arXiv subjects

Pouyan Ghaemi

Publications and source records attributed to Pouyan Ghaemi.

53 records · Page 3Linked to original sources

arXiv2013

Tuning thermoelectric power factor by crystal-field and spin-orbit couplings in Kondo lattice materials

We study thermoelectric transport at low temperatures in correlated Kondo insulators, motivated by the recent observation of a high thermoelectric figure of merit(ZT) in $FeSb_2$ at $T \sim 10 K$. Even at room temperature, correlations have the potential to lead to high ZT, as in $YbAl_3$, one of the most widely used thermoelectric metals. At low temperature correlation effects are especially worthy of study because fixed band structures are unlikely to give rise to the very small energy gaps $E_g \sim 5 kT$ necessary for a weakly correlated material to function efficiently at low temperature. We explore the possibility of improving the thermoelectric properties of correlated Kondo insulators through tuning of crystal field and spin-orbit coupling and present a framework to design more efficient low-temperature thermoelectrics based on our results.

cond-mat.mtrl-sci↗

arXiv2012

Stability of edge states in strained graphene

Spatially inhomogeneous strains in graphene can simulate the effects of valley-dependent magnetic fields. As demonstrated in recent experiments, the realizable magnetic fields are large enough to give rise to well-defined flat pseudo-Landau levels, potentially having counter-propagating edge modes. In the present work we address the conditions under which such edge modes are visible. We find that, whereas armchair edges do not support counter-propagating edge modes, zigzag edges do so, through a novel selective-hybridization mechanism. We then discuss effects of interactions on the stability of counter-propagating edge modes, and find that, for the experimentally relevant case of Coulomb interactions, interactions typically decrease the stability of the edge modes. Finally, we generalize our analysis to address the case of spontaneous valley polarization, which is expected to occur in charge-neutral strained graphene.

cond-mat.mes-hall↗

arXiv2012

Stabilization of Majorana modes in vortices in the superconducting phase of topological insulators using topologically trivial bands

If superconductivity is induced in the metallic surface states of topological insulators via proximity, Majorana modes will be trapped on the vortex cores. The same effects hold for doped topological insulators which become bulk s-wave superconductors as long as the doping does not exceed a critical values $ μ^{\pm}_c.$ It is this critical chemical potential at which the material forgets it arose from a band-inverted topological insulator; it loses its topological \emph{imprint.} For the most common classes of topological insulators, which can be modeled with a minimal 4-band Dirac model the values of $μ^{\pm}_c$ can be easily calculated, but for materials with more complicated electronic structures such as HgTe or ScPtBi the result is unknown. We show that due to the hybridization with an additional Kramers' pair of topologically trivial bands the topological imprint of HgTe-like electronic structures (which includes the ternary Heusler compounds) can be widely extended for p-doping. As a practical consequence we consider the effects of such hybridization on the range of doping over which Majorana modes will be bound to vortices in superconducting topological insulators and show that the range is strongly extended for p-doping, and reduced for n-doping. This leaves open the possibility that other topological phenomena may be stabilized over a wider range of doping.

cond-mat.mes-hall↗

arXiv2012

Vortex Lattices in the Superconducting Phases of Doped Topological Insulators and Heterostructures

Majorana fermions are predicted to play a crucial role in condensed matter realizations of topological quantum computation. These heretofore undiscovered quasiparticles have been predicted to exist at the cores of vortex excitations in topological superconductors and in heterostructures of superconductors and materials with strong spin-orbit coupling. In this work we examine topological insulators with bulk s-wave superconductivity in the presence of a vortex-lattice generated by a perpendicular magnetic field. Using self-consistent Bogoliubov-de Gennes, calculations we confirm that beyond the semi-classical, weak-pairing limit that the Majorana vortex states appear as the chemical potential is tuned from either side of the band edge so long as the density of states is sufficient for superconductivity to form. Further, we demonstrate that the previously predicted vortex phase transition survives beyond the semi-classical limit. At chemical potential values smaller than the critical chemical potential, the vortex lattice modes hybridize within the top and bottom surfaces giving rise to a dispersive low-energy mid-gap band. As the chemical potential is increased, the Majorana states become more localized within a single surface but spread into the bulk toward the opposite surface. Eventually, when the chemical potential is sufficiently high in the bulk bands, the Majorana modes can tunnel between surfaces and eventually a critical point is reached at which modes on opposite surfaces can freely tunnel and annihilate leading to the topological phase transition previously studied in the work of Hosur et al.

cond-mat.mes-hall↗

arXiv2012

Designer quantum spin Hall phase transition in molecular graphene

Graphene was the first material predicted to be a time-reversal-invariant topological insulator; however, the insulating gap is immeasurably small owing to the weakness of spin-orbit interactions in graphene. A recent experiment [1] demonstrated that designer honeycomb lattices with graphene-like "Dirac" band structures can be engineered by depositing a regular array of carbon monoxide atoms on a metallic substrate. Here, we argue that by growing such designer lattices on metals or semiconductors with strong spin-orbit interactions, one can realize an analog of graphene with strong intrinsic spin-orbit coupling, and hence a highly controllable two-dimensional topological insulator. We estimate the range of substrate parameters for which the topological phase is achievable, and consider the experimental feasibility of some candidate substrates.

