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arXiv2013

Intrinsic high-temperature superconductivity in ternary iron selenides

We examine superconductivity in the mesoscopically mixed antiferromagnetic(AF) and superconducting (SC) phases of ternary iron selenides K$_{y}$Fe$_{2-x}$Se$_{2}$. It is shown that the interlayer hopping and AF order are key factors to determine $T_{c}$ of the SC phase. In general, the hopping will produce deformed Fermi surfaces (FS's) that tend to suppress superconductivity. However, contrary to the common expectation, we find that larger AF order actually results in larger SC order, which explains the observed relatively high $T_{c}$ in these phases. Furthermore our results indicate that by reducing the interlayer hopping appropriately, phase-separated K$_{y}$Fe$_{2-x}$Se$_{2}$ may exhibit its intrinsic SC phase in the two dimensional limit with a much higher $T_{c}$ ($\sim 65K$) than what has been observed.

cond-mat.supr-con↗

arXiv2013

Josephson surface plasmons in spatially confined cuprate superconductors

In this work, we generalize the theory of localized surface plasmons to the case of high-Tc cuprate superconductors, spatially confined in the form of small spherical particles. At variance from ordinary metals, cuprate superconductors are characterized by a low-energy bulk excitation known as the Josephson plasma wave (JPW), arising from interlayer tunneling of the condensate along the c-axis. The effect of the JPW is revealed in a characteristic spectrum of surface excitations, which we call Josephson surface plasmons. Our results, which apply to any material with a strongly anisotropic electromagnetic response, are worked out in detail for the case of multilayered superconductors supporting both low-frequency (acoustic) and transverse-optical JPW. Spatial confinement of the Josephson plasma waves may represent a new degree of freedom to engineer their frequencies and to explore the link between interlayer tunnelling and high-Tc superconductivity.

cond-mat.supr-con↗

arXiv2013

Magnetic structures and the Ce-Fe coupling induced Fe spin reorientation in CeFeAsO single crystal

Neutron and synchrotron resonant X-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in single crystal CeFeAsO. The RXMS of magnetic reflections at the Ce $L_{\rm II}$-edge shows a magnetic transition that is specific to the Ce antiferromagnetic long-range ordering at $T_\texttt{Ce}\approx$ 4 K with short-range Ce ordering above $T_\texttt{Ce}$, whereas neutron diffraction measurements of a few magnetic reflections indicate a transition at $T^{*}\approx$ 12 K with unusual order parameter. Detailed order parameter measurements on several magnetic reflections by neutrons show a weak anomaly at 4 K which we associate with the Ce ordering. The successive transitions at $T_\texttt{Ce}$ and $T^{*}$ can also be clearly identified by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at $T^{*}\approx$ 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the \textit{ab} plane. The Fe spin reorientation transition and short-range Ce ordering above $T_\texttt{Ce}$ reflect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through $T^{*}$ and $T_\texttt{Ce}$ is proposed.

cond-mat.str-el↗

arXiv2013

Measuring Majorana fermions qubit state and non-Abelian braiding statistics in quenched inhomogeneous spin ladders

We study the Majorana fermions (MFs) in a spin ladder model. We propose and numerically show that the MFs qubit state can be read out by measuring the fusion excitation in the quenched inhomogeneous spin ladders. Moreover, we construct an exactly solvable T-junction spin ladder model, which can be used to implement braiding operations of MFs. With the braiding processes simulated numerically as non-equilibrium quench processes, we verify that the MFs in our spin ladder model obey the non-Abelian braiding statistics. Our scheme not only provides a promising platform to study the exotic properties of MFs, but also has broad range of applications in topological quantum computation.

cond-mat.mes-hall↗

arXiv2013

Absence of broken time reversal symmetry beneath the surface of YBCO films

We report the results of a search for spontaneous magnetism due to a time reversal symmetry breaking phase in the superconducting state of (110)-oriented YBCO films, expected near the surface in this geometry. Zero field and weak transverse field measurements performed using the low-energy muon spin rotation technique with muons implanted few nm inside optimally-doped YBCO-(110) films show no appearance of spontaneous magnetic fields below the superconducting temperature down to 2.9 K. Our results give an upper limit of ~0.02 mT for putative spontaneous internal fields.

