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Sep 22, 2013
09/13

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M. Franz

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This is a nontechnical Perspective Commentary on the recent developments in cuprate STM and their ramifications for the theories of the pseudogap state, focusing on the paper by Hanaguri et al. [Nature 430, 1001 (2004)].

Source: http://arxiv.org/abs/cond-mat/0409431v1

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Sep 22, 2013
09/13

by
M. Franz

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A critical overview is given, aimed at non-specialist audience, of the recent efforts to detect and manipulate Majorana fermions in solid state devices. It is argued that the experiments on semiconductor quantum wires proximity coupled to superconducting leads present tantalizing hints but not a definitive evidence for these elusive particles.

Source: http://arxiv.org/abs/1302.3641v2

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Sep 20, 2013
09/13

by
M. Franz

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A continuum version of the vortex-boson duality in (3+1) dimensions is formulated and its implications studied in the context of a pair Wigner crystal in underdoped cuprate superconductors. The dual theory to a phase fluctuating superconductor (or superfluid) is shown to be a theory of bosonic strings interacting through a Kalb-Ramond rank-2 tensorial gauge field. String condensation produces Higgs mass for the gauge field and the expected Wigner crystal emerges as an interesting space-time...

Source: http://arxiv.org/abs/cond-mat/0607310v2

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Mar 1, 2016
03/16

by
M Franz Kohl

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Sep 24, 2013
09/13

by
A. Iyengar; M. Franz

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A variety of experiments suggest that in the cuprates, the low-energy superconducting quasiparticles undergo forward scattering from extended impurity potentials. We argue that when such potentials dominate the scattering, the penetration depth may be computed in a simple zero-angle scattering approximation (ZSA), in which the vertex corrections to the Meissner effect may be summed exactly. We find a remarkably simple relationship between the normal fluid density and the quasiparticle density...

Source: http://arxiv.org/abs/cond-mat/0412261v1

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Jun 29, 2018
06/18

by
Anffany Chen; M. Franz

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We study the proximity effect between an s-wave superconductor (SC) and the surface states of a Weyl semimetal. An interesting two-dimensional SC forms in such an interface with properties resembling in certain aspects the Fu-Kane superconductor with some notable differences. In a Weyl semimetal with unbroken time reversal symmetry the interface SC supports completely flat Majorana bands in a linear Josephson junction with a {\pi} phase difference. We discuss stability of these bands against...

Topics: Mesoscale and Nanoscale Physics, Superconductivity, Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1601.01727

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Sep 23, 2013
09/13

by
C. Weeks; M. Franz

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We construct a simple model for electrons in a three-dimensional crystal where a combination of short-range hopping and spin-orbit coupling results in nearly flat bands characterized by a non-trivial Z2 topological index. The flat band is separated from other bands by a bandgap much larger than the bandwidth. We discuss the fate of the many-body ground state of electrons in the flat band in the presence of repulsive interactions at partial filling and conjecture that it may become a...

Source: http://arxiv.org/abs/1111.1447v2

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Sep 18, 2013
09/13

by
M. Franz; Z. Tesanovic

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A novel singular gauge transformation is developed for quasiparticles in the mixed state of a strongly type-II superconductor which permits a full solution of the problem at low and intermediate fields. For a periodic vortex lattice the natural low-energy quasiparticle states are Bloch waves rather than Landau levels discussed in the recent literature. The new representation elucidates the physics considerably and provides fresh insights into the spectral properties of such systems.

Source: http://arxiv.org/abs/cond-mat/9903152v3

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Sep 19, 2013
09/13

by
M. Franz; Z. Tesanovic

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We discuss the evolution of the local quasiparticle spectral density and the related tunneling conductance measurable by the scanning tunneling microscope, as a function of distance r and angle \theta from the vortex core in a d_{x^2-y^2} superconductor. We consider the effects of electronic disorder and of a strongly anisotropic tunneling matrix element, and show that in real materials they will likely obscure the ~1/r asymptotic tail in the zero-bias tunneling conductance expected from the...

Source: http://arxiv.org/abs/cond-mat/9812126v1

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Jul 20, 2013
07/13

by
M. Franz; O. Vafek

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We formulate an effective low energy theory for the fermionic excitations in d-wave superconductors in the presence of periodic vortex lattices. These can be modeled by an effective free Dirac Hamiltonian with renormalized velocities and possibly a small mass term. In the presence of random nonmagnetic impurities this will result in universal (i.e. field and disorder strength independent) thermal and spin conductivities with values different from those occurring in the Meissner state.

