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arXiv2013

Connectivity of the Icosahedral Network and a Dramatically Growing Static Length Scale in Cu-Zr Binary Metallic Glasses

We report on and characterize, via molecular dynamics (MD) studies, the evolution of the structure of Cu50Zr50 and Cu64Zr36 metallic glasses (MGs) as temperature is varied. Interestingly, a percolating icosahedral network appears in the Cu64Zr36 system as it is supercooled. This leads us to introduce a static length scale, which grows dramatically as this three dimensional system approaches the glass transition. Amidst interpenetrating connections, non-interpenetrating connections between icosahedra are shown to become prevalent upon supercooling and to greatly enhance the connectivity of the MG's icosahedral network. Additionally, we characterize the chemical compositions of the icosahedral networks and their components. These findings demonstrate the importance of non-interpenetrating connections for facilitating extensive structural networks in Cu-Zr MGs, which in turn drive dynamical slowing in these materials.

cond-mat.mtrl-sci↗

arXiv2012

Thermodynamic properties of the Yb2Ti2O7 pyrochlore as a function of temperature and magnetic field: validation of a quantum spin ice exchange Hamiltonian

The thermodynamic properties of the pyrochlore Yb2Ti2O7 material are calculated using the numericallinked-cluster (NLC) calculation method for an effective anisotropic-exchange spin-1/2 Hamiltonian with parameters recently determined by fitting the neutron scattering spin wave data obtained at high magnetic field h. Magnetization, M(T,h), as a function of temperature T and for different magnetic fields h applied along the three high symmetry directions [100], [110] and [111], are compared with experimental measurements on the material for temperature T>1.8K. The excellent agreement between experimentally measured and calculated M(T,h) over the entire temperature and magnetic field range considered provides strong quantitative validation of the effective Hamiltonian. It also confirms that fitting the high-field neutron spin wave spectra in the polarized paramagnetic state is an excellent method for determining the microscopic exchange constants of rare-earth insulating magnets that are described by an effective spin-1/2 Hamiltonian. Finally, we present results which demonstrate that a recent analysis of the polarized neutron scattering intensity of Yb2Ti2O7 using a random phase approximation (RPA) method [Chang et al., Nature Communications {3}, 992 (2012)] does not provide a good description of M(T,h) for $T\lesssim 10$ K, that is in the entire temperature regime where correlations become non-negligible.

cond-mat.str-el↗

arXiv2013

Quantum magnetooscillations in the ac conductivity of disordered graphene

The dynamic conductivity σ(ω) of graphene in the presence of diagonal white noise disorder and quantizing magnetic field B is calculated. We obtain analytic expressions for σ(ω) in various parametric regimes ranging from the quasiclassical Drude limit corresponding to strongly overlapping Landau levels (LLs) to the extreme quantum limit where the conductivity is determined by the optical selection rules of the clean graphene. The nonequidistant LL spectrum of graphene renders its transport characteristics quantitatively different from conventional 2D electron systems with parabolic spectrum. Since the magnetooscillations in the semiclassical density of states are anharmonic and are described by a quasi-continuum of cyclotron frequencies, both the ac Shubnikov-de Haas oscillations and the quantum corrections to σ(ω) that survive to higher temperatures manifest a slow beating on top of fast oscillations with the local energy-dependent cyclotron frequency.Both types of quantum oscillations possess nodes whose index scales as ω^2. In the quantum regime of separated LLs, we study both the cyclotron resonance transitions, which have a rich spectrum due to the nonequidistant spectrum of LLs, and disorder-induced transitions which violate the clean selection rules of graphene. We identify the strongest disorder-induced transitions in recent magnetotransmission experiments. We also compare the temperature- and chemical potential-dependence of σ(ω) in various frequency ranges from the dc limit allowing intra-LL transition only to the universal high-frequency limit where the Landau quantization provides a small B-dependent correction to the universal value of the interband conductivity σ=e^2/4 \hbar of the clean graphene.

cond-mat.mes-hall↗

arXiv2013

Criticality of the metal-topological insulator transition driven by disorder

Employing scaling analysis of the localization length, we deduce the critical exponent of the metal-topological insulator (TI) transitions induced by disorder. The obtained exponent nu~2.7 shows no conspicuous deviation from the value established for metal-ordinary insulator transitions in systems of the symplectic class. We investigate the topological phase diagram upon carrier doping to reveal the nature of the so-called topological Anderson insulator (TAI) region. The critical exponent of the metal-TAI transition is also first estimated, shown to be undistinguishable from the above value within the numerical error. By symmetry considerations we determine the explicit form of Rashba spin-orbit coupling in systems of C4v point group symmetry.

