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arXiv2013

Study of interface phenomena in a topological-insulator/Mott-insulator heterostructure

We theoretically investigate a two-dimensional heterostructure composed of a topological insulator (TI) and a Mott insulator (MI), and clarify what kind of electronic states can be realized at the interface. By using inhomogeneous dynamical mean-field theory, we confirm that the topological edge state penetrating into the MI region induces a heavy-fermion like mid-gap state. We further elucidate the nature of the spatially-modulