arXiv subjects

Zlatko Tesanovic

Publications and source records attributed to Zlatko Tesanovic.

At least 19 recordsLinked to original sources

arXiv2013

Pairing instabilities in topological insulator quantum wells

Topological insulator quantum wells with induced attractive interactions between electrons are candidate systems for the realization of novel vortex lattice states with time-reversal symmetry, and incompressible quantum vortex liquids with fractional excitations. We analyze the competition between different pairing channels stimulated by the superconducting proximity effect in these quantum wells, and calculate the helical triplet pairing instability that can produce the mentioned phases using perturbation theory. We discuss the phase diagram tunable by gate voltage.

cond-mat.str-el↗

arXiv2013

Interaction proximity effect at the interface between a superconductor and a topological insulator quantum well

A material whose electrons are correlated can affect electron dynamics across the interface with another material. Such a "proximity effect" can have several manifestations, from order parameter leakage to generated effective interactions. The resulting combination of induced electron correlations and their intrinsic dynamics at the surface of the affected material can give rise to qualitatively new quantum states. For example, the leaking of a superconducting order parameter into certain Rashba spin-orbit-coupled materials has been recently identified as a path to creating "topological superconductors" that can host Majorana particles of use in quantum computing. Here we analyze the other aspects of the superconducting proximity effect. The proximity-induced interactions are a promising path to incompressible quantum liquids with non-Abelian fractional quasiparticles in topological insulator quantum wells, which could also find applications in topological quantum computing. We discuss the operational and design principles of a heterostructure device that could realize such states. We apply field-theoretical methods to characterize the properties of induced interactions via the electron-phonon coupling and Cooper pair tunneling across the interface. We argue that bound-state Cooper pairs can be stabilized by the interaction proximity effect inside a topological insulator quantum well at experimentally observable energy scales. The condensation of spinful triplet pairs, enabled by the Rashba spin-orbit coupling and tunable by gate voltage, would lead to novel superconducting states and fractional topological insulators.

cond-mat.str-el↗

arXiv2013

Fractional topological insulators of Cooper pairs induced by proximity effect

Certain insulating materials with strong spin-orbit coupling can conduct currents along their edges or surfaces. This phenomenon arises from the non-trivial topological properties of the electronic band-structure, and is somewhat similar to the integer quantum Hall effect of electrons in strong magnetic fields. Topological insulators analogous to the fractional quantum Hall effect are also possible, but have not yet been observed in any material. Here we show that a quantum well made from a topological band insulator such as Bi2Se3 or Bi2Te3, placed in contact with a superconductor, can be used to realize a two-dimensional topological state with macroscopic many-body quantum entanglement whose excitations carry fractional amounts of electron's charge and spin. This fractional topological insulator is a "pseudogap" state of induced spinful p-wave Cooper pairs, fundamentally significant to quantum field theory and applicable to spintronic devices and quantum computing.

cond-mat.str-el↗

arXiv2013

Theory of Charge Order and Heavy-Electron Formation in the Mixed-Valence Compound KNi$_2$Se$_2$

The material KNi$_2$Se$_2$ has recently been shown to possess a number of striking physical properties, many of which are apparently related to the mixed valency of this system, in which there is on average one quasi-localized electron per every two Ni sites. Remarkably, the material exhibits a charge density wave (CDW) phase that disappears upon cooling, giving way to a low-temperature coherent phase characterized by an enhanced electron mass, reduced resistivity, and an enlarged unit cell free of structural distortion. Starting from an extended periodic Anderson model and using the slave-boson formulation, we develop a model for this system and study its properties within mean-field theory. We find a reentrant first-order transition from a CDW phase, in which the localized moments form singlet dimers, to a heavy Fermi liquid phase as temperature is lowered. The magnetic susceptibility is Pauli-like in both the high- and low-temperature regions, illustrating the lack of a single-ion Kondo regime such as that usually found in heavy-fermion materials.

cond-mat.str-el↗

arXiv2012

Dimer Impurity Scattering, "Reconstructed" Nesting and Density-Wave Diagnostics in Iron Pnictides

While the impurity-induced nanoscale electronic disorder has been extensively reported in the underdoped iron pnictides, its microscopic origins remain elusive. Recent scanning tunneling microscopy (STM) measurements reveal a dimer-type resonant structure induced by cobalt doping. These dimers are randomly distributed but uniformly aligned with the antiferromagnetic a axis. A theory of the impurity-induced quasiparticle interference patterns is presented that shows the local density of states developing an oscillatory pattern characterized by both geometry and orbital content of the {\em reconstructed} Fermi pockets, occasioned by the pocket density-wave (PoDW) order along the b axis. This pattern breaks the $C_4$ symmetry and its size and orientation compare well with the dimer resonances found in the STM experiments, hinting at the presence of a "hidden" PoDW order. More broadly, our theory spotlights such nanoscale structures as a useful diagnostic tool for various forms of order in iron pnictides.