cond-mat.mes-hall↗

arXiv2013

Ideal strength of random alloys from first-principles theory

The all-electron exact muffin-tin orbitals method in combination with the coherent-potential appproximation has been employed to investigate the ideal tensile strengths of elemental V, Mo solids and V- and Mo-based random solid solutions. The present ideal tensile strengths, calculated assuming isotropic Poisson contraction, are 16.1, 26.7 and 37.6 GPa for bcc V in the [001], [111] and [110] directions, respectively, and 26.7 GPa for bcc Mo in the [001] direction, which are all in good agreement with the available theoretical data. When a few percent Tc is introduced in Mo, it is found that the ideal strength decreases in the [001] direction. For the V-based alloys, Cr increases and Ti decreases the ideal tensile strength in all principal directions. Adding the same concentration of Cr and Ti to V leads to ternary alloys with similar ideal strength values as that of pure V. The alloying effects on the ideal strength is explained using the electronic band structure.

cond-mat.dis-nn↗

arXiv2012

Eigenfrequencies of the randomly pinned drum and conductivity of graphene

Graphene is convenient material for nanomechanichal applications since high-frequency oscillations are easily accessible. In this Article, we consider graphene on a rough substrate attached to imperfections at random locations. We explore the statistics of low-lying phonon modes, which exert most influence on the conductivity of graphene. We find that the nearest neighbor spacings of low lying eigenfrequencies have the Wigner-Dyson probability distribution after averaging over the random configurations of disorder. Due to interaction of electrons with the oscillations of the membrane, an electron can be transfered to higher or lower energies, which is a manifestation of the phonon-assisted Tien-Gordon effect. The Tien-Gordon effect suppresses the conductivity of graphene. In the regime of low Fermi energies and small sizes of the sample an increase of conductivity is observed which we refer to Klein tunneling and electron-hole pair creation. Eventually, when the increase of the transmission becomes too prominent, the pair creation changes the ground state of the system, signalizing the limit of applicability of the single-particle Dirac equation used in this paper.

cond-mat.mes-hall↗

arXiv2013

Paraelectric and Ferroelectric States in a Model for Relaxor Ferroelectrics

We study the free energy landscape of a minimal model for relaxor ferroelectrics. Using a variational method which includes leading correlations beyond the mean-field approximation as well as disorder averaging at the level of a simple replica theory, we find metastable paraelectric states with a stability region that extends to zero temperature. The free energy of such states exhibits an essential singularity for weak compositional disorder pointing to their necessary occurrence. Ferroelectric states appear as local minima in the free energy at high temperatures and become stable below a coexistence temperature $T_c$. We calculate the phase diagram in the electric field-temperature plane and find a coexistence line of the polar and non-polar phases which ends at a critical point. First-order phase transitions are induced for fields sufficiently large to cross the region of stability of the metastable paraelectric phase. These polar and non-polar states have distinct structure factors from those of conventional ferroelectrics. We use this theoretical framework to compare and to gain physical understanding of various experimental results in typical relaxors.

cond-mat.dis-nn↗

arXiv2013

Breakdown of self-averaging in the Bose glass

We study the square-lattice Bose-Hubbard model with bounded random on-site energies at zero temperature. Starting from a dual representation obtained from a strong-coupling expansion around the atomic limit, we employ a real-space block decimation scheme. This approach is non-perturbative in the disorder and enables us to study the renormalization-group flow of the induced random-mass distribution. In both insulating phases, the Mott insulator and the Bose glass, the average mass diverges, signaling short range superfluid correlations. The relative variance of the mass distribution distinguishes the two phases, renormalizing to zero in the Mott insulator and diverging in the Bose glass. Negative mass values in the tail of the distribution indicate the presence of rare superfluid regions in the Bose glass. The breakdown of self-averaging is evidenced by the divergent relative variance and increasingly non-Gaussian distributions. We determine an explicit phase boundary between the Mott insulator and Bose glass.