cond-mat.mes-hall↗

arXiv2012

Non-Abelian SU(2) gauge fields through density-wave order and strain in graphene

Spatially varying strain patterns can qualitatively alter the electronic properties of graphene, acting as effective valley-dependent magnetic fields and giving rise to pseudo-Landau-level (PLL) quantization. Here, we show that the strain-induced magnetic field is one component of an SU(2) non-Abelian gauge field within the low-energy theory of graphene, and identify the other two components as period-3 charge-density waves. We show that these density-waves, if spatially varied, give rise to PLL quantization. We also argue that strain-induced magnetic fields can induce density-wave order in graphene, thus dynamically gapping out the lowest PLL; moreover, the ordering should generically be accompanied by dislocations. We discuss experimental signatures of these effects.

cond-mat.mes-hall↗

arXiv2012

Competing orders in the Dirac-like electronic structure and the non-linear sigma model with the topological terms

The Dirac-like electronic structure can host a large number of competing orders in the form of mass terms. In particular, two different order parameters can be said to be dual to each other, when a static defect in one of them traps a quantum number (or "charge") of the other. We discuss that such complementary nature of the pair of the order parameters shows up in their correlation functions and dynamical properties when a quantum phase transition is driven by fluctuations of the one of the order parameters. Approaching the transition from the disordered (paramagnetic) side, the order parameter correlation function at the critical point is reduced, while such fluctuations enhance the correlation of the dual order parameter. Such complementary behaviors in the correlation function can be used to diagnose the nature of quantum fluctuations that is the driving force of the quantum phase transition.

cond-mat.str-el↗

arXiv2011

Near-Zero Modes in Superconducting Graphene

Vortices in the simplest superconducting state of graphene contain very low energy excitations, whose existence is connected to an index theorem that applies strictly to an approximate form of the relevant Bogoliubov-deGennes equations. When Zeeman interactions are taken into account, the zero modes required by the index theorem are (slightly) displaced. Thus the vortices acquire internal structure, that plausibly supports interesting dynamical phenomena.

cond-mat.supr-con↗

arXiv2011

Majorana modes at the ends of superconductor vortices in doped topological insulators

Recent experiments have observed bulk superconductivity in doped topological insulators. Here we ask whether vortex Majorana zero modes, previously predicted to occur when superconductivity is induced on the surface of topological insulators, survive even in these doped systems with metallic normal states. Assuming inversion symmetry, we find that Majorana zero modes indeed appear but only below a critical doping. The critical doping is associated with a topological phase transition of the vortex line, where it supports gapless excitations along its length. The critical point depends only on the orientation of the vortex line, and a Berry phase property, the SU(2) Berry phase of the Fermi surface in the metallic normal state. By calculating this phase for available band structures we determine that materials candidates like $p$-doped Bi$_{2}$Te$_{3}$ under pressure supports vortex end Majorana modes. Surprisingly, even superconductors derived from topologically trivial band structures can support Majorana modes, providing a promising route to realizing them.

cond-mat.supr-con↗

arXiv2010

Anomalous Zeeman response in coexisting phase of superconductivity and spin-density wave as a probe of extended $s$-wave pairing structure in ferro-pnictide

In several members of the ferro-pnictides, spin density wave (SDW) order coexists with superconductivity over a range of dopings. In this letter we study the anomalous magnetic Zeeman response of this coexistence state and show that it can be used to confirm the extended s-wave gap structure as well as structure of superconducting (SC) gap in coexisting phase. On increasing the field, a strongly anisotropic reduction of SC gap is found. The anisotropy is directly connected to the gap structure of superconducting phase. The signature of this effect in quasiparticle interference measured by STM, as well as heat transport in magnetic field is discussed. For the compounds with the nodal SC gap we show that the nodes are removed upon formation of SDW. Interestingly the size of the generated gap in the originally nodal areas is anisotropic in the position of the nodes over the Fermi surface in direct connection with the form of SC pairing.

cond-mat.supr-con↗

arXiv2010

In-plane transport and enhanced thermoelectric performance in thin films of the topological insulators Bi_2Te_3 and Bi_2Se_3

Several small-bandgap semiconductors are now known to have protected metallic surface states as a consequence of the topology of the bulk electron wavefunctions. The known "topological insulators" with this behavior include the important thermoelectric materials Bi_2Te_3 and Bi_2Se_3, whose surfaces are observed in photoemission experiments to have an unusual electronic structure with a single Dirac cone. We study in-plane (i.e., horizontal) transport in thin films made of these materials. The surface states from top and bottom surfaces hybridize, and conventional diffusive transport predicts that the tunable hybridization-induced band gap leads to increased thermoelectric performance at low temperatures. Beyond simple diffusive transport, the conductivity shows a crossover from the spin-orbit induced anti-localization at a single surface to ordinary localization.