cond-mat.supr-con↗

arXiv2013

Superconductivity from orbital nematic fluctuations

Recent experiments suggest that besides of antiferromagnetic fluctuations nematic fluctuations may contribute to the occurrence of superconductivity in iron pnictides. Motivated by this observation we study superconductivity from nematic fluctuations in a minimal two-band model. The employed band parameters are appropriate for iron pnictides and lead to four pockets for the Fermi line. It is shown that low-energy, long-wavelength nematic fluctuations within the pockets give rise to strong-coupling superconductivity whereas the large momenta density fluctuations between pockets are rather irrelevant. The obtained transition temperatures are similar to those typically found in the pnictides and are rather robust against repulsive Coulomb interactions. The superconducting and nematic states coexist in a large region of the phase diagram.

cond-mat.supr-con↗

arXiv2013

Two-dimensional p-wave superconducting states with magnetic moments on a conventional s-wave superconductor

Unconventional superconductivity induced by the magnetic moments in a conventional spin-singlet s-wave superconductor is theoretically studied. By choosing the spin directions of these moments, one can design spinless pairing states appearing within the s-wave superconducting energy gap. It is found that the helix spins produce px + py-wave state while the skyrmion crystal configuration px + ipy-wave like state. Nodes in the energy gap and the zero energy flat band of Majorana edge states exist in the former one, while the chiral Majorana channels along edges of the sample and the zero energy Majorana bound state at the core of the vortex appear in the latter case.

cond-mat.supr-con↗

arXiv2013

Electrical detection of topological quantum phase transitions in disordered Majorana nanowires

We study a disordered superconducting nanowire, with broken time-reversal and spin-rotational symmetry, which can be driven into a topological phase with end Majorana bound states by an externally applied magnetic field. As a function of disorder strength, it is known that the Majorana nanowire has a delocalization quantum phase transition from a topologically nontrivial phase, which supports Majorana bound states, to a nontopological insulating phase without them. On both sides of the transition, the system is localized at zero energy albeit with very different topological properties. We exploit this deep connection between topology and localization properties to propose an electrical transport measurement to detect the localization-delocalization transition occurring in the bulk of the nanowire. The basic idea consists of measuring the difference of conductance at one end of the wire obtained at different values of the coupling to the opposite lead. We show that this measurement reveals the nonlocal correlations emergent only at the topological transition. Hence, while the proposed experiment does not directly probe the end Majorana bound states, it can provide direct evidence for the bulk topological quantum phase transition itself.

cond-mat.mes-hall↗

arXiv2013

Majorana Excitations, Spin- and Mass Currents on the Surface of Topological Superfluid $^3$He-B

The B-phase of superfluid $^3$He is a 3D time-reversal invariant (TRI) topological superfluid with an isotropic energy gap separating the ground-state and bulk continuum states. We report calculations of surface spectrum, spin- and mass current densities originating from the Andreev surface states for confined $^3$He-B. The surface states are Majorana Fermions with their spins polarized transverse to their direction of propagation along the surface. The negative energy states give rise to a ground-state helical spin current confined on the surface. The spectral functions reveal the subtle role of the spin-polarized surface states in relation to the ground-state spin current. By contrast, these states do not contribute to the $T=0$ mass current. Superfluid flow through a channel of confined $^3$He-B is characterized by the flow field, $p_s$. The flow field breaks 2D rotational symmetry and time reversal (T). However, the Bogoliubov-Nambu Hamiltonian remains invariant under $T$ combined with a 180 degree rotation about the normal to the film. As a result the B-phase in the presence of a superflow remains a topological phase with a gapless spectrum of Majorana modes on the surface. Thermal excitation of the Doppler shifted Majorana branches leads to a power law suppression of the superfluid mass current, providing a direct signature of the Majorana branches of surface excitations in the fully gapped 3D topological superfluid, $^3$He-B. Results are reported for the superfluid fraction (mass current) and helical spin current for confined $^3$He-B, including the temperature dependences, as well as dependences on confinement, interactions between quasiparticles and pressure.