cond-mat.supr-con↗

arXiv2012

s-wave Cooper pair insulators and theory of correlated superconductors

The pseudogap state of cuprate high-temperature superconductors has been often viewed as either a yet unknown competing order or a precursor state to superconductivity. While awaiting the resolution of the pseudogap problem in cuprates, we demonstrate that local pairing fluctuations, vortex liquid dynamics and other precursor phenomena can emerge quite generally whenever fermionic excitations remain gapped across the superconducting transition, regardless of the gap origin. Our choice of a tractable model is a lattice band insulator with short-range attractive interactions between fermions in the s-wave channel. An effective crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensate (BEC) regimes can be identified in any band insulator above two dimensions, while in two dimensions only the BEC regime exists. The superconducting transition is "unconventional" (non-pair-breaking) in the BEC regime, identified by either the bosonic mean-field or XY universality class. The insulator adjacent to the superconductor in the BEC regime is a bosonic Mott insulator of Cooper pairs, which may be susceptible to charge density wave ordering. We construct a function of the many-body excitation spectrum whose non-analytic changes define a sharp distinction between band and Mott insulators. The corresponding "second order transition" can be observed out of equilibrium by driving a Cooper pair laser in the Mott insulator. We explicitly show that the gap for charged bosonic excitations lies below the threshold for Cooper pair breakup in any BEC regime, despite quantum fluctuations. Our discussion ends with a view of possible consequences for cuprates, where antinodal pair dynamics has certain features in common with our simple s-wave picture.

cond-mat.supr-con↗

arXiv2012

Quantum Criticality for Extended Nodes on a Bethe Lattice in the Large Connectivity Limit

Theoretical description of anisotropic systems, such as layered superconductors and coupled spin chains, is often a challenge due to the different natures of interactions along different directions. As a model of such a system, we present an analytical study of d-dimensional "nodes" arranged as the vertices of a Bethe lattice, where each node has nonzero spatial dimension and is described by an O(N) quantum rotor model, and there is hopping between neighboring nodes. In the limit of large connectivity on the Bethe lattice, the hopping can be treated by constructing a self-consistent effective action for a single node. This procedure is akin to dynamical mean field theory, but generalized so that spatial as well as quantum fluctuations are taken into account on each node. The quantum phase transition is studied using this effective action for both infinite and finite N. The importance of the Perron-Frobenius uniform mode on the Bethe lattice is discussed, and its elimination via an "infinite range hopping" term shifts the transition, leading to nontrivial critical behavior. We calculate critical exponents and find that the internode hopping reduces the upper and lower critical dimensions each by one, indicating that--at least for the purposes of quantum criticality--the large number of internode couplings is similar to adding a single extra dimension to the theory describing a single node.

cond-mat.str-el↗

arXiv2011

Isolated Vortex and Vortex Lattice in a Holographic p-wave Superconductor

Using the holographic gauge-gravity duality, we find a solution for an isolated vortex and a vortex lattice in a 2+1 dimensional p-wave superconductor, which is described by the boundary theory dual to an SU(2) gauge theory in 3+1 dimensional anti-de Sitter space. Both $p_x+ip_y$ and $p_x-ip_y$ components of the superconducting order parameter, as well as the effects of a magnetic field on these components, are considered. The isolated vortex solution is studied, and it is found that the two order parameter components have different amplitudes due to the time reversal symmetry breaking. The vortex lattice for large magnetic fields is also studied, where it is argued that only one order parameter component will be nonzero sufficiently close to the upper critical field. The upper critical field exhibits a characteristic upward curvature, reflecting the effects of field-induced correlations captured by the holographic theory. The free energy is calculated perturbatively in this region of the phase diagram, and it is shown that the triangular vortex lattice is the thermodynamically preferred solution.

hep-th↗

arXiv2011

Robust accidental nodes and zeroes and critical quasiparticle scaling in iron-based multiband superconductors

We study multigap superconductivity, with strong angular variations of one of the gaps, as appropriate for certain iron-based high-temperature superconductors. We solve the gap equations of this model and find that the nodes or zeroes in the gap function present at Tc - although purely accidental -- typically survive down to T=0. Based on this result, we investigate the line of quantum transitions at which gap zeroes first appear. The peculiar "zero-point" critical scaling emanating from this line dominates quasiparticle thermodynamics and transport properties over much of the phase diagram, and supplants more familiar forms of scaling associated with accidental nodes.

