arXiv subjects

M. M. Maska

Publications and source records attributed to M. M. Maska.

14 recordsLinked to original sources

arXiv2018

Interplay between pairing and correlations in spin-polarized bound states

We investigate the single and multiple defects embedded in a superconducting host, studying interplay between the proximity induced pairing and interactions. We explore influence of the spin-orbit coupling on energies, polarization and spatial patterns of the bound (Yu-Shiba-Rusinov) states of magnetic impurities in 2-dimensional square lattice. We also address the peculiar bound states in the proximitized Rashba chain, resembling the Majorana quasiparticles, focusing on their magnetic polarization which has been recently reported by S. Jeon et al., [Science 358, 772 (2017)]. Finally, we study leakage of these polarized Majorana quasiparticles on the side-attached nanoscopic regions and confront them with the subgap Kondo effect near to the singlet-doublet phase transition.

cond-mat.mes-hall↗

arXiv2018

The effective increase in atomic scale disorder by doping and superconductivity in Ca$_3$Rh$_4$Sn$_{13}$

The comprehensive research of the electronic structure, thermodynamic and electrical transport properties reveals the existence of inhomogeneous superconductivity due to structural disorder in Ca$_3$Rh$_4$Sn$_{13}$ doped with La (Ca$_{3-x}$La$_x$Rh$_4$Sn$_{13}$) or Ce (Ca$_{3-x}$Ce$_x$Rh$_4$Sn$_{13}$) with superconducting critical temperatures $T_c^{\star}$ higher than those ($T_c$) observed in the parent compounds. The $T-x$ diagrams and the entropy $S(x)_T$ isotherms well document the relation between degree of an atomic disorder and separation of the {\it high-temperature} $T_c^{\star}$ and $T_c$-bulk phases. In these dirty superconductors with the mean free path much smaller than the coherence length, the Werthamer-Helfand-Hohenber theoretical model does not well fits the $H_{c2}(T)$ data. We suggest that this can result from two-band superconductivity or from the presence of strong inhomogeneity in these systems. The multiband model very well describes the $H-T$ dependencies, but the present results as well as our previous studies give arguments for the scenario based on the presence of nanoscopic inhomogeneity of the superconducting state. We also revisited the nature of structural phase transition at $T^{\star}\sim 130-170$ K and documented that there might be another precursor transition at higher temperatures. The impact of the magnetic Ce-Ce correlations on the increase of $T_c$ in respect to the critical temperatures of Ca$_{3-x}$La$_x$Rh$_4$Sn$_{13}$ is also discussed.

cond-mat.supr-con↗

arXiv2017

Spin-polarized Andreev tunneling through the Rashba chain

We demonstrate that the selective equal spin Andreev reflection (SESAR) spectroscopy can be used in STM experiments to distinguish the zero-energy Majorana quasiparticles from the ordinary fermionic states of the Rashba chain. Such technique, designed for probing the p-wave superconductivity, could be applied to the intersite pairing of equal-spin electrons in the chain of magnetic Fe atoms deposited on the superconducting Pb substrate. Our calculations of the effective pairing amplitude for individual spin components imply the magnetically polarized Andreev conductance, which can be used to `filter' the Majorana quasiparticles from the ordinary in-gap states, although the pure spin current (i.e., perfect polarization) is impossible.

cond-mat.mes-hall↗

arXiv2016

Majorana quasiparticles of inhomogeneous Rashba chain

We investigate the inhomogeneous Rashba chain coupled to a superconducting substrate, hosting the Majorana quasiparticles near its edges. We discuss its subgap spectrum and study how robust are the zero-energy quasiparticles against the diagonal and off-diagonal disorder. Studying the $\mathbb{Z}_2$ topological invariant we show that disorder induced transition from the topologically non-trivial to trivial phases is manifested by characteristic features in the spatially-resolved quasiparticle spectrum at zero energy. We provide evidence for the non-local nature of the zero-energy Majorana quasiparticles, that are well preserved upon partitioning the chain into separate pieces. Even though the Majorana quasiparticles are not completely immune to inhomogeneity we show that they can spread onto other (normal) nanoscopic objects via the proximity effect.

cond-mat.mes-hall↗

arXiv2016

Effect of atomic disorder and Ce doping on superconductivity of Ca3Rh4Sn13: Electric transport properties under high pressure