cond-mat.supr-con↗

arXiv2013

The effect of amplitude fluctuations on the BKT transition

The Berezinskii-Kosterlitz-Thouless (BKT) transition is a generic transition describing the loss of coherence in two dimensional systems, and has been invoked, for example, to describe the superconductor-insulator transition in thin films. However, recent experiments have shown that the BKT transition, driven by phase fluctuations, is not sufficient to describe the loss of superconducting order, and amplitude fluctuations have also to be taken into account. The standard models that are extensively used to model two-dimensional superconductors are the Hubbard and XY models. Whereas the XY model allows only phase fluctuations, the Hubbard model has an extra degree of freedom: amplitude fluctuations. In this paper we compare two Hubbard models with the same critical temperature but with different interaction, and deduce the role of the amplitude fluctuations in the superconducting transition. For this purpose, a novel approximation is presented and used. We derive an effective phase-only (XY) Hamiltonian, incorporating amplitude fluctuations in an explicit temperature dependence of the phase rigidity. We study the relation between amplitude fluctuations and coupling strength. Our results support existing claims about the suppression of phase rigidity due to amplitude fluctuations not present in the XY model.

cond-mat.supr-con↗

arXiv2013

Phases in two dimensional $p_x+ip_y$ superconducting systems with next-nearest-neighbor interactions

A chiral $p_x+ip_y$ superconductor on a square lattice with nearest and next-nearest hopping and pairing terms is considered. Gap closures, as various parameters of the system are varied, are found analytically and used to identify the topological phases. The phases are characterized by Chern numbers (ranging from -3 to 3), and (numerically) by response to introduction of weak disorder, edges, and magnetic fields in an extreme type-II limit, focusing on the low-energy modes (which presumably become zero-energy Majorana modes for large lattices and separations). Several phases are found, including a phase with Chern number 3 that cannot be thought of in terms of a single range of interaction, and phase with Chern number 2 that may host an additional, disorder resistant, Majorana mode. The energies of the vortex quasiparticle modes were found to oscillate as vortex position varied. The spatial length scale of these oscillations was found for various points in the Chern number 3 phase which increased as criticality was approached.

cond-mat.supr-con↗

arXiv2013

Effective crystal field and Fermi surface topology: a comparison of d- and dp-orbital models

The effective crystal field in multi-orbital correlated materials can be either enhanced or reduced by electronic correlations with crucial consequences for the topology of the Fermi surface and, hence, on the physical properties of these systems. In this respect, recent local density approximation (LDA) plus dynamical mean-field theory (DMFT) studies of Ni-based heterostructure have shown contradicting results, depending on whether the less correlated $p$-orbitals are included or not. We investigate the origin of this problem and identify the key parameters controlling the Fermi surface properties of these systems. Without the $p$-orbitals the model is quarter filled, while the $d$ manifold moves rapidly towards half-filling when the $p$-orbitals are included. This implies that the local Hund's exchange, while rather unimportant for the former case, can play a predominant role in controlling the orbital polarization for the extended basis-set by favoring the formation of a larger local magnetic moment.

cond-mat.str-el↗

arXiv2013

Superconductivity with intrinsic topological order induced by pure Coulomb interaction and time-reversal symmetry breaking

Recently, in certain flat band lattice systems at commensurate fillings, fractional quantum Hall states have been found -- which have anyonic excitations. We study such systems away from commensuration, i.e. the ground state of an anyon gas in such a system. The presence of the underlying lattice allows access to an entirely new regime where the anyon kinetic energy can be larger than their interaction energy. Within the flux-attachment approach, using mean-field then adding fluctuations, we find several possible superfluid states. Two have intrinsic topological order, i.e. fractionalized quasiparticles with a fusion structure of (Z_2)^4 and (Z_8)^2 respectively, and a third has no fractionalized excitations similar to a BCS-type state. This represents a mechanism for superconductivity driven purely by strong repulsion and complex hopping of electrons.

