed generated. We decouple the subsystems via: (a) the single step, and (b) the continuous canonical transformation. With the second one we investigate the feedback effects effectively leading to the finite amplitude of the scattering strength. We study them in detail in: the normal $T>T_{c}$ and superconducting $T \leq T_{c}$ states.

cond-mat.supr-con↗

arXiv2002

Effect of the on-site Coulomb repulsion on superconductivity in the boson-fermion model

We study the influence of the repulsive Coulomb interactions on thermodynamic properties of the boson fermion model with an anisotropic (d-wave, and extended s-wave) order parameter. Superconductivity is induced in this model from the anisotropic charge exchange interaction between the conduction band fermions (electrons or holes) and the immobile hard-core bosons (the localized electron pairs). The on-site Coulomb repulsion competes with this pairing interaction and hence is expected to have a detrimental influence on superconductivity. We analyze this effect in some detail considering the two opposite limits of: the weak and strong repulsion. A possible crossover between both these regimes is also discussed.

cond-mat.supr-con↗

arXiv2002

Thermodynamics and tunneling spectroscopy in the pseudogap regime of the boson fermion model

Motivated by the STM experimental data on Bi_2 Sr_2 CaCU_2 O_{8+x} which indicate the tunneling conductance asymmetry sigma(-V) not equal sigma(V), we report that such a behavior can be explained in terms of the boson fermion model. It has been shown in the recent studies, based on various selfconsistent techniques to capture the many-body effects, that the low energy spectrum of the boson fermion model is featured by an appearance of the pseudogap at T^* > T_c. We argue that the pseudogap structure has to exhibit a particle-hole asymmetry. This asymmetry may eventually depend on the boson concentration.

cond-mat.supr-con↗

arXiv2002

Effect of disorder on superconductivity in the boson-fermion model

We study how a randomness of either boson or fermion site energies affects the superconducting phase of the boson fermion model. We find that, contrary to what is expected for s-wave superconductors, the non-magnetic disorder is detrimental to the s-wave superconductivity. However, depending in which subsystem the disorder is located, we can observe different channels being affected. Weak disorder of the fermion subsystem is responsible mainly for renormalization of the single particle density of states while disorder in the boson subsystem directly leads to fluctuation of the strength of the effective pairing between fermions.

cond-mat.supr-con↗

arXiv2002

Renormalization group approach to the interacting boson fermion systems

We study a pseudogap region of the mixed boson fermion system using a recent formulation of the renormalization group technique through the set of infinitesimal unitary transformations. Renormalization of fermion energies gives rise to a depletion of the low energy states (pseudogap) for temperatures $T^{*}>T>T_{c}$ which foreshadows appearance of the pairwise correlations with their long range phase coherence being missed. With a help of the flow equations for boson and fermion operators we analyze spectral weights and finite life times of these quasiparticles caused by interactions.

cond-mat.supr-con↗

arXiv2001

Continuous canonical transformation for the double exchange model

The method of continuous canonical transformation is applied to the double exchange model with a purpose to eliminate the interaction term responsible for non conservation of magnon number. Set of differential equations for the effective Hamiltonian parameters is derived. Within the lowest order (approximate) solution we reproduce results of the standard (single step) canonical transformation. Results of the selfconsistent numerical treatment are compared with the other known studies for this model.

cond-mat.str-el↗

arXiv2000

Non-linear feedback effects in coupled Boson-Fermion systems

We address ourselves to a class of systems composed of two coupled subsystems without any intra-subsystem interaction: itinerant Fermions and localized Bosons on a lattice. Switching on an interaction between the two subsystems leads to feedback effects which result in a rich dynamical structure in both of them. Such feedback features are studied on the basis of the flow equation technique - an infinite series of infinitesimal unitary transformations - which leads to a gradual elimination of the inter-subsystem interaction. As a result the two subsystems get decoupled but their renormalized kinetic energies become mutually dependent on each other. Choosing for the inter - subsystem interaction a charge exchange term (the Boson-Fermion model) the initially localized Bosons acquire itinerancy through their dependence on the renormalized Fermion dispersion. This latter evolves from a free particle dispersion into one showing a pseudogap structure near the chemical potential. Upon lowering the temperature both subsystems simultaneously enter a macroscopic coherent quantum state. The Bosons become superfluid, exhibiting a soundwave like dispersion while the Fermions develop a true gap in their dispersion. The essential physical features described by this technique are already contained in the renormalization of the kinetic terms in the respective Hamiltonians of the two subsystems. The extra interaction terms resulting in the process of iteration only strengthen this physics. We compare the results with previous calculations based on selfconsistent perturbative approaches.

cond-mat.supr-con↗

arXiv1997

The Atomic Limit of the Boson-Fermion Model

The Boson-Fermion model, describing a mixture of hybridized localized Bosons and itinerant Fermions on a lattice, is known to exhibit spectral properties for the Fermions which upon lowering the temperature develop into a three pole structure in the vicinity of the Fermi level. These spectral features go hand in hand with the opening of a pseudogap in the density of states upon approaching the critical temperature Tc when superconductivity sets in. In the present work we study this model, in the atomic limit where the three pole structure arises naturally from the local bonding, anti-bonding and non-bonding states between the Bosons and Fermions.

cond-mat.str-el↗