arXiv subjects

Yafis Barlas

Publications and source records attributed to Yafis Barlas.

At least 55 records · Page 3Linked to original sources

arXiv2013

Superconducting States in pseudo-Landau Levels of Strained Graphene

We describe the formation of superconducting states in graphene in the presence of pseudo-Landau levels induced by strain, when time reversal symmetry is preserved. We show that superconductivity in strained graphene is quantum critical when the pseudo-Landau levels are completely filled, whereas at partial fillings superconductivity survives at weak coupling. In the weak coupling limit, the critical temperature scales \emph{linearly} with the coupling strength and shows a sequence of quantum critical points as a function of the filling factor that can be accessed experimentally. We argue that superconductivity can be induced by electron-phonon coupling and that the transition temperature can be controlled with the amount of strain and with the filling fraction of the Landau levels.

cond-mat.supr-con↗

arXiv2012

Collective modes in singlet superconductors

A brief summary of collective mode excitations that can exist in singlet superconductors with irreducible representation $L$ is given. Such excitations may be classified as the coupled excitations of the charge density $ρ$ and the phase $ϕ$ of the order parameter, or of the amplitude $Δ$ of order parameter. Each of these classes may be further characterized in the long wavelength limit by the irreducible representation $\ell$ of the excitation, which may or may not be the same as the ground state $L$.

cond-mat.supr-con↗

arXiv2012

Phase diagram of insulating crystal and quantum Hall states in ABC-stacked trilayer graphene

In the presence of a perpendicular magnetic field, ABC-stacked trilayer graphene's chiral band structure supports a 12-fold degenerate N=0 Landau level (LL). Along with the valley and spin degrees of freedom, the zeroth LL contains additional quantum numbers associated with the LL orbital index $% n=0,1,2$. Remote inter-layer hopping terms and external potential difference $Δ_{B}$ between the layers lead to LL splitting by introducing a gap $% Δ_{LL}$ between the degenerate zero-energy triplet LL orbitals. Assuming that the spin and valley degrees of freedom are frozen, we study the phase diagram of this system resulting from competition of the single particle LL splitting and Coulomb interactions within the Hartree-Fock approximation at integer filling factors. Above a critical value $Δ_{LL}^{c}$ of the external potential difference i,e, for $|Δ_{LL}| >Δ_{LL}^{c}$, the ground state is a uniform quantum Hall state where the electrons occupy the lowest unoccupied LL orbital index. For $|Δ_{LL}| <Δ_{LL}^{c}$ (which corresponds to large positive or negative values of $Δ_{B}$) the uniform QH state is unstable to the formation of a crystal state at integer filling factors. This phase transition should be characterized by a Hall plateau transition as a function of $Δ_{LL}$ at a fixed filling factor. We also study the properties of this crystal state and discuss its experimental detection.

cond-mat.mes-hall↗

arXiv2012

Quantum Hall to charge-density-wave phase transitions in ABC-trilayer graphene

ABC-stacked trilayer graphene's chiral band structure results in three ($n=0,1,2$) Landau level orbitals with zero kinetic energy. This unique feature has important consequences on the interaction driven states of the 12-fold degenerate (including spin and valley) N=0 Landau level. In particular, at many filling factors $ν_{T} =\pm5,\pm4,\pm2,\pm1$ a quantum phase transition from a quantum Hall liquid state to a triangular charge density wave occurs as a function of the single-particle induced LL orbital splitting $Δ_{LL}$. This phase transition should be characterized by a re-entrant integer quantum Hall effect with the Hall conductivity corresponding to the {\it adjacent} interaction driven integer quantum Hall plateau.

cond-mat.mes-hall↗

arXiv2012

Higgs Bosons in D-wave Superconductors

The concept of "broken symmetry", that the symmetry of the vacuum may be lower than the Hamiltonian of a quantum theory, plays an important role in modern physics. A manifestation of this phenomena is the Higgs boson in particle physics whose long awaited discovery is imminent. An equivalent mode in superconductors is implicit in the early theories of their collective fluctuations. Spurred by some mysterious experimental results, the theory of the oscillation of the amplitude of superconductivity order parameter, which is the equivalent to the Higgs modes in s-wave superconductors and its identification in the experiments, was explicitly provided. It was also shown that a necessary condition for this to occur is the emergent Lorentz invariance in the superconducting state while the metallic state and the region just below $T_c$ is manifestly non-Lorentz invariant. Here we show that d-wave superconductors, such as the high temperature Cuprate superconductors, should have a rich assortment of Higgs bosons, each in a different irreducible representation of the point-group symmetries of the lattice. We also show that these modes have a characteristic singular spectral structure which can be discovered in Raman scattering experiments.

cond-mat.supr-con↗

arXiv2012

Thermopower of Quantum Hall States in Corbino Geometry as a Measure of Quasiparticle Entropy