cond-mat.mes-hall↗

arXiv2009

The quantum valley Hall effect in proximity-induced superconducting graphene: an experimental window for deconfined quantum criticality

We argue that by inducing superconductivity in graphene via the proximity effect, it is possible to observes the "quantum valley Hall effect". In the presence of magnetic field, supercurrent causes "valley pseudospin" to accumulate at the edges of the superconducting strip. This, and the structure of the superconducting vortex core, provide possibilities to experimentally observe aspects of the "deconfined quantum criticality".

cond-mat.supr-con↗

arXiv2008

Andreev Bound states as a phase sensitive probe of the pairing symmetry of the iron pnictide superconductors

A leading contender for the pairing symmetry in the Fe-pnictide high temperature superconductors is extended s-wave $s_\pm$, a nodeless state in which the pairing changes sign between Fermi surfaces. Verifying such a pairing symmetry requires a special phase sensitive probe that is also momentum selective. We show that the sign structure of $s_\pm$ pairing leads to surface Andreev bound states at the sample edge. In the clean limit they only occur when the edge is along the nearest neighbor Fe-Fe bond, but not for a diagonal edge or a surface orthogonal to the c-axis. In contrast to d-wave Andreev bound states, they are not at zero energy and, in general, do not produce a zero bias tunneling peak. Consequences for tunneling measurements are derived, within a simplified two band model and also for a more realistic five band model.

cond-mat.supr-con↗

arXiv2007

Angle dependent quasiparticle weights in correlated metals

The variation in the quasiparticle weight $Z$ on moving around the fermi surface in correlated metals is studied theoretically. Our primary example is a heavy Fermi liquid treated within the standard hybridization mean field theory. The most dramatic variation in the quasiparticle weight happens in situations where the hybridization vanishes along certain directions in momentum space. Such a "hybridization node" is demonstrated for a simplified model of a Cerium-based cubic heavy electron metal. We show that the quasiparticle weight varies from almost unity in some directions, to values approaching zero in others. This is accompanied by a similar variation in the quasiparticle effective mass. Some consequences of such hybridization nodes and the associated angle dependence are explored. Comparisons to somewhat similar phenomena in the normal metallic state of the cuprate materials are discussed. A phenomenological picture of the pseudogap state in the cuprates with a large Fermi surface with a severely anisotropic spectral weight is explored.

cond-mat.str-el↗

arXiv2006

Higher angular momentum Kondo liquids

Conventional heavy Fermi liquid phases of Kondo lattices involve the formation of a "Kondo singlet" between the local moments and the conduction electrons. This Kondo singlet is usually taken to be in an internal s-wave angular momentum state. Here we explore the possibility of phases where the Kondo singlet has internal angular momentum that is d-wave. Such states are readily accessed in a slave boson mean field formulation, and are energetically favorable when the Kondo interaction is between a local moment and an electron at a nearest neighbor site. The properties of the d-wave Kondo liquid are studied. Effective mass and quasiparticle residue show large angle dependence on the Fermi surface. Remarkably in certain cases, the quasiparticle residue goes to zero at isolated points (in two dimensions) on the Fermi surface. The excitations at these points then include a free fractionalized spinon. We also point out the possibility of quantum Hall phenomena in two dimensional Kondo {\em insulators}, if the Kondo singlet has complex internal angular momentum. We suggest that such d-wave Kondo pairing may provide a useful route to thinking about correlated Fermi liquids with strong anisotropy along the Fermi surface.

cond-mat.str-el↗

arXiv2005

Neel order, quantum spin liquids and quantum criticality in two dimensions

This paper is concerned with the possibility of a direct second order transition out of a collinear Neel phase to a paramagnetic spin liquid in two dimensional quantum antiferromagnets. Contrary to conventional wisdom, we show that such second order quantum transitions can potentially occur to certain spin liquid states popular in theories of the cuprates. We provide a theory of this transition and study its universal properties in an $ε$ expansion. The existence of such a transition has a number of interesting implications for spin liquid based approaches to the underdoped cuprates. In particular it considerably clarifies existing ideas for incorporating antiferromagnetic long range order into such a spin liquid based approach.

cond-mat.str-el↗

arXiv2004

Finite temperature properties of quantum Lifshitz transitions between valence bond solid phases: An example of `local' quantum criticality

We study the finite temperature properties of quantum magnets close to a continuous quantum phase transition between two distinct valence bond solid phases in two spatial dimension. Previous work has shown that such a second order quantum `Lifshitz' transition is described by a free field theory and is hence tractable, but is nevertheless non-trivial. At $T>0$, we show that while correlation functions of certain operators exhibit $ω/T$ scaling, they do not show analogous scaling in space. In particular, in the scaling limit, all such correlators are purely {\em local} in space, although the same correlators at T=0 decay as a power law. This provides a valuable microscopic example of a certain kind of `local' quantum criticality. The local form of the correlations arise from the large density of soft modes present near the transition that are excited by temperature. We calculate exactly the autocorrelation function for such operators in the scaling limit. Going beyond the scaling limit by including irrelevant operators leads to finite spatial correlations which are also obtained.

cond-mat.str-el↗