cond-mat.str-el↗

arXiv2013

Enhancement of critical temperatures in disordered bipartite lattices

We study the strong enhancement, induced by random hopping, of the critical temperatures characterizing the transitions to superconductivity, charge-density wave and antiferromagnetism, which can occur in bipartite lattice models at half-filling, like graphene, by means of an extended Finkel'stein non-linear $σ$-model renormalization group approach. We show that, if Cooper channel interaction dominates, superconducting critical temperature can be enhanced at will, since superconductivity cannot be broken by entering any Anderson insulating phase. If, instead, staggered interactions are relevant, antiferromagnetic order is generated by disorder at a temperature well above that expected for a clean system.

cond-mat.dis-nn↗

arXiv2013

Electronic phase separation in iron pnictides

A mechanism for electronic phase separation in iron pnictides is proposed. It is based on the competition between commensurate and incommensurate spin-density-wave phases in a system with an imperfect doping-dependent nesting of a multi-sheeted Fermi surface. We model the Fermi surface by two elliptical electron pockets and three circular hole pockets. The interaction between a charge carrier in a hole band and a carrier in an electron band leads to the formation of spin-density-wave order. The commensurate spin density wave in the parent compound transforms to the incommensurate phase when doping is introduced. We show that, for certain parameter values, the uniform state is unstable with respect to phase separation. The resulting inhomogeneous state consists of regions of commensurate and incommensurate spin-density-wave phases. Our results are in qualitative agreement with recent observations of incommensurate spin density waves and electronic inhomogeneity in iron pnictides.

cond-mat.str-el↗

arXiv2013

Squeezing light with Majorana fermions

Coupling a semiconducting nanowire to a microwave cavity provides a powerfull means to assess the presence or absence of isolated Majorana fermions in the nanowire. These exotic bound states can cause a significant cavity frequency shift but also a strong cavity nonlinearity leading for instance to light squeezing. The dependence of these effects on the nanowire gate voltages gives direct signatures of the unique properties of Majorana fermions, such as their self-adjoint character and their exponential confinement.

cond-mat.mes-hall↗

arXiv2013

The influence of spin-phonon coupling on antiferromagnetic spin fluctuations in FeSe under pressure: the First-principles calculations with van der Waals corrections

The electronic structures, lattice dynamics, and magnetic properties of crystal β-FeSe under hydrostatic pressure have been studied by using the first-principles electronic structure calculations with van der Waals (vdW) corrections. With applied pressures, the energy bands around the Fermi energy level consisting mainly of Fe-3d orbitals show obvious energy shifts and occupation variations, and meanwhile the frequencies of all optical phonon modes increase. Among these phonon modes, the A1g mode, which relates to the Se height from the Fe-Fe plane, shows a clear frequency jump in the range between 5 and 6 GPa. This is also the pressure range within which the highest superconducting transition temperature Tc of FeSe is reached in experiments. In comparison with the other phonon modes, the zero-point vibration of the A1g mode induces the strongest fluctuation of local magnetic moment on Fe under a pressure from 0 to 9 GPa, and the induced fluctuation reaches the maximum around 5 GPa. These results suggest that the effect of phonon via spin-phonon coupling need to be considered when exploring the superconducting mechanism in iron-based superconductors.

cond-mat.supr-con↗

arXiv2012

Shiba impurity bound states as a probe of topological superconductivity and Fermion parity changing quantum phase transitions

Spin-orbit coupled superconductors are potentially interesting candidates for realizing topological and potentially non-Abelian states with Majorana Fermions. We argue that time-reversal broken spin-orbit coupled superconductors generically can be characterized as having sub-gap states that are bound to localized non-magnetic impurities. Such bound states, which are referred to as Shiba states, can be detected as sharp resonances in the tunneling spectrum of the spin-orbit coupled superconductors. The Shiba state resonance can be tuned using a gate-voltage or a magnetic field from being at the edge of the gap at zero magnetic fields to crossing zero energy when the Zeeman splitting is tuned into the topological superconducting regime. The zero-crossing signifies a Fermion parity changing first order quantum phase transition, which is characterized by a Pfaffian topological invariant. These zero-crossings of the impurity level can be used to locally characterize the topological superconducting state from tunneling experiments.

cond-mat.supr-con↗