Using the Onsager relation between electric and heat transport coefficients, and considering the very different roles played by the quantum Hall condensate and quasiparticles in transport, we argue that near the center of a quantum Hall plateau thermopower in a Corbino geometry measures {\it "entropy per quasiparticle per quasiparticle charge"}. This relation indicates that thermopower measurement in a Corbino setup is a more direct measure of quasiparticle entropy than in a Hall bar. Treating disorder within the self-consistent Born approximation, we show through an explicit microscopic calculation that this relation holds on an integer quantum Hall plateau at low temperatures. Applying this to non-Abelian quantum Hall states, we argue that Corbino thermopower at sufficiently low temperature becomes temperature-independent, and measures the quantum dimension of non-Abelian quasiparticles that determines the topological entropy they carry.

cond-mat.mes-hall↗

arXiv2012

Coulomb impurity under magnetic field in graphene: a semiclassical approach

We address the problem of a Coulomb impurity in graphene in the presence of a perpendicular uniform magnetic field. We show that the problem can be solved below the supercritical impurity magnitude within the WKB approximation. Without impurity the semiclassical energies correctly reproduce the Landau level spectrum. For a given Landau level the WKB energy depends on the absolute value of angular momentum in a way which is consistent with the exact diagonalization result. Below the supercritical impurity magnitude, the WKB solution can be expanded as a convergent series in powers of the effective fine structure constant. Relevance of our results to validity of the widely used Landau level projection approximation is discussed.

cond-mat.str-el↗

arXiv2011

Quantum Hall Effects in Graphene-Based Two-Dimensional Electron Systems

In this article we review the quantum Hall physics of graphene based two-dimensional electron systems, with a special focus on recent experimental and theoretical developments. We explain why graphene and bilayer graphene can be viewed respectively as J=1 and J=2 chiral two-dimensional electron gases (C2DEGs), and why this property frames their quantum Hall physics. The current status of experimental and theoretical work on the role of electron-electron interactions is reviewed at length with an emphasis on unresolved issues in the field, including assessing the role of disorder in current experimental results. Special attention is given to the interesting low magnetic field limit and to the relationship between quantum Hall effects and the spontaneous anomalous Hall effects that might occur in bilayer graphene systems in the absence of a magnetic field.

cond-mat.mes-hall↗

arXiv2011

Charge and Spin Reconstruction in Quantum Hall Strips

We study the effect of electron-electron interactions on the charge and spin structures of a Quantum Hall strip in a triangularly confined potential. We find that the strip undergoes a spin-unpolarized to spin-polarized transition as a function of magnetic field perpendicular to the strip. For sharp confinements the spin-polarization transition is spontaneous and first develops at the softer side of the triangular potential which shows up as an "eye-structure" in the electron dispersion. For sufficiently weak confinements this spin-polarization transition is preceded by a charge reconstruction of a single spin species, which creates a spin-polarized strip of electrons with a width of the order of the magnetic length detached from the rest of the system. Relevance of our findings to the recent momentum resolved tunneling experiments is also discussed.

cond-mat.mes-hall↗

arXiv2010

Fractionalization via $\mathbb{Z}_{2}$ Gauge Fields at a Cold Atom Quantum Hall Transition

We study a single species of fermionic atoms in an "effective" magnetic field at total filling factor $ν_{f}=1$, interacting through a p-wave Feshbach resonance, and show that the system undergoes a quantum phase transition from a $ν_{f} =1 $ fermionic integer quantum Hall state to $ν_{b} =1/4 $ bosonic fractional quantum Hall state as a function of detuning. The transition is in the $(2+1)$-D Ising universality class. We formulate a dual theory in terms of quasiparticles interacting with a $\mathbb{Z}_{2}$ gauge field, and show that charge fractionalization follows from this topological quantum phase transition. Experimental consequences and possible tests of our theoretical predictions are discussed.

cond-mat.str-el↗

arXiv2010

Orbital and interlayer Skyrmions crystals in bilayer graphene

A graphene bilayer in a transverse magnetic field has a set of Landau levels with energies $E=\pm \sqrt{N(N+1)}\hslash ω_{c}^{\ast}$ where $ω_{c}^{\ast}$ is the effective cyclotron frequency and $% N=0,1,2,...$ All Landau levels but N=0 are four times degenerate counting spin and valley degrees of freedom. The Landau level N=0 has an extra degeneracy due to the fact that orbitals $n=0$ and $n=1$ both have zero kinetic energies. At integer filling factors, Coulomb interactions produce a set of broken-symmetry states with partial or full alignement in space of the valley and orbital pseudospins. These quantum Hall pseudo-ferromagnetic states support topological charged excitations in the form of orbital and valley Skyrmions. Away from integer fillings, these topological excitations can condense to form a rich variety of Skyrme crystals with interesting properties. We study in this paper different crystal phases that occur when an electric field is applied between the layers. We show that orbital Skyrmions, in analogy with spin Skyrmions, have a texture of electrical dipoles that can be controlled by an in-plane electric field. Moreover, the modulation of electronic density in the crystalline phases are experimentally accessible through a measurement of their local density of states