Source: http://arxiv.org/abs/cond-mat/0107521v2

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Sep 22, 2013
09/13

by
G. Rosenberg; M. Franz

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It has been noted a long time ago that a term of the form theta (e^2/2\pi h) B dot E may be added to the standard Maxwell Lagrangian without modifying the familiar laws of electricity and magnetism. theta is known to particle physicists as the 'axion' field and whether or not it has a nonzero expectation value in vacuum remains a fundamental open question of the Standard Model. A key manifestation of the axion term is the Witten effect: a unit magnetic monopole placed inside a medium with...

Source: http://arxiv.org/abs/1001.3179v3

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Sep 19, 2013
09/13

by
M. Franz; Z. Tesanovic

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The idea of spin-charge separation in cuprate superconductors has been recently energized by Senthil and Fisher who formulated a Z_2 gauge theory and, within its context, proposed a ``vison detection'' experiment as a test for topological order in a sample with multiply connected geometry. Here we show that the same experiment can be performed to test for the spin-charge separation in U(1) [but not in SU(2)] theory and argue that vortex core spectroscopy can in fact distinguish between the...

Source: http://arxiv.org/abs/cond-mat/0010436v1

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Sep 17, 2013
09/13

by
Ion Garate; M. Franz

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We derive compact analytical expressions for the coupled spin-charge susceptibility of a clean helical metal at the surface of a three dimensional topological insulator (TI). These expressions lead to unconventional non-collinear RKKY interactions between two impurity magnetic moments placed on the surface of a TI, and predict the generation of electric currents by time-dependent magnetic moments. We determine the influence of gate and bias voltages on the interlayer exchange coupling between...

Source: http://arxiv.org/abs/1003.2008v1

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Sep 23, 2013
09/13

by
A. Cook; M. Franz

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A topological insulator nanowire, proximity-coupled to an ordinary bulk s-wave superconductor and subject to a longitudinal applied magnetic field, is shown to realize a one-dimensional topological superconductor with unpaired Majorana fermions localized at both ends. This situation occurs under a wide range of conditions and constitutes what is possibly the most easily accessible physical realization of the elusive Majorana particles in a solid-state system.

Source: http://arxiv.org/abs/1105.1787v2

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Sep 22, 2013
09/13

by
Ion Garate; M. Franz

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We present a theoretical study of interacting electron-hole pairs located on a magnetized surface of a strong topological insulator (TI). The excitonic energy levels and the optical absorption on such surface display unique and potentially measurable features such as (i) an enhanced binding energy for excitons whose total angular momentum is aligned with the magnetic exchange field, (ii) a stark dependence of the optical absorption on the direction of the magnetic exchange field as well as on...

Source: http://arxiv.org/abs/1103.5059v2

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Sep 21, 2013
09/13

by
M. Franz; S. Teitel

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Monte Carlo simulations of 2D vortex lattice melting in a thin superconducting film (or alternatively an array of Josephson junctions) are performed in the London limit. Finite size scaling analyses are used to make a detailed test of the dislocation mediated melting theory of KTNHY. We find that the melting transition is weakly first order, with a jump in the shear modulus very close to that predicted by the KTNHY theory. No hexatic liquid phase is found.

Source: http://arxiv.org/abs/cond-mat/9403071v3

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May 5, 2022
05/22

by
M. Franz; M. Döring

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Sep 18, 2013
09/13

by
M. Franz; M. Ichioka

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In a recent Letter Morita, Kohmoto and Maki analyzed the structure of quasiparticle states near a single vortex in a d-wave superconductor using an approximate version of the Bogoliubov - de Gennes theory. Their principal result is the existence of a bound state within the core region at finite energy with full rotational symmetry, which they assert explains the recent scanning tunneling microscopy results on YBCO single crystals. Here we argue that the approximation used in this work is...

Source: http://arxiv.org/abs/cond-mat/9710243v1

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Sep 18, 2013
09/13

by
M. Franz; Z. Tešanović

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We investigate quasiparticle states associated with an isolated vortex in a d-wave superconductor using a self-consistent Bogoliubov-de Gennes formalism. For a pure $d_{x^2-y^2}$ superconductor we find that there exist no bound states in the core; all the states are extended with continuous energy spectrum. This result is inconsistent with the existing experimental data on cuprates. We propose an explanation for this data in terms of a magnetic-field-induced $d_{x^2-y^2}+id_{xy}$ state recently...