cond-mat.dis-nn↗

arXiv2013

Magnetic responses of randomly depleted spin ladders

The magnetic responses of a spin-1/2 ladder doped with non-magnetic impurities are studied using various methods and including the regime where frustration induces incommensurability. Several improvements are made on the results of the seminal work of Sigrist and Furusaki [J. Phys. Soc. Jpn. 65, 2385 (1996)]. Deviations from the Brillouin magnetic curve due to interactions are also analyzed. First, the magnetic profile around a single impurity and effective interactions between impurities are analyzed within the bond-operator mean-field theory and compared to density-matrix renormalization group calculations. Then, the temperature behavior of the Curie constant is studied in details. At zero-temperature, we give doping-dependent corrections to the results of Sigrist and Furusaki on general bipartite lattice and compute exactly the distribution of ladder cluster due to chain breaking effects. Using exact diagonalization and quantum Monte-Carlo methods on the effective model, the temperature dependence of the Curie constant is compared to a random dimer model and a real-space renormalization group scenario. Next, the low-part of the magnetic curve corresponding to the contribution of impurities is computed using exact diagonalization. The random dimer model is shown to capture the bulk of the curve, accounting for the deviation from the Brillouin response. At zero-temperature, the effective model prediction agrees relatively well with density-matrix renormalization group calculations. Finite-temperature effects are displayed within the effective model and for large depleted ladder models using quantum Monte-Carlo simulations. In all, the effect of incommensurability does not display a strong qualitative effect on both the magnetic susceptibility and the magnetic curve. Consequences for experiments on the BiCu2PO6 compound and other spin-gapped materials are briefly discussed.

cond-mat.str-el↗

arXiv2013

Diluted antiferromagnets in a field seem to be in a different universality class than the random-field Ising model

We perform large-scale Monte Carlo simulations using the Machta-Newman-Chayes algorithms to study the critical behavior of both the diluted antiferromagnet in a field with 30% dilution and the random-field Ising model with Gaussian random fields for different field strengths. Analytical calculations by Cardy [Phys. Rev. B 29, 505 (1984)] predict that both models map onto each other and share the same universality class in the limit of vanishing fields. However, a detailed finite-size scaling analysis of both the Binder cumulant and the two-point finite-size correlation length suggests that even in the limit of small fields, where the mapping is expected to work, both models are not in the same universality class. Therefore, care should be taken when interpreting (experimental) data for diluted antiferromagnets in a field using the random-field Ising model. Based on our numerical data, we present analytical expressions for the phase boundaries of both models.

cond-mat.dis-nn↗

arXiv2013

Systematic investigation of the effects of disorder at the lowest order throughout the BCS-BEC crossover

A systematic investigation of the effects of disorder on the BCS-BEC crossover at the lowest order in the impurity potential is presented for the normal phase above the critical temperature Tc. Starting with the t-matrix approach for the clean system, by which pairing correlations between opposite-spin fermions evolve from the weak-coupling (BCS) to the strong-coupling (BEC) limits by increasing the strength of the attractive inter-particle interaction, all possible diagrammatic processes are considered where the effects of a disordered potential are retained in the self-energy at the lowest order. An accurate numerical investigation is carried out for all these diagrammatic terms, to determine which of them are mostly important throughout the BCS-BEC crossover. Explicit calculations for the values of Tc, the chemical potential, and the Tan's contact are carried out. In addition, the effect of disorder on the single-particle spectral function is analyzed, and a correlation is found between an increase of Tc and a widening of the pseudo-gap energy at Tc on the BCS side of unitarity in the presence of disorder, while on the BEC side of unitarity the presence of disorder favors the collapse of the underlying Fermi surface. The present investigation is meant to orient future studies when the effects of disorder will be considered at higher orders, with the purpose of limiting the proliferation of diagrammatic terms in which interaction and disorder are considered simultaneously.

cond-mat.quant-gas↗

arXiv2013

Electrical detection of topological quantum phase transitions in disordered Majorana nanowires

We study a disordered superconducting nanowire, with broken time-reversal and spin-rotational symmetry, which can be driven into a topological phase with end Majorana bound states by an externally applied magnetic field. As a function of disorder strength, it is known that the Majorana nanowire has a delocalization quantum phase transition from a topologically nontrivial phase, which supports Majorana bound states, to a nontopological insulating phase without them. On both sides of the transition, the system is localized at zero energy albeit with very different topological properties. We exploit this deep connection between topology and localization properties to propose an electrical transport measurement to detect the localization-delocalization transition occurring in the bulk of the nanowire. The basic idea consists of measuring the difference of conductance at one end of the wire obtained at different values of the coupling to the opposite lead. We show that this measurement reveals the nonlocal correlations emergent only at the topological transition. Hence, while the proposed experiment does not directly probe the end Majorana bound states, it can provide direct evidence for the bulk topological quantum phase transition itself.

cond-mat.mes-hall↗

arXiv2013

Interaction-disorder competition in a spin system evaluated through the Loschmidt Echo

The interplay between interactions and disorder in closed quantum many-body systems is relevant for thermalization phenomenon. In this article, we address this competition in an infinite temperature spin system, by means of the Loschmidt echo (LE), which is based on a time reversal procedure. This quantity has been formerly employed to connect quantum and classical chaos, and in the present many-body scenario we use it as a dynamical witness. We assess the LE time scales as a function of disorder and interaction strengths. The strategy enables a qualitative phase diagram that shows the regions of ergodic and nonergodic behavior of the polarization under the echo dynamics.

