arXiv subjects

Gregory A. Fiete

Publications and source records attributed to Gregory A. Fiete.

At least 127 records · Page 7Linked to original sources

arXiv2010

Coulomb drag between helical edge states

We theoretically investigate the Coulomb drag between the edge states of two quantum spin Hall systems. Using an interacting theory of the one-dimensional helical edge modes, we show that the drag vanishes at second order in the inter-edge interaction, where it is typically finite in other systems, due to the absence of backscattering within the edges. However, in the presence of a small external magnetic field the drag is finite and scales as the fourth power of the magnetic field, a behavior that sharply distinguishes it from other systems. We obtain the temperature dependence of the drag for regimes of both linear and quadratic edge dispersion in the presence of a finite field.

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arXiv2010

Spin-incoherent behavior in the ground state of strongly correlated systems

It is commonly believed that strongly interacting one-dimensional Fermi systems with gapless excitations are effectively described by Luttinger liquid theory. However, when the temperature of the system is high compared to the spin energy, but small compared to the charge energy, the system becomes "spin-incoherent". We present numerical evidence showing that the one-dimensional "t-J-Kondo" lattice, consisting of a t-J chain interacting with localized spins, displays all the characteristic signatures of spin-incoherent physics, but in the {\it ground state}. We argue that similar physics may be present in a wide range of strongly interacting systems.

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arXiv2010

Exotic resonant level models in non-Abelian quantum Hall states coupled to quantum dots

In this paper we study the coupling between a quantum dot and the edge of a non-Abelian fractional quantum Hall state. We assume the dot is small enough that its level spacing is large compared to both the temperature and the coupling to the spatially proximate bulk non-Abelian fractional quantum Hall state. We focus on the physics of level degeneracy with electron number on the dot. The physics of such a resonant level is governed by a $k$-channel Kondo model when the quantum Hall state is a Read-Rezayi state at filling fraction $ν=2+k/(k+2)$ or its particle-hole conjugate at $ν=2+2/(k+2)$. The $k$-channel Kondo model is channel symmetric even without fine tuning any couplings in the former state; in the latter, it is generically channel asymmetric. The two limits exhibit non-Fermi liquid and Fermi liquid properties, respectively, and therefore may be distinguished. By exploiting the mapping between the resonant level model and the multichannel Kondo model, we discuss the thermodynamic and transport properties of the system. In the special case of $k=2$, our results provide a novel venue to distinguish between the Pfaffian and anti-Pfaffian states at filling fraction $ν=5/2$. We present numerical estimates for realizing this scenario in experiment.

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arXiv2010

Interaction-driven topological insulators on the kagome and the decorated honeycomb lattices

We study the spinless and spinful extended Hubbard models with repulsive interactions on the kagome and the decorated honeycomb ("star") lattice. Using Hartree-Fock mean-field theory, we show that interaction-driven insulating phases with non-trivial topological invariants (Chern number or $Z_2$ invariant) exist for an experimentally reasonable range of parameters. These phases occur at filling fractions which involve either Dirac points or quadratic band crossing points in the non-interacting limit. We present comprehensive mean-field phase diagrams for these lattices and discuss the competition between topologically non-trivial phases and numerous other ordered states, including various charge, spin, and bond orderings. Our results suggest that $Z_2$ topological insulators should be found in a number of systems with either little or no intrinsic spin-orbit coupling.

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arXiv2010

Topological phases and phase transitions on the square-octagon lattice

We theoretically investigate a tight binding model of fermions hopping on the square-octagon lattice which consists of a square lattice with plaquette corners themselves decorated by squares. Upon the inclusion of second neighbor spin-orbit coupling or non-Abelian gauge fields, time-reversal symmetric topological Z_2 band insulators are realized. Additional insulating and gapless phases are also realized via the non-Abelian gauge fields. Some of the phase transitions involve topological changes to the Fermi surface. The stability of the topological phases to various symmetry breaking terms is investigated via the entanglement spectrum. Our results enlarge the number of known exactly solvable models of Z_2 band insulators, and are potentially relevant to the realization and identification of topological phases in both the solid state and cold atomic gases.

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arXiv2010

Topological Insulators on the Decorated Honeycomb Lattice

We show that the decorated honeycomb lattice supports a number of topological insulating phases with a non-trivial Z_2 invariant and time-reversal symmetry protected gapless edge modes. We investigate the stability of these phases with respect to various symmetry breaking perturbations and demonstrate the connection to the recently discovered exactly solvable S=1/2 chiral spin liquid model [Phys. Rev. Lett. 99, 247203 (2007)] with non-Abelian and Abelian excitations on the same lattice. Our work highlights the relationship between topological band insulators and topologically ordered spin systems, and points to promising avenues for enlarging the number of known examples of both.