Source: http://arxiv.org/abs/cond-mat/9710258v2

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Jun 28, 2018
06/18

by
Tianyu Liu; M. Franz

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Certain types of topological superconductors and superfluids are known to host protected Majorana zero modes in cores of Abrikosov vortices. When such vortices are arranged in a dense periodic lattice one expects zero modes from neighboring vortices to hybridize and form dispersing bands. Understanding the structure of these bands is essential for the schemes that aim to employ the zero modes in quantum computation applications and in studies of their strongly interacting phases. We investigate...

Topics: Superconductivity, Mesoscale and Nanoscale Physics, Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1506.05084

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Sep 23, 2013
09/13

by
G. Rosenberg; M. Franz

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We show that a three dimensional topological insulator doped with magnetic impurities in the bulk can have a regime where the surface is magnetically ordered but the bulk is not. This is in contrast to conventional materials where bulk ordered phases are typically more robust than surface ordered phases. The difference originates from the topologically protected gapless surface states characteristic of topological insulators. We study the problem using a mean field approach in two concrete...

Source: http://arxiv.org/abs/1202.1850v1

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Sep 18, 2013
09/13

by
M. Franz; Z. Tesanovic

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Within the phase fluctuation model for the pseudogap state of cuprate superconductors we identify a novel statistical "Berry phase" interaction between the nodal quasiparticles and fluctuating vortices. The effective action describing this model assumes the form of an anisotropic Euclidean quantum electrodynamics in (2+1) dimensions (QED_3) and naturally generates the marginal Fermi liquid behavior for its fermionic excitations. The doping axis in the x-T phase diagram emerges as a...

Source: http://arxiv.org/abs/cond-mat/0012445v4

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Sep 19, 2013
09/13

by
C. Weeks; M. Franz

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We explore the possible particle-hole instabilities that can arise in a system of massless Dirac fermions on both the honeycomb and pi-flux square lattices with short range interactions. Through analytical and numerical studies we show that these instabilities can result in a number of interesting phases. In addition to the previously identified charge and spin density wave phases and the exotic `quantum anomalous Hall' (Haldane) phase, we establish the existence of the dimerized Kekule phase...

Source: http://arxiv.org/abs/0910.3450v1

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Sep 21, 2013
09/13

by
B. Seradjeh; M. Franz

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We show that fractional charges bound to topological defects in the recently proposed time-reversal-invariant models on honeycomb and square lattices obey fractional statistics. The effective low-energy description is given in terms of a `doubled' level-2 Chern-Simons field theory, which is parity and time-reversal invariant and implies two species of semions (particles with statistical angle pi/2) labeled by a new emergent quantum number that we identify as the fermion axial charge.

Source: http://arxiv.org/abs/0709.4258v1

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Sep 18, 2013
09/13

by
M. Franz; Z. Tesanovic

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We analyze the recent vortex core spectroscopy experiments in cuprate superconductor and discuss what can be learned from them about the nature of the ground state in these compounds. We argue that the data are inconsistent with the assumption of a simple metallic ground state and exhibit characteristics of a doped Mott insulator. A theory for a vortex core in such a doped Mott insulator is developed based on the U(1) gauge field slave boson model and is shown to exhibit properties...

Source: http://arxiv.org/abs/cond-mat/0002137v2

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Sep 22, 2013
09/13

by
M. Franz; S. Teitel

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We carry out MC studies of 2D superconducting networks, in an applied magnetic field, for square and honeycomb geometries. We consider both dilute systems (f=1/q) and systems near full frustration (f=1/2-1/q). For the dilute case (which models a film as q->infinity), we find two transitions: at T_c(f)~1/q there is a depinning transition from a pinned to a floating vortex lattice; at T_m(f)~constant the floating vortex lattice melts into an isotropic liquid. We analyze this melting according...

Source: http://arxiv.org/abs/cond-mat/9409007v1

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Sep 22, 2013
09/13

by
C. Weeks; M. Franz

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Electrons hopping on the sites of a two-dimensional Lieb lattice and three-dimensional edge centered cubic (perovskite) lattice are shown to form topologically non-trivial insulating phases when spin-orbit coupling is introduced. These simple models on lattices with cubic symmetry show a Dirac-like structure in the excitation spectrum but with the unusual feature that there is a dispersionless band through the center of the spectrum and only a single Dirac cone per Brillouin zone.