cond-mat.stat-mech↗

arXiv2013

Enhancement of critical temperatures in disordered bipartite lattices

We study the strong enhancement, induced by random hopping, of the critical temperatures characterizing the transitions to superconductivity, charge-density wave and antiferromagnetism, which can occur in bipartite lattice models at half-filling, like graphene, by means of an extended Finkel'stein non-linear $σ$-model renormalization group approach. We show that, if Cooper channel interaction dominates, superconducting critical temperature can be enhanced at will, since superconductivity cannot be broken by entering any Anderson insulating phase. If, instead, staggered interactions are relevant, antiferromagnetic order is generated by disorder at a temperature well above that expected for a clean system.

cond-mat.dis-nn↗

arXiv2013

Susceptibility at the Superfluid-Insulator Transition for One-Dimensional Disordered Bosons

A pair of recent Monte Carlo studies have reported evidence for and against a crossover from weak to strong-disorder criticality in the one-dimensional dirty boson problem. The Monte Carlo analyses rely on measurement of two observables: the effective Luttinger parameter K_{eff} and the superfluid susceptibility chi. The former quantity was previously calculated analytically, using the strong-disorder renormalization group (SDRG), by Altman, Kafri, Polkovnikov, and Refael. Here, we use an extension of the SDRG framework to find a non-universal anomalous dimension eta_{sd} characterizing the divergence of the susceptibility with system size: chi ~ L^(2-eta_{sd}). We show that eta_{sd} obeys the hyperscaling relation eta_{sd} = 1/2K_{eff}. We also identify an important obstacle to measuring this exponent on finite-size systems and comment on the implications for numerics and experiments.

cond-mat.dis-nn↗

arXiv2013

Numerical evidence for strong randomness scaling at a superfluid-insulator transition of one-dimensional bosons

We present numerical evidence from Monte Carlo simulations that the superfluid-insulator quantum phase transition of interacting bosons subject to strong disorder in one dimension is controlled by the strong-randomness critical point. At this critical point the distribution of superfluid stiffness over disorder realizations develops a power-law tail reflecting a universal distribution of weak links. The Luttinger parameter on the other hand does not take on a universal value at this critical point, in marked contrast to the known Berezinskii-Kosterlitz-Thouless-like superfluid-insulator transition in weakly disordered systems. We develop a finite-size scaling procedure which allows us to directly compare the numerical results from systems of linear size up to 1024 sites with theoretical predictions obtained by Altman et al. [ Phys. Rev. Lett. 93 150402 (2004)] using a strong disorder renormalization group approach. The data shows good agreement with the scaling expected at the strong-randomness critical point.

cond-mat.dis-nn↗

arXiv2013

Interlayer excitonic superfluidity in graphene

We discuss the conditions under which the predicted (but not yet observed) zero-field interlayer excitonic condensation in double layer graphene has a critical temperature high enough to allow detection. Crucially, disorder arising from charged impurities and corrugation in the lattice structure --- invariably present in all real samples --- affects the formation of the condensate via the induced charge inhomogeneity. In the former case, we use a numerical Thomas-Fermi-Dirac theory to describe the local fluctuations in the electronic density in double layer graphene devices and estimate the effect these realistic fluctuations have on the formation of the condensate. To make this estimate, we calculate the critical temperature for the interlayer excitonic superfluid transition within the mean-field BCS theory for both optimistic (unscreened) and conservative (statically screened) approximations for the screening of the interlayer Coulomb interaction. We also estimate the effect of allowing dynamic contributions to the interlayer screening. We then conduct similar calculations for double quadratic bilayer graphene, showing that the quadratic nature of the low-energy bands produces pairing with critical temperature of the same order of magnitude as the linear bands of double monolayer graphene.

cond-mat.mes-hall↗

arXiv2013

Effect of weak impurities on electronic properties of graphene: functional renormalization-group analysis

We consider an effect of weak impurities on electronic properties of graphene within the functional renormalization-group approach. The energy dependences of the electronic self-energy and density of states near the neutrality point are discussed. Depending on the symmetry of the impurities, the electronic damping $Γ$ and density of states $ρ$ can deviate substantially from those given by the self-consistent Born approximation. We investigate the crossover from the results of the self-consistent Born approximation, which are valid far from the neutrality point to the strong-coupling (diffusive) regime near the neutrality point. For impurities, which are diagonal in both, valley and sublattice indices, we obtain finite density of states at the Fermi level with the values, which are much bigger than the results of the self-consistent Born approximation.

cond-mat.str-el↗