We report the observation of a superconducting state below 8K coexistent with a spin-glass state caused by atomic disorder in Ce substituted Ca3Rh4Sn13. Measurements of specific heat, resistivity, and magnetism reveal the existence of inhomogeneous superconductivity in samples doped with Ce with superconducting critical temperatures Tc higher than those observed in the parent compound. For Ca3Rh4Sn13, the negative value of the change in resistivity with pressure P, correlates well with the calculated decrease in the density of states at the Fermi energy with P. Based on band structure calculations performed under pressure, we demonstrate how the change in DOS would affect Tc of Ca3Rh4Sn13 under negative lattice pressure in samples that are strongly defected by quenching.

cond-mat.supr-con↗

arXiv2015

Superconductivity in the presence of disorder in skutterudite-related La$_3$Co$_4$Sn$_{13}$ and La$_3$Ru$_4$Sn$_{13}$ compounds; electrical transport and magnetic studies

La$_3$Co$_4$Sn$_{13}$ and La$_3$Ru$_4$Sn$_{13}$ were categorized as BCS superconductors. In a plot of the critical field $H_{c2}$ vs $T$, La$_3$Ru$_4$Sn$_{13}$ displays a second superconducting phase at the higher critical temperature $T_c^{\star}$, characteristic of inhomogeneous superconductors, while La$_3$Co$_4$Sn$_{13}$ shows bulk superconductivity below $T_c$. We observe a decrease in critical temperatures with external pressure and magnetic field for both compounds with $\frac{dT_c^{\star}}{dP} > \frac{dT_c}{dP}$. The pressure dependences of $T_c$ are interpreted according to the McMillan theory and understood to be a consequence of lattice stiffening. The investigation of the superconducting state of La$_3$Co$_x$Ru$_{4-x}$Sn$_{13}$ shows a $T_c^{\star}$ that is larger then $T_c$ for $x<4$. This unique and unexpected observation is discussed as a result of the local disorder and/or the effect of chemical pressure when Ru atoms are partially replaced by smaller Co atoms.

cond-mat.supr-con↗

arXiv2014

Superconductivity of La$_3$Co$_4$Sn$_{13}$ and La$_3$Rh$_4$Sn$_{13}$: A comparative study

We report the electric transport and thermodynamic properties of the skutterudite-related La$_3$Co$_4$Sn$_{13}$ and La$_3$Rh$_4$Sn$_{13}$ superconductors. Applying an external pressure to La$_3$Rh$_4$Sn$_{13}$, the resistive superconducting critical temperature Tc decreases, while the critical temperature of La$_3$Co$_4$Sn$_{13}$ is enhanced with increasing pressure. The positive pressure coefficient dTc/dP correlates with a subtle structural transition in La$_3$Co$_4$Sn$_{13}$ and is discussed in the context of lattice instabilities. Specific-heat data show that both compounds are typical BCS superconductors. However, La$_3$Rh$_4$Sn$_{13}$ also exhibits a second superconducting phase at higher temperatures, which is characteristic of inhomogeneous superconductors. We calculate the specific heat for an inhomogeneous superconducting phase, which agrees well with experimental C(T) data for La$_3$Rh$_4$Sn$_{13}$. We also found that an applied pressure reduces this second superconducting phase.

cond-mat.supr-con↗

arXiv2012

Effective approach to the Nagaoka regime of the two dimensional t-J model

We argue that the t-J model and the recently proposed Ising version of this model give the same physical picture of the Nagaoka regime for J/t << 1. In particular, both models are shown to give compatible results for a single Nagaoka polaron as well as for a Nagaoka bipolaron. When compared to the standard t-J or t-Jz models, the Ising version allows for a numerical analysis on much larger clusters by means of classical Monte Carlo simulations. Taking the advantage of this fact, we study the low doping regime of t-J model for J/t << 1 and show that the ground state exhibits phase separation into hole-rich ferromagnetic and hole-depleted antiferromagnetic regions. This picture holds true up to a threshold concentration of holes, δ< δ_t ~ 0.44 \sqrt{J/t}. Analytical calculations show that δ_t=\sqrt{J/2πt}.

cond-mat.str-el↗

arXiv2011

Momentum distribution and ordering in mixtures of ultracold light and heavy fermionic atoms

The momentum distribution is one of the most important quantities which provides information about interactions in many-body systems. At the same time it is a quantity that can easily be accessed in experiments on ultracold atoms. In this paper, we consider mixtures of light and heavy fermionic atoms in an optical lattice described effectively by the Falicov-Kimball model. Using a Monte Carlo method, we study how different ordered density-wave phases can be detected by measurement of the momentum distribution of the light atoms. We also demonstrate that ordered phases can be seen in Bragg scattering experiments. Our results indicate that the main factor that determines the momentum distribution of the light atoms is the trap confinement. On the other hand, the pattern formed by the heavy atoms seen in the Bragg scattering experiments is very sensitive to the temperature and possibly can be used in low-temperature thermometry.

