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Hamiltonian theory of fractionally filled Chern bands  [PDF]
Ganpathy Murthy,R. Shankar
Physics , 2012, DOI: 10.1103/PhysRevB.86.195146
Abstract: There is convincing numerical evidence that fractional quantum Hall (FQH)-like ground states arise in fractionally filled Chern bands (FCB). Here we show that the Hamiltonian theory of Composite Fermions (CF) can be as useful in describing the FCB as it was in describing the FQHE in the continuum. We are able to introduce CFs into the FCB problem even though there is no external magnetic field by following a two-stage process. First we construct an algebraically exact mapping which expresses the electron density projected to the Chern band, ${\rho}_{{\tiny FCB}}$, as a sum of Girvin-MacDonald-Platzman density operators, ${\rho}_{{\tiny GMP}}$, that obey the Magnetic Translation Algebra. Next, following our Hamiltonian treatment of the FQH problem, we rewrite the GMP operators in terms of CF variables which reproduce the same algebra. This naturally produces a unique Hartree-Fock ground state for the CFs, which can be used as a springboard for computing gaps, response functions, temperature-dependent phenomena, and the influence of disorder. We give two concrete examples, one of which has no analog in the continuum FQHE with $\nu= {1 \over 5}$ and $\sigma_{xy}={2\over 5}$. Our approach can be easily extended to fractionally filled, strongly interacting two-dimensional time-reversal-invariant topological insulators.
Composite Fermions for Fractionally Filled Chern Bands  [PDF]
Ganpathy Murthy,R. Shankar
Physics , 2011,
Abstract: We address the question of whether fractionally filled bands with a nontrivial Chern index in zero external field could also exhibit a Fractional Quantum Hall Effect (FQHE). Numerical works suggest this is possible. Analytic treatments are complicated by a non-vanishing band dispersion and a non-constant Berry flux. We propose embedding the Chern band in an auxiliary lowest Landau level (LLL) and then using composite fermions. We find some states which have no analogue in the continuum, and dependent on the interplay between interactions and the lattice. The approach extends to two-dimensional time-reversal invariant topological insulators.
Competing periodicities in fractionally filled one-dimensional bands  [PDF]
P. C. Snijders,S. Rogge,H. H. Weitering
Physics , 2005, DOI: 10.1103/PhysRevLett.96.076801
Abstract: We present a variable temperature Scanning Tunneling Microscopy and Spectroscopy (STM and STS) study of the Si(553)-Au atomic chain reconstruction. This quasi one-dimensional (1D) system undergoes at least two charge density wave (CDW) transitions at low temperature, which can be attributed to electronic instabilities in the fractionally-filled 1D bands of the high-symmetry phase. Upon cooling, Si(553)-Au first undergoes a single-band Peierls distortion, resulting in period doubling along the imaged chains. This Peierls state is ultimately overcome by a competing tripleperiod CDW, which in turn is accompanied by a x2 periodicity in between the chains. These locked-in periodicities indicate small charge transfer between the nearly half-filled and quarter-filled 1D bands. The presence and the mobility of atomic scale dislocations in the x3 CDW state indicates the possibility of manipulating phase solitons carrying a (spin,charge) of (1/2,+-e/3) or (0,+-2e/3).
Orbital Magnetism in Small Quantum Dots with Closed Shells  [PDF]
W. D. Heiss,R. G. Nazmitdinov
Physics , 1998, DOI: 10.1134/1.567955
Abstract: It is found that various kind of shell structure which occurs at specific values of the magnetic field leads to the disappearance of the orbital magnetization for particular magic numbers of small quantum dots with an electron number $A < 30$.
