Origin of anomalous Fano resonance explained in Scientific Reports

J. Barański, T. Zienkiewicz, M. Barańska, K. J. Kapcia,
Anomalous Fano Resonance in Double Quantum Dot System Coupled to Superconductor,
Scientific Reports 10, 2881 (2020)
Article is free to read: article published under Gold Open Access


We analyze the influence of a local pairing on the quantum interference in nanoscopic systems. As a model system we choose the double quantum dot coupled to one metallic and one superconducting electrode in the T-shape geometry. The analysis is particularly valuable for systems containing coupled objects with considerably different broadening of energy levels. In such systems, the scattering of itinerant electrons on a discrete (or narrow) energy level gives rise to the Fano-type interference. Systems with induced superconducting order, along well understood Fano resonances, exhibit also another features on the opposite side of the Fermi level. The lineshape of these resonances differs significantly from their reflection on the opposite side of the Fermi level, and their origin was not fully understood. Here, considering the spin-polarized tunneling model, we explain a microscopic mechanism of a formation of these resonances and discuss the nature of their uncommon lineshapes. We show that the anomalous Fano profiles originate solely from the pairing of nonscattered electrons with scattered ones. We investigate also the interplay of each type of resonances with the Kondo physics and discuss the resonant features in differential conductivity.

Scholarships of the Minister of Science and Higher Education (Poland) for outstanding young scientists in 2019

Stypendia Ministra Nauki i Szkolnictwa Wyższego dla wybitnych młodych naukowców w 2019

211 wybitnych młodych naukowców, w tym 29 doktorantów, otrzymało stypendia Ministra Nauki i Szkolnictwa Wyższego. Minister uhonorował stypendiami młodych naukowców, którzy prowadzą innowacyjne badania naukowe na wysokim poziomie i cieszą się imponującym dorobkiem naukowym o wysokim prestiżu i międzynarodowym zasięgu.

Ministerstwo Nauki i Szkolnictwa Wyższego [dostęp: 15/11/2019]
Forum Akademickie [dostęp: 15/11/2019]
Lista laureatów:
plik pdf na stronie ministerstwa [dostęp: 15/11/2019]
Kopia lokalna [z dnia 15/11/2019]

Scholarships of the Minister of Science and Higher Education (Poland) for outstanding young scientists in 2019

211 outstanding young scientists, including 29 doctoral students, received scholarships from the Minister of Science and Higher Education. Minister awarded scholarships to young scientists who conduct innovative, high-quality scientific research and have impressive scientific achievements of high prestige and international reach.

Ministry of Science and Higher Education (Poland) [access: 15/11/2019]
List of the winners:
pdf file on the Ministry website [access: 15/11/2019]
Local copy [from 15/11/2019]

Noncentrosymmetric superconductors ThTSi: ab initio study of electronic and lattice properties

A. Ptok, K. Domieracki, K.J. Kapcia, J. Łażewski, P.T. Jochym, M. Sternik, P. Piekarz, D. Kaczorowski, Electronic and lattice properties of noncentrosymmetric superconductors ThTSi (T = Co, Ir, Ni, and Pt),
Phys. Rev. B 100 (16), 165130 (1-8) (2019)
pre-print also available on arXiv (free access): arXiv [cond-mat.str-el]

Theoretical studies on the electronic and lattice properties of the series of noncentrosymmetric superconductors ThTSi, where T = Co, Ni, Ir, and Pt, are presented. The electronic band-structure and crystal parameters were optimized within the density functional theory. Spin-orbit coupling leads to the splitting of the electronic bands and Fermi surfaces, with the stronger effect observed for the compounds with the heavier atoms Ir and Pt. The possible mixing of the spin-singlet and spin-triplet pairing in the superconducting state is discussed. The phonon dispersion relations and phonon density of states were obtained using the direct method. The dispersion curves in ThCoSi and ThIrSi exhibit low-energy modes along the S−N−S0 line with the tendency for softening and dynamic instability. Additionally, we calculate and analyze the contributions of phonon modes to lattice heat capacity.

Full phase diagram of extended Falicov-Kimball model

K.J. Kapcia, R. Lemański, S. Robaszkiewicz,
Extended Falicov-Kimball model: Exact solution for finite temperatures,
Phys. Rev. B 99 (24), 245143 (1-15) (2019)
pre-print also available on arXiv (free access): 1903.08092 [cond-mat.str-el]