Source: http://arxiv.org/abs/1004.5172v1

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Sep 19, 2013
09/13

by
T. Pereg-Barnea; M. Franz

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Within the framework of the QED3 theory of cuprates it is argued that bulk antiferromagnetic (AF) order can coexist with d-wave superconductivity in the underdoped region, in agreement with recent experiments. The AF order arises from the phase fluctuating d-wave superconductor via the mechanism of spontaneous chiral symmetry breaking, provided that fluctuations are sufficiently strong. The phase diagram for this coexistence is mapped out by means of analytical and numerical solutions of the...

Source: http://arxiv.org/abs/cond-mat/0209301v2

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Sep 23, 2013
09/13

by
M. M. Vazifeh; M. Franz

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The Lagrangian describing the bulk electromagnetic response of a three-dimensional strong topological insulator contains a topological `axion' term of the form '\theta E dot B'. It is often stated (without proof) that the corresponding action is quantized on periodic space-time and therefore invariant under '\theta -> \theta +2\pi'. Here we provide a simple, physically motivated proof of the axion action quantization on the periodic space-time, assuming only that the vector potential is...

Source: http://arxiv.org/abs/1006.3355v2

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Sep 18, 2013
09/13

by
M. M. Vazifeh; M. Franz

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The surface of a topological insulator hosts a very special form of a quasi-two dimensional metallic system when it is embedded in a topologically trivial medium like the vacuum. The electronic properties of this unusual 2D metal are distinct in many aspects from both the conventional two-dimensional electron gas systems in quantum well heterostructures as well as those of a single layer graphene. In this paper, we study one of these distinct features i.e., the response of the electronic spins...

Source: http://arxiv.org/abs/1201.2424v1

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Sep 22, 2013
09/13

by
H. -M. Guo; M. Franz

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Electrons hopping on the sites of a three-dimensional pyrochlore lattice are shown to form topologically non-trivial insulating phases when the spin-orbit (SO) coupling and lattice distortions are present. Of 16 possible topological classes 9 are realized for various parameters in this model. Specifically, at half-filling undistorted pyrochlore lattice with SO term yields a 'pristine' strong topological insulator with Z2 index (1;000). At quarter filling various strong and weak topological...

Source: http://arxiv.org/abs/0908.0698v2

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Sep 18, 2013
09/13

by
T. Pereg-Barnea; M. Franz

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Electrons, when scattered by static random disorder, form standing waves that can be imaged using scanning tunneling microscopy. Such interference patterns, observable by the recently developed technique of Fourier transform scanning tunneling spectroscopy (FT-STS), are shown to carry unique fingerprints characteristic of the electronic order present in a material. We exploit this feature of the FT-STS technique to propose a test for the nature of the enigmatic pseudogap phase in the high-$T_c$...

Source: http://arxiv.org/abs/cond-mat/0401594v2

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Jul 20, 2013
07/13

by
T. Pereg-Barnea; M. Franz

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When quantum fluctuations in the phase of the superconducting order parameter destroy the off-diagonal long range order, duality arguments predict the formation of a Cooper pair crystal. This effect is thought to be responsible for the static checkerboard patterns observed recently in various underdoped cuprate superconductors by means of scanning tunneling spectroscopy. Breaking of the translational symmetry in such a Cooper pair Wigner crystal may, under certain conditions, lead to the...

Source: http://arxiv.org/abs/cond-mat/0604559v3

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Sep 22, 2013
09/13

by
M. Franz; A. J. Millis

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We consider the effect of classical phase fluctuations on the quasiparticle spectra of underdoped high-$T_c$ cuprate superconductors in the pseudogap regime above $T_c$. We show that photoemission and tunneling spectroscopy data are well accounted for by a simple model in which mean field d-wave quasiparticles are semiclasically coupled to supercurrents induced by fluctuating unbound vortex-antivortex pairs. We argue that the data imply that transverse phase fluctuations are important at...