Magnetism in Closed-shell Quantum Dots: Emergence of Magnetic Bipolarons  [PDF]
R. Oszwa?dowski,I. ?uti?,A. G. Petukhov
Physics , 2011, DOI: 10.1103/PhysRevLett.106.177201
Abstract: Similar to atoms and nuclei, semiconductor quantum dots exhibit formation of shells. Predictions of magnetic behavior of the dots are often based on the shell occupancies. Thus, closed-shell quantum dots are assumed to be inherently nonmagnetic. Here, we propose a possibility of magnetism in such dots doped with magnetic impurities. On the example of the system of two interacting fermions, the simplest embodiment of the closed-shell structure, we demonstrate the emergence of a novel broken-symmetry ground state that is neither spin-singlet nor spin-triplet. We propose experimental tests of our predictions and the magnetic-dot structures to perform them.
Emergent Many-Body Translational Symmetries of Abelian and Non-Abelian Fractionally Filled Topological Insulators  [PDF]
B. Andrei Bernevig,N. Regnault
Physics , 2011, DOI: 10.1103/PhysRevB.85.075128
Abstract: The energy and entanglement spectrum of fractionally filled interacting topological insulators exhibit a peculiar manifold of low energy states separated by a gap from a high energy set of spurious states. In the current manuscript, we show that in the case of fractionally filled Chern insulators, the topological information of the many-body state developing in the system resides in this low-energy manifold. We identify an emergent many-body translational symmetry which allows us to separate the states in quasi-degenerate center of mass momentum sectors. Within one center of mass sector, the states can be further classified as eigenstates of an emergent (in the thermodynamic limit) set of many-body relative translation operators. We analytically establish a mapping between the two-dimensional Brillouin zone for the Fractional Quantum Hall effect on the torus and the one for the fractional Chern insulator. We show that the counting of quasi-degenerate levels below the gap for the Fractional Chern Insulator should arise from a folding of the states in the Fractional Quantum Hall system at identical filling factor. We show how to count and separate the excitations of the Laughlin, Moore-Read and Read-Rezayi series in the Fractional Quantum Hall effect into two-dimensional Brillouin zone momentum sectors, and then how to map these into the momentum sectors of the Fractional Chern Insulator. We numerically check our results by showing the emergent symmetry at work for Laughlin, Moore-Read and Read-Rezayi states on the checkerboard model of a Chern insulator, thereby also showing, as a proof of principle, that non-Abelian Fractional Chern Insulators exist.
Transient stability of empty and fluid-filled cylindrical shells
Gon?alves, Paulo B.;Silva, Frederico M. A. da;del Prado, Zenon J. G. N.;
Journal of the Brazilian Society of Mechanical Sciences and Engineering , 2006, DOI: 10.1590/S1678-58782006000300011
Abstract: in the present work a qualitatively accurate low dimensional model is used to study the non-linear dynamic behavior of shallow cylindrical shells under axial loading. the dynamic version of the donnell non-linear shallow shell equations are discretized by the galerkin method. the shell is considered to be initially at rest, in a position corresponding to a pre-buckling configuration. then, a harmonic excitation is applied and conditions to escape from this configuration are sought. by defining steady state and transient stability boundaries, frequency regimes of instability may be identified such that they may be avoided in design. initially a steady state analysis is performed; resonance response curves in the forcing plane are presented and the main instabilities are identified. finally, the global transient response of the system is investigated in order to quantify the degree of safety of the shell in the presence of small perturbations. since the initial conditions, or even the shell parameters, may vary widely, and indeed are often unknown, attention is given to all possible transient motions. as parameters are varied, transient basins of attraction can undergo quantitative and qualitative changes; hence a stability analysis which only considers the steady-state and neglects this global transient behavior, may be seriously non-conservative.
Transient stability of empty and fluid-filled cylindrical shells  [cached]
Gon?alves Paulo B.,Silva Frederico M. A. da,del Prado Zenon J. G. N.