The extended Falicov-Kimball model is analyzed exactly for finite temperatures in the limit of large dimensions. The onsite, as well as the intersite density-density interactions represented by the coupling constants U and V, respectively, are included in the model. Using the dynamical mean field theory (DMFT) formalism on the Bethe lattice we find rigorously the temperature dependent density of states (DOS) at half-filling. At zero temperature (T=0) the system is ordered to form the checkerboard pattern and the DOS has the gap $\Delta(\varepsilon_F) > 0$ at the Fermi level, if only U≠0 or V≠0. With an increase of T the DOS evolves in various ways that depend both on U and V. If U2V, two additional subbands develop inside the principal energy gap. They become wider with increasing T and at a certain U- and V-dependent temperature TMI they join with each other at $\varepsilon_F$. Since above TMI the DOS is positive at $\varepsilon_F$, we interpret TMI as the transformation temperature from insulator to metal. It appears, that TMI approaches the order-disorder phase transition temperature TOD for |U|=2 and $0<U\lesssim 2V$, but otherwise TMI is substantially lower than TOD. Moreover, we show that if $V\lesssim 0.54$ then TMI at two quasi-quantum critical points $U_{cr}^{\pm}$ (one positive and the other negative), whereas for $V\gtrsim 0.54$ there is only one negative $U^-_{cr}$. Having calculated the temperature dependent DOS we study thermodynamic properties of the system starting from its free energy F and then we construct the phase diagrams in the variables T and U for a few values of V. Our calculations give that inclusion of the intersite coupling V causes the finite temperature phase diagrams to become asymmetric with respect to a change of sign of U. On these phase diagrams we detected stability regions of eight different kinds of ordered phases, where both charge-order and antiferromagnetism coexists (five of them are insulating and three are conducting) and three different nonordered phases (two of them are insulating and one is conducting). Moreover, both continuous and discontinuous transitions between various phases were found.

Core-shell structures revisited in Scientific Reports

A.Cichy, K. J. Kapcia, A. Ptok, Phase separations induced by a trapping potential in one-dimensional fermionic systems as a source of core-shell structures,
Scientific Reports 9, 6719 (1-9) (2019); arXiv
Article is free to read: article published under Gold Open Access


Ultracold fermionic gases in optical lattices give a great opportunity for creating different types of novel states. One of them is phase separation induced by a trapping potential between different types of superfluid phases. The core-shell structures, occurring in systems with a trapping potential, are a good example of such separations. The types and the sequences of phases which emerge in such structures can depend on spin-imbalance, shape of the trap and on-site interaction strength. In this work, we investigate the properties of such structures within an attractive Fermi gas loaded in the optical lattice, in the presence of the trapping potential and their relations to the phase diagram of the homogeneous system. Moreover, we show how external and internal parameters of the system and parameters of the trap influence their properties. In particular, we show a possible occurrence of the core-shell structure in a system with a harmonic trap, containing the BCS and FFLO states. Additionally, we find a spatial separation of two superfuild states in the system, one in the BCS limit as well as the other one in the tightly bound local pairs (BEC) regime.

Isostructural phase transition under pressure: ab initio calculations for iron-based superconductor

A. Ptok, M. Sternik, K.J. Kapcia, P. Piekarz, Structural, electronic, and dynamical properties of the tetragonal and collapsed tetragonal phases of KFe2As2, Phys. Rev. B 99 (13), 134103 (1-16) (2019)
Open Access: pre-print also available on arXiv: 1903.07953 [cond-mat.supr-con]

Compounds with a tetragonal ThCr2Si2-type structure are characterized by the possibility of the isostructural phase transition from the tetragonal phase to the collapsed tetragonal phase induced by the external pressure. An example of a compound with such a phase transition is KFe2As2, which belongs to the family of the iron-based superconductors. In this paper, we investigate the effects of the phase transition on the structural, electronic, and dynamical properties of this compound. Performing the ab initio calculations, we reproduce the dependence of the lattice constants on pressure and analyze the changes of the interatomic distances in the tetragonal and collapsed tetragonal phases. Using the tight binding model with maximally localized Wannier orbitals, we calculate and discuss the influence of pressure on the electronic band structure as well as on the shape of the Fermi surface. We found a precursor of the phase transition in the form of enhancement of overlapping between two Wannier orbitals of As atoms. In order to better understand the superconducting properties of KFe2As2, we study the orbital-projected Cooper pairs susceptibility as a function of pressure. We found a decrease of susceptibility with the increasing pressure in a good qualitative agreement with experimental observation. The structural transition also influences the phonon spectrum of KFe2As2, which exhibits pronounced changes induced by pressure. Some phonon modes related with the vibrations of Fe and As atoms show an anomalous, nonmonotonic dependence on pressure close to the phase transition.

Our work in new APS journal

A. Ptok, K. Rodríguez, K.J. Kapcia, Superconducting monolayer deposited on substrate: Effects of the spin-orbit coupling induced by proximity effects,
Phys. Rev. Materials 2(2), 024801 (1-10) (2018),
(pre-print also available on arXiv: 1801.08055 [cond-mat.supr-con])

Spin-orbit coupling can lead to exotic states of matter and unexpected behavior of the system properties. In this paper, we investigate the influence of spin-orbit coupling induced by proximity effects on a monolayer of superconductor (with s-wave or d-wave pairing) placed on an insulating bulk. We show that the critical temperatures Tc of the superconducting states can be tuned by the spin-orbit coupling both in the case of on-site and intersite pairing. Moreover, we discuss a possibility of changing the location of the maximal Tc from the half filling into the underdoped or overdoped regimes.