cond-mat.supr-con↗

arXiv2011

Theory of Valley-Density Wave and Hidden Order in Iron-Pnictides

In the limit of perfect nesting, the physics of iron-pnictides is governed by the density wave formation at the zone-edge vector M. At high energies, various spin- (SDW), charge- (CDW), orbital/pocket- (PDW) density waves, and their linear combinations, all appear equally likely, unified within the unitary order parameter of U(4)XU(4) symmetry. Nesting imperfections and low-energy interactions reduce this symmetry to that of real materials. Nevertheless, the generic ground state preserves a distinct signature of its highly symmetric origins: a SDW along one axis of the iron lattice is predicted to coexist with a perpendicular PDW, accompanied by weak charge currents. This "hidden" order induces the structural transition in our theory, naturally insures T_s >= T_N, and leads to orbital ferromagnetism and other observable consequences.

cond-mat.supr-con↗

arXiv2010

Large $D-2$ Theory of Superconducting Fluctuations in a Magnetic Field and its Application to Iron-Pnictides

A Ginzburg-Landau approach to fluctuations of a layered superconductor in a magnetic field is used to show that the interlayer coupling can be incorporated within an interacting self-consistent theory of a single layer, in the limit of a large number of neighboring layers. The theory exhibits two phase transitions -- a vortex liquid-to-solid transition is followed by a Bose-Einstein condensation into the Abrikosov lattice -- illustrating the essential role of interlayer coupling. Using this theory, explicit expressions for magnetization, specific heat, and fluctuation conductivity are derived. We compare our results with recent experimental data on the iron-pnictide superconductors.

cond-mat.supr-con↗

arXiv2010

Three-band superconductivity and the order parameter that breaks time-reversal symmetry

We consider a model of multiband superconductivity, inspired by iron pnictides, in which three bands are connected via repulsive pair-scattering terms. Generically, three distinct superconducting states arise within such a model. Two of them are straightforward generalizations of the two-gap order parameter while the third one corresponds to a time-reversal symmetry breaking order parameter, altogether absent within the two-band model. Potential observation of such a genuinely frustrated state would be a particularly vivid manifestation of the repulsive interactions being at the root of iron-based high temperature superconductivity. We construct the phase diagram of this model and discuss its relevance to the iron pnictides family of high temperature superconductors. We also study the case of the Josephson coupling between a two-band s' (or extended s-wave) superconductor and a single-gap s-wave superconductor, and the associated phase diagram.

cond-mat.supr-con↗

arXiv2009

Valley density-wave and multiband superconductivity in Fe-pnictides

The key feature of the Fe-based superconductors is their quasi 2D multiband Fermi surface. By relating the problem to a negative U Hubbard model and its superconducting ground state, we show that the defining instability of such a Fermi surface is the valley density-wave (VDW), a combined spin/charge density-wave at the wavevector connecting the electron and hole valleys. As the valley parameters change by doping or pressure, the fictitious superconductor experiences "Zeeman splitting", eventually going into a non-uniform "Fulde-Ferrell-Larkin-Ovchinikov" (FFLO) state, an itinerant and often incommensurate VDW of the real world, characterized by the metallic conductivity from the ungapped remnants of the Fermi surface. When "Zeeman splitting" exceeds the "Chandrasekhar-Clogston" limit, the "FFLO" state disappears, and the VDW is destabilized. Near this point, the VDW fluctuations and interband pair repulsion are essential ingredients of high-Tc superconductivity in Fe-pnictides.

cond-mat.supr-con↗

arXiv2009

Multiband magnetism and superconductivity in Fe-based compounds

Recent discovery of high T_c superconductivity in Fe-based compounds may have opened a new pathway to the room temperature superconductivity. The new materials feature FeAs layers instead of the signature CuO_2 planes of much-studied cuprates. A model Hamiltonian describing FeAs layers is introduced, highlighting the crucial role of puckering of As atoms in promoting d-electron itinerancy and warding off large local-moment magnetism of Fe ions, the main enemy of superconductivity. Quantum many-particle effects in charge, spin and multiband channels are explored and a nesting-induced spin density-wave order is found in the parent compund. We argue that this largely itinerant antiferromagnetism and high T_c itself are essentially tied to the multiband nature of the Fermi surface.