Journal of the Brazilian Society of Mechanical Sciences and Engineering , 2006,
Abstract: In the present work a qualitatively accurate low dimensional model is used to study the non-linear dynamic behavior of shallow cylindrical shells under axial loading. The dynamic version of the Donnell non-linear shallow shell equations are discretized by the Galerkin method. The shell is considered to be initially at rest, in a position corresponding to a pre-buckling configuration. Then, a harmonic excitation is applied and conditions to escape from this configuration are sought. By defining steady state and transient stability boundaries, frequency regimes of instability may be identified such that they may be avoided in design. Initially a steady state analysis is performed; resonance response curves in the forcing plane are presented and the main instabilities are identified. Finally, the global transient response of the system is investigated in order to quantify the degree of safety of the shell in the presence of small perturbations. Since the initial conditions, or even the shell parameters, may vary widely, and indeed are often unknown, attention is given to all possible transient motions. As parameters are varied, transient basins of attraction can undergo quantitative and qualitative changes; hence a stability analysis which only considers the steady-state and neglects this global transient behavior, may be seriously non-conservative.
Magnetism in Na-filled Fe-based skutterudites  [PDF]
Guangzong Xing,Xiaofeng Fan,Weitao Zheng,Yanming Ma,Hongliang Shi,David J. Singh
Physics , 2015, DOI: 10.1038/srep10782
Abstract: The interplay of superconductivity and magnetism is a subject of ongoing interest, stimulated most recently by the discovery of Fe-based superconductivity and the recognition that spin-fluctuations near a magnetic quantum critical point may provide an explanation for the superconductivity and the order parameter. Here we investigate magnetism in the Na filled Fe-based skutterudites using first principles calculations. NaFe4Sb12 is a known ferromagnet near a quantum critical point. We find a ferromagnetic metallic state for this compound driven by a Stoner type instability, consistent with prior work. In accord with prior work, the magnetization is overestimated, as expected for a material near an itinerant ferromagnetic quantum critical point. NaFe4P12 also shows a ferromagnetic instability at the density functional level, but this instability is much weaker than that of NaFe4Sb12, possibly placing it on the paramagnetic side of the quantum critical point. NaFe4As12 shows intermediate behavior. We also present results for skutterudite FeSb3, which is a metastable phase that has been reported in thin film form.
Orbital Magnetism of Graphene Nanostructures: Bulk and Confinement Effects  [PDF]
Lisa He?e,Klaus Richter
Physics , 2014, DOI: 10.1103/PhysRevB.90.205424
Abstract: We consider the orbital magnetic properties of non-interacting charge carriers in graphene-based nanostructures in the low-energy regime. The magnetic response of such systems results both, frombulk contributions and from confinement effects that can be particularly strong in ballistic quantum dots. First we provide a comprehensive study of the magnetic susceptibility $\chi$ of bulk graphene in a magnetic field for the different regimes arising from the relative magnitudes of the energy scales involved, i.e. temperature, Landau level spacing and chemical potential. We show that for finite temperature or chemical potential, $\chi$ is not divergent although the diamagnetic contribution $\chi_{0}$ from the filled valance band exhibits the well-known $-B^{-1/2}$ dependence. We further derive oscillatory modulations of $\chi$, corresponding to de Haas-van Alphen oscillations of conventional two-dimensional electron gases. These oscillations can be large in graphene, thereby compensating the diamagnetic contribution $\chi_{0}$ and yielding a net paramagnetic susceptibility for certain energy and magnetic field regimes. Second, we predict and analyze corresponding strong, confinement-induced susceptibility oscillations in graphene-based quantum dots with amplitudes distincly exceeding the corresponding bulk susceptibility. Within a semiclassical approach we derive generic expressions for orbital magnetism of graphene quantum dots with regular classical dynamics. Graphene-specific features can be traced back to pseudospin interference along the underlying periodic orbits. We demonstrate the quality of the semiclassical approximation by comparison with quantum mechanical results for two exemplary mesoscopic systems, a graphene disk with infinite mass-type edges and a rectangular graphene structure with armchair and zigzag edges, using numerical tight-binding calculations in the latter case.
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