Source: http://arxiv.org/abs/cond-mat/9805401v2

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51

Sep 18, 2013
09/13

by
M. Franz; A. P. Iyengar

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A new phenomenology is proposed for the superfluid density of strongly underdoped cuprate superconductors based on recent data for ultra-clean single crystals of YBCO. The data feature a puzzling departure from Uemura scaling and a decline of the slope as the T_c = 0 quantum critical point is approached. We show that this behavior can be understood in terms of the renormalization of quasiparticle effective charge by quantum fluctuations of the superconducting phase as described by a...

Source: http://arxiv.org/abs/cond-mat/0504735v3

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Sep 19, 2013
09/13

by
H. -M. Guo; M. Franz

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Electrons on the surface of a strong topological insulator, such as Bi2Te3 or Bi1-xSnx, form a topologically protected helical liquid whose excitation spectrum contains an odd number of massless Dirac fermions. A theoretical survey and classification is given of the universal features, observable by the ordinary and spin-polarized scanning tunneling spectroscopy, in the interference patterns resulting from the quasiparticle scattering by magnetic and non-magnetic impurities in such a helical...

Source: http://arxiv.org/abs/0910.4946v2

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Sep 23, 2013
09/13

by
M. M. Vazifeh; M. Franz

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It has been suggested recently, based on subtle field-theoretical considerations, that the electromagnetic response of Weyl semimetals and the closely related Weyl insulators can be characterized by an axion term E.B with space and time dependent axion angle. Here we construct a minimal lattice model of the Weyl medium and study its electromagnetic response by a combination of analytical and numerical techniques. We confirm the existence of the anomalous Hall effect expected on the basis of the...

Source: http://arxiv.org/abs/1303.5784v2

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Jun 30, 2018
06/18

by
S. R. Elliott; M. Franz

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Ettore Majorana (1906-1938) disappeared while traveling by ship from Palermo to Naples in 1938. His fate has never been fully resolved and several articles have been written that explore the mystery itself. His demise intrigues us still today because of his seminal work, published the previous year, that established symmetric solutions to the Dirac equation that describe a fermionic particle that is its own anti-particle. This work has long had a significant impact in neutrino physics, where...

Topics: High Energy Physics - Experiment, Superconductivity, Mesoscale and Nanoscale Physics, Nuclear...

Source: http://arxiv.org/abs/1403.4976

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Sep 20, 2013
09/13

by
T. Pereg-Barnea; M. Franz

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Quasiparticle interference in a d-wave superconductor with weak disorder produces distinctive peaks in the Fourier-transformed local density of states measured by scanning tunneling spectroscopy. We predict that amplitudes of these peaks can be enhanced or suppressed by applied magnetic field according to a very specific pattern governed by the symmetry of the superconducting order parameter. This calculated pattern agrees with the recent experimental measurement and suggests that the technique...

Source: http://arxiv.org/abs/0802.3936v2

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55

Sep 18, 2013
09/13

by
T. Pereg-Barnea; M. Franz

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A method is proposed to test for the nature of the pseudogap phase in cuprates using the recently developed technique of Fourier transform scanning tunneling spectroscopy. We show that the observed quasiparticle interference patterns depend critically on the quasiparticle coherence factors, making it possible to distinguish between the pseudogap dominated by superconducting fluctuations and by various particle-hole condensates.

Source: http://arxiv.org/abs/cond-mat/0306712v2

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Sep 21, 2013
09/13

by
H. -M. Guo; M. Franz

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Itinerant electrons in a two-dimensional Kagome lattice form a Dirac semi-metal, similar to graphene. When lattice and spin symmetries are broken by various periodic perturbations this semi-metal is shown to spawn interesting non-magnetic insulating phases. These include a two-dimensional topological insulator with a non-trivial Z_2 invariant and robust gapless edge states, as well as dimerized and trimerized `Kekule' insulators. The latter two are topologically trivial but the Kekule phase...

Source: http://arxiv.org/abs/0905.3385v2

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Sep 18, 2013
09/13

by
B. Seradjeh; H. Weber; M. Franz

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A real-space formulation is given for the recently discussed exciton condensate in a symmetrically biased graphene bilayer. We show that in the continuum limit an oddly-quantized vortex in this condensate binds exactly one zero mode per valley index of the bilayer. In the full lattice model the zero modes are split slightly due to intervalley mixing. We support these results by an exact numerical diagonalization of the lattice Hamiltonian. We also discuss the effect of the zero modes on the...