cond-mat.supr-con↗

arXiv2008

Spin Fluctuation Dynamics and Multiband Superconductivity in Iron Pnictides

Multiband superconductivity, involving resonant pair scattering between different bands, has emerged as a possible explanation of some of the main characteristics of the recently discovered iron pnictides. A key feature of such interband pairing mechanism is that it can generate or enhance superconducting pairing irrespective of whether it is attractive or repulsive. The latter case typically leads to the superconducting gap switching its sign among different sections of the Fermi surface. In iron pnictides, the natural scenario is that the gap changes sign between the hole and the electron Fermi surfaces. However, the macroscopic symmetry of such an extended s'-wave state still belongs to the general s-wave category, raising the question of how to distinguish it from an ordinary s-wave. In such a quest, it is essential to use experimental techniques that have a momentum space resolution and can probe momenta of order M, the wavevector that separates the hole and the electron Fermi surfaces in the Brillouin zone. Here we study experimental signatures in the spin fluctuation dynamics of the fully-gapped s- and s'-wave superconducting states, as well as those of the nodal d- and p-wave. The coupling between spin fluctuations of the incipient nearly-nested spin density-wave (SDW) and the Bogoliubov-deGennes quasiparticles of the superconducting state leads to the Landau-type damping of the former. The intrinsic structure of the superconducting gap leaves a distinctive signature in the form of this damping, allowing it to be used to diagnose the nature of iron-based superconductivity in neutron scattering and other experiments sensitive to spin fluctuations in momentum space. We also discuss the coexistence between superconductivity and SDW order.

cond-mat.supr-con↗

arXiv2008

Restoration of the magnetic hc/e-periodicity in unconventional superconductors

We consider the energy of the filled quasiparticle's Fermi sea of a macroscopic superconducting ring threaded by an hc/2e-vortex, when the material of the ring is of an unconventional pairing symmetry. The energy relative to the one for the hc/e-vortex configuration is finite, positive, and inversely proportional to ring's inner radius. We argue that the existence of this energy in unconventional superconductors removes the commonly assumed degeneracy between the odd and the even vortices, with the loss of the concomitant hc/2e periodicity in external magnetic field as a consequence. This macroscopic quantum effect should be observable in nanosized unconventional superconductors with a small phase stiffness, such as deeply underdoped YBCO with Tc < 5K.

cond-mat.supr-con↗

arXiv2007

Emergence of Cooper pairs, d-wave duality and the phase diagram of cuprate superconductors

BCS theory describes the formation of Cooper pairs and their instant "Bose condensation" into a superconducting state. Helium atoms are preformed bosons and, in addition to their condensed superfluid state, can also form a quantum solid, lacking phase-coherence. Here we show that the fate of Cooper pairs can be more varied than the BCS or helium paradigms. In copper-oxide d-wave superconductors (dSC) Cooper pairs are intrinsically non-local objects, with both center-of-mass and relative motions. As doping decreases, the center-of mass fluctuations force a correlated dSC into a state with enhanced diamagnetism and robust but short-ranged superconducting order. At extreme underdoping, the relative fluctuations take over and two pseudogaps -- ``small'' (charge) and ``large'' (spin) -- emerge naturally from the theory, as Cooper pairs ``disintegrate'' and charge ``detaches'' from spin-singlet bonds. The ensuing ground state(s) are governed by diagonal (mostly antiferromagnetic) rather than by superconducting (off-diagonal) correlations. The theory is used to account for several recent experiments and to draw general conclusions about the phase diagram.

cond-mat.str-el↗

arXiv2006

Mixed state of a lattice d-wave superconductor

We study the mixed state in an extreme type-II lattice d-wave superconductor in the regime of intermediate magnetic fields H_{c1} << H << H_{c2}. We analyze the low energy spectrum of the problem dominated by nodal Dirac-like quasiparticles with momenta near k_F=(\pm k_D,\pm k_D) and find that the spectrum exhibits characteristic oscillatory behavior with respect to the product of k_D and magnetic length l. The Simon-Lee scaling, predicted in this regime, is satisfied only on average, with the magnitude of the oscillatory part of the spectrum displaying the same 1/l dependence as its monotonous ``envelope'' part. The oscillatory behavior of the spectrum is due to the inter-nodal interference enhanced by the singular nature of the low energy eigenfunctions near vortices. We also study a separate problem of a single vortex piercing an isolated superconducting grain of size L by L. Here we find that the periodicity of the quasiparticle energy oscillations with respect to k_D L is doubled relative to the case where the field is zero and the vortex is absent, both such oscillatory behaviors being present at the leading order in 1/L. Finally, we review the overall features of the tunneling conductance experiments in YBCO and BSCCO, and suggest an interpretation of the peaks at 5-20 meV observed in the tunneling local density of states in these materials.

cond-mat.supr-con↗