cond-mat.quant-gas↗

arXiv2009

Improving the efficiency of ultracold dipolar molecule formation by first loading onto an optical lattice

Ultracold ground state dipolar 40K-87Rb molecules have recently been produced in a loose harmonic trap by employing a magnetic field sweep across a Feshbach resonance followed by stimulated Raman adiabatic passage [K.-K. Ni et al., Science 322, 231 (2008)]. The overall experimental efficiency for molecule formation was around 20%. We show that the efficiency can be increased to nearly 100% if one first loads the atomic gases into an optical lattice of the appropriate depth and tunes the interspecies attraction to have exactly one atom of each species at an occupied lattice site. Our proposed scheme provides a large enhancement to the dipolar molecule density even at relatively high temperatures, and avoids three-body recombination loss by preventing lattice sites from being triply occupied.

cond-mat.quant-gas↗

arXiv2008

Ising t-J model close to half filling: A Monte Carlo study

Within the recently proposed doped-carrier representation of the projected lattice electron operators we derive a full Ising version of the t-J model. This model possesses the global discrete Z_2 symmetry as a maximal spin symmetry of the Hamiltonian at any values of the coupling constants, t and J. In contrast, in the spin anisotropic limit of the t-J model, usually referred to as the t-J_z model, the global SU(2) invariance is fully restored at J_z=0, so that only the spin-spin interaction has in that model the true Ising form. We discuss a relationship between those two models and the standard isotropic t-J model. We show that the low-energy quasiparticles in all three models share the qualitatively similar properties at low doping and small values of J/t. The main advantage of the proposed Ising t-J model over the t-J_z one is that the former allows for the unbiased Monte Carlo calculations on large clusters of up to 10^3 sites. Within this model we discuss in detail the destruction of the antiferromagnetic order by doping as well as the interplay between the AF order and hole mobility. We also discuss the effect of the exchange interaction and that of the next nearest neighbour hoppings on the destruction of the AF order at finite doping. We show that the short-range AF order is observed in a wide range of temperatures and dopings, much beyond the boundaries of the AF phase. We explicitly demonstrate that the local no double occupancy constraint plays the dominant role in destroying the magnetic order at finite doping. Finally, a role of inhomogeneities is discussed.

cond-mat.str-el↗

arXiv2008

Superconducting state of quasiparticles with spin dependent mass and their distinguishability for Cooper-pair state

Spin dependence of quasiparticle mass has been observed recently in CeCoIn5 and other systems. It emerges from strong electronic correlations in a magnetically polarized state and was predicted earlier. Additionally, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)phase has also been discovered in CeCoIn5 and therefore, the question arises as to what extent these two basic phenomena are interconnected, as it appears in theory. Here we show that the appearance of the spin-split masses essentially extends the regime of temperature and applied magnetic field, in which FFLO state is stable, and thus, it is claimed to be very important for the phase detectability. Furthermore, in the situation when the value of the spin z-component σdifferentiates masses of the particles, the fundamental question is to what extent the two mutually bound particles are indistinguishable quantum mechanically? By considering first the Cooper-pair state we show explicitly that the antisymmetry of the spin-pair wave function in the ground state may be broken when the magnetic field is applied.

cond-mat.str-el↗

arXiv2008

Pattern formation in mixtures of ultracold atoms in optical lattices

Regular pattern formation is ubiquitous in nature; it occurs in biological, physical, and materials science systems. Here we propose a set of experiments with ultracold atoms that show how to examine different types of pattern formation. In particular, we show how one can see the analog of labyrinthine patterns (so-called quantum emulsions) in mixtures of light and heavy atoms (that tend to phase separate) by tuning the trap potential and we show how complex geometrically ordered patterns emerge (when the mixtures do not phase separate), which could be employed for low-temperature thermometry. The complex physical mechanisms for the pattern formation at zero temperature are understood within a theoretical analysis called the local density approximation.

cond-mat.str-el↗

arXiv2004

Possible singlet to triplet pairing transition in NaxCoO2 H2O

We present precise measurements of the upper critical field (Hc2) in the recently discovered cobalt oxide superconductor. We have found that the critical field has an unusual temperature dependence; namely, there is an abrupt change of the slope of Hc2(T) in a weak field regime. In order to explain this result we have derived and solved Gor'kov equations on a triangular lattice. Our experimental results may be interpreted in terms of the field-induced transition from singlet to triplet superconductivity.

cond-mat.supr-con↗