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arXiv2010

Seeing Quantum Fractals

This Perspectives article highlights some of the interesting features and open questions presented by recent scanning tunneling microscope (STM) experiments on GaMnAs near the metal-insulator transition.

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arXiv2010

Visualizing Quantum Well State Perturbations of Metallic Thin Films near Stacking Fault Defects

We demonstrate that quantum well states (QWS) of thin Pb films are highly perturbed within the proximity of intrinsic film defects. Scanning Tunneling Spectroscopy (STM/STS) measurements indicate that the energy of these states have a strong distance dependence within 4 nm of the defect with the strongest energetic fluctuations equaling up to 100 meV. These localized perturbations show large spatially-dependent asymmetries in the LDOS around the defect site for each corresponding quantum well state. These energetic fluctuations can be described by a simple model which accounts for fluctuations in the confinement potential induced by topographic changes.

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arXiv2009

Singular responses of spin-incoherent Luttinger liquids

When a local potential changes abruptly in time, an electron gas shifts to a new state which at long times is orthogonal to the one in the absence of the local potential. This is known as Anderson's orthogonality catastrophe and it is relevant for the so-called X-ray edge or Fermi edge singularity, and for tunneling into an interacting one dimensional system of fermions. It often happens that the finite frequency response of the photon absorption or the tunneling density of states exhibits a singular behavior as a function of frequency: $(\frac{ω_{\rm th}}{ω-ω_{\rm th}})^αΘ(ω-ω_{\rm th})$ where $ω_{\rm th}$ is a threshold frequency and $α$ is an exponent characterizing the singular response. In this paper singular responses of spin-incoherent Luttinger liquids are reviewed. Such responses most often do not fall into the familiar form above, but instead typically exhibit logarithmic corrections and display a much higher universality in terms of the microscopic interactions in the theory. Specific predictions are made, the current experimental situation is summarized, and key outstanding theoretical issues related to spin-incoherent Luttinger liquids are highlighted.

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arXiv2008

Junctions of Spin-Incoherent Luttinger Liquids with Ferromagnets and Superconductors

We discuss the properties of a strongly interacting spin-charge separated one dimensional system coupled to ferromagnets and/or superconductors. Our results are valid for arbitrary temperatures with respect to the spin energy, but require temperature be small compared to the charge energy. We focus mainly on the spin-incoherent regime where temperature is large compared to the spin energy, but small compared to the charge energy. In the case of a ferromagnet we study spin pumping and the renormalized dynamics of a precessing magnetic order parameter. We find the interaction-dependent temperature dependence of the spin pumping can be qualitatively different in the spin-incoherent regime from the Luttinger liquid regime, allowing an identification of the former. Likewise, the temperature dependence of the renormlized magnetization dynamics can be used to identify spin-incoherent physics. For the case of a spin-incoherent Luttinger liquid coupled to two superconductors, we compute the ac and dc Josephson current for a wire geometry in the limit of tunnel coupled superconductors. Both the ac and dc response contain "smoking gun" signatures that can be used to identify spin-incoherent behavior. Experimental requirements for the observation of these effects are laid out.

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arXiv2008

Multi-channel Kondo Models in non-Abelian Quantum Hall Droplets

We study the coupling between a quantum dot and the edge of a non-Abelian fractional quantum Hall state which is spatially separated from it by an integer quantum Hall state. Near a resonance, the physics at energy scales below the level spacing of the edge states of the dot is governed by a $k$-channel Kondo model when the quantum Hall state is a Read-Rezayi state at filling fraction $ν=2+k/(k+2)$ or its particle-hole conjugate at $ν=2+2/(k+2)$. The $k$-channel Kondo model is channel isotropic even without fine tuning in the former state; in the latter, it is generically channel anisotropic. In the special case of $k=2$, our results provide a new venue, realized in a mesoscopic context, to distinguish between the Pfaffian and anti-Pfaffian states at filling fraction $ν=5/2$.

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arXiv2008

Quantum Hall States at $ν=\frac{2}{k+2}$

We study the $ν=\frac{2}{k+2}$ quantum Hall states which are particle-hole conjugates of the $ν=\frac{k}{k+2}$ Read-Rezayi states. We find that equilibration between the different modes at the edge of such a state leads to an emergent SU(2)$_k$ algebra in the counter-propagating neutral sector. Heat flow along the edges of these states will be in the opposite direction of charge flow. In the $k=3$ case, which may be relevant to $ν=2+{2/5}$, the thermal Hall conductance and the exponents associated with quasiparticle and electron tunneling distinguish this state from competing states such as the hierarchy/Jain state.