Source: http://arxiv.org/abs/0806.0849v2

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Sep 21, 2013
09/13

by
B. Seradjeh; C. Weeks; M. Franz

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We propose a two-dimensional time-reversal invariant system of essentially non-interacting electrons on a square lattice that exhibits configurations with fractional charges e/2. These are vortex-like topological defects in the dimerization order parameter describing spatial modulation in the electron hopping amplitudes. Charge fractionalization is established by a simple counting argument, analytical calculation within the effective low-energy theory, and by an exact numerical diagonalization...

Source: http://arxiv.org/abs/0706.1559v2

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Sep 18, 2013
09/13

by
D. J. J. Marchand; M. Franz

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A surface of a strong topological insulator (STI) is characterized by an odd number of linearly dispersing gapless electronic surface states. It is well known that such a surface cannot be described by an effective two-dimensional lattice model (without breaking the time-reversal symmetry), which often hampers theoretical efforts to quantitatively understand some of the properties of such surfaces, including the effect of strong disorder, interactions and various symmetry-breaking...

Source: http://arxiv.org/abs/1209.4055v2

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Sep 21, 2013
09/13

by
M. Franz; Z. Tesanovic; O. Vafek

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High-$T_c$ cuprates differ from conventional superconductors in three crucial aspects: the superconducting state descends from a strongly correlated Mott-Hubbard insulator, the order parameter exhibits d-wave symmetry and superconducting fluctuations play an all important role. We formulate a theory of the pseudogap state in the cuprates by taking the advantage of these unusual features. The effective low energy theory within the pseudogap phase is shown to be equivalent to the (anisotropic)...

Source: http://arxiv.org/abs/cond-mat/0203333v3

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Sep 18, 2013
09/13

by
Z. Tesanovic; O. Vafek; M. Franz

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A d-wave superconductor, its phase coherence progressively destroyed by unbinding of vortex-antivortex pairs, suffers an instability related to chiral symmetry breaking in two-flavor QED$_3$. The chiral manifold exhibits large degeneracy spanned by physical states acting as inherent ``competitors'' of d-wave superconductivity. Two of these states are associated with antiferromagnetic insulator and ``stripe'' phases, known to be stable in the pseudogap regime of cuprates near half-filling. The...

Source: http://arxiv.org/abs/cond-mat/0110253v1

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Sep 20, 2013
09/13

by
M. Franz; Z. Tesanovic; O. Vafek

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In a recent preprint [cond-mat/0204040] Khveshchenko questioned the validity of our computation of the gauge invariant fermion propagator in QED3, which we employed as an effective theory of high-T_c cuprate superconductors [cond-mat/0203333]. We take this opportunity to further clarify our procedure and to show that criticism voiced in the above preprint is unwarranted.

Source: http://arxiv.org/abs/cond-mat/0204536v1

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Sep 21, 2013
09/13

by
O. Vafek; Z. Tesanovic; M. Franz

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We show that at long lengthscales and low energies and to leading order in 1/N expansion, the anisotropic QED in 2+1 dimensions renormalizes to an isotropic limit. Consequently, the (Euclidean) relativistic invariance of the theory is spontaneously restored at the isotropic critical point, characterized by the anomalous dimension exponent of the Dirac fermion propagator $\eta$. We find $\eta=16/3\pi^2 N$.

Source: http://arxiv.org/abs/cond-mat/0203047v2

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Jun 29, 2018
06/18

by
D. I. Pikulin; Anffany Chen; M. Franz

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Dirac and Weyl semimetals form an ideal platform for testing ideas developed in high energy physics to describe massless relativistic particles. One such quintessentially field-theoretic idea of chiral anomaly already resulted in the prediction and subsequent observation of the pronounced negative magnetoresistance in these novel materials for parallel electric and magnetic fields. Here we predict that the chiral anomaly occurs - and has experimentally observable consequences - when real...

Topics: Mesoscale and Nanoscale Physics, Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1607.01810

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Sep 21, 2013
09/13

by
B. Seradjeh; J. E. Moore; M. Franz

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An odd number of gapless Dirac fermions is guaranteed to exist at a surface of a strong topological insulator. We show that in a thin-film geometry and under external bias, electron-hole pairs that reside in these surface states can condense to form a coherent exciton condensate, similar in general terms to the exciton condensate recently argued to exist in a biased graphene bilayer, but with different topological properties. Such a `topological' exciton condensate (TEC) exhibits a host of...

Source: http://arxiv.org/abs/0902.1147v2