cond-mat.mes-hall↗

arXiv2010

Orbital order in bilayer graphene at filling factor $ν=-1 $

In a graphene bilayer with Bernal stacking both $n=0$ and $n=1$ orbital Landau levels have zero kinetic energy. An electronic state in the N=0 Landau level consequently has three quantum numbers in addition to its guiding center label: its spin, its valley index $K$ or $K^{\prime}$, and an orbital quantum number $n=0,1.$ The two-dimensional electron gas (2DEG) in the bilayer supports a wide variety of broken-symmetry states in which the pseudospins associated these three quantum numbers order in a manner that is dependent on both filling factor $ν$ and the electric potential difference between the layers. In this paper, we study the case of $ν=-1$ in an external field strong enough to freeze electronic spins. We show that an electric potential difference between layers drives a series of transitions, starting from interlayer-coherent states (ICS) at small potentials and leading to orbitally coherent states (OCS) that are polarized in a single layer. Orbital pseudospins carry electric dipoles with orientations that are ordered in the OCS and have Dzyaloshinskii-Moriya interactions that can lead to spiral instabilities. We show that the microwave absorption spectra of ICSs, OCSs, and the mixed states that occur at intermediate potentials are sharply distinct.

cond-mat.mes-hall↗

arXiv2009

Non-Fermi Liquid behavior in Neutral Bilayer Graphene

We calculate the density-density response function and electron self-energy for undoped bilayer graphene, within the Random Phase Approximation (RPA). We show that the quasiparticle decay rate scales linearly with the quasiparticle energy, and quasiparticle weight vanishes logarithmically in the low-energy limit, indicating non-Fermi liquid behavior. This is a consequence of the absence of a Fermi surface for neutral bilayer graphene and corresponding larger phase space available for scattering processes. Experimental consequences of our results as well as their differences from those of single-layer graphene are discussed.

cond-mat.mes-hall↗

arXiv2009

Renormalized Landau Levels and Particle-Hole Symmetry in Graphene

In this proceedings paper we report on a calculation of graphene's Landau levels in a magnetic field. Our calculations are based on a self-consistent Hartree-Fock approximation for graphene's massless-Dirac continuum model. We find that because of graphene's chiral band structure interactions not only shift Landau-level energies, as in a non-relativistic electron gas, but also alter Landau level wavefunctions. We comment on the subtle continuum model regularization procedure necessary to correctly maintain the lattice-model's particle hole symmetry properties.

cond-mat.mes-hall↗

arXiv2009

Anomalous Exciton-Condensation in Graphene Bilayers

In ordinary semiconductor bilayers, exciton condensates appear at total Landau level filling factor $ν_{T}=1$. We predict that similar states will occur in Bernal stacked graphene bilayers at many non-zero integer filling factors. For $ν_{T} = -3,1$ we find that the superfluid density of the exciton condensate vanishes and that a finite-temperature fluctuation induced first order isotropic-smectic phase transition occurs when the layer densities are not balanced. These anomalous properties of bilayer graphene exciton condensates are due to the degeneracy of Landau levels with $n=0$ and $n=1$ orbital character.

cond-mat.mes-hall↗

arXiv2008

Intra-Landau level Cyclotron Resonance in Bilayer Graphene

Interaction driven integer quantum Hall effects are anticipated in graphene bilayers because of the near-degeneracy of the eight Landau levels which appear near the neutral system Fermi level. We predict that an intra-Landau-level cyclotron resonance signal will appear at some odd-integer filling factors, accompanied by collective modes which are nearly gapless and have approximate $k^{3/2}$ dispersion. We speculate on the possibility of unususal localization physics associated with these modes.

cond-mat.mes-hall↗

arXiv2007

The Role of Electron-electron Interactions in Graphene ARPES Spectra

We report on a theoretical study of the influence of electron-electron interactions on ARPES spectra in graphene that is based on the random-phase-approximation and on graphene's massless Dirac equation continuum model. We find that level repulsion between quasiparticle and plasmaron resonances gives rise to a gap-like feature at small k. ARPES spectra are sensitive to the electron-electron interaction coupling strength $α_{\rm gr}$ and might enable an experimental determination of this material parameter.

cond-mat.mes-hall↗

arXiv2007

Chirality and Correlations in Graphene

Graphene is described at low-energy by a massless Dirac equation whose eigenstates have definite chirality. We show that the tendency of Coulomb interactions in lightly doped graphene to favor states with larger net chirality leads to suppressed spin and charge susceptibilities. Our conclusions are based on an evaluation of graphene's exchange and random-phase-approximation (RPA) correlation energies. The suppression is a consequence of the quasiparticle chirality switch which enhances quasiparticle velocities near the Dirac point.

cond-mat.str-el↗