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arXiv2007

Universal Periods in Quantum Hall Droplets

Using the hierarchy picture of the fractional quantum Hall effect, we study the the ground state periodicity of a finite size quantum Hall droplet in a quantum Hall fluid of a different filling factor. The droplet edge charge is periodically modulated with flux through the droplet and will lead to a periodic variation in the conductance of a nearby point contact, such as occurs in some quantum Hall interferometers. Our model is consistent with experiment and predicts that superperiods can be observed in geometries where no interfering trajectories occur. The model may also provide an experimentally feasible method of detecting elusive neutral modes and otherwise obtaining information about the microscopic edge structure in fractional quantum Hall states.

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arXiv2007

Tunneling Density of States, Pair Correlation, and Josephson Current in Spin-Incoherent Luttinger Liquid-Superconductor Hybrid Systems

We study a hybrid system consisting of a spin-incoherent Luttinger liquid adjoined at one or both ends to a superconductor. We find the tunneling density of states diverges at low energies and exhibits a universal frequency dependence independent of the strength of the interactions in the system. We show that in spite of exponentially decaying pair correlations with distance into the spin-incoherent Luttinger liquid, the Josephson current remains robust. Compared to the zero temperature Luttinger liquid case there is a factor of 2 reduction in the critical current and a halving of the period in the phase difference between the superconductors. Our results open the way for a new class of experiments in the spin-incoherent regime of one dimensional systems.

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arXiv2007

The spin-incoherent Luttinger liquid

In contrast to the well known Fermi liquid theory of three dimensions, interacting one-dimensional and quasi one-dimensional systems of fermions are described at low energy by an effective theory known as Luttinger liquid theory. This theory is expressed in terms of collective many-body excitations that show exotic behavior such as spin-charge separation. Luttinger liquid theory is commonly applied on the premise that "low energy" describes both the spin and charge sectors. However, when the interactions in the system are very strong, as they typically are at low particle densities, the ratio of spin to charge energy may become exponentially small. It is then possible at very low temperatures for the energy to be low compared to the characteristic charge energy, but still high compared to the characteristic spin energy. This energy window of near ground-state charge degrees of freedom, but highly thermally excited spin degrees of freedom is called a spin-incoherent Luttinger liquid. The spin-incoherent Luttinger liquid exhibits a higher degree universality than the Luttinger liquid and its properties are qualitatively distinct. In this colloquium I detail some of the recent theoretical developments in the field and describe experimental indications of such a regime in gated semiconductor quantum wires.

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arXiv2007

Non-equilibrium charge noise and dephasing from a spin-incoherent Luttinger liquid

We theoretically investigate the charge noise and dephasing in a metallic device in close proximity to a spin incoherent Luttinger liquid with a small but finite current. The frequency dependence of the charge noise exhibits a loss of frequency peaks corresponding to the $2k_F$ part of the density correlations in the electron liquid when the temperature $T$ is increased from values below the magnetic exchange energy $J$ of the electron gas to values above it. The dephasing rate in a nearby metallic nanostructure also shows a cross-over for $T \sim J$ and may exhibit a non-monotonic temperature dependence. For a range of temperatures the dephasing rate {\em decreases} with increasing temperatures. The proposed experiments provide a convenient approach to probe the spin-incoherent Luttinger liquid and should be implementable in a wide variety of systems.

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arXiv2007

Fermi-edge singularity in a spin-incoherent Luttinger liquid

We theoretically investigate the Fermi edge singularity in a spin incoherent Luttinger liquid. Both cases of finite and infinite core hole mass are explored, as well as the effect of a static external magnetic field of arbitrary strength. For a finite mass core hole the absorption edge behaves as $(ω-ω_{\rm th})^α/ \sqrt{|\ln(ω-ω_{\rm th})|}$ for frequencies $ω$ just above the threshold frequency $ω_{\rm th}$. The exponent $α$ depends on the interaction parameter $g$ of the interacting one dimensional system, the electron-hole coupling, and is independent of the magnetic field strength, the momentum, and the mass of the excited core hole (in contrast to the spin coherent case). In the infinite mass limit, the spin incoherent problem can be mapped onto an equivalent problem in a spinless Luttinger liquid for which the logarithmic factor is absent, and backscattering from the core hole leads to a universal contribution to the exponent $α$.

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arXiv2006

Effective Hamiltonians for some highly frustrated magnets

In prior work, the authors developed a method of degenerate perturbation theory about the Ising limit to derive an effective Hamiltonian describing quantum fluctuations in a half-polarized magnetization plateau on the pyrochlore lattice. Here, we extend this formulation to an arbitrary lattice of corner sharing simplexes of $q$ sites, at a fraction $(q-2k)/q$ of the saturation magnetization, with $0<k<q$. We present explicit effective Hamiltonians for the examples of the checkerboard, kagome, and pyrochlore lattices. The consequent ground states in these cases for $k=1$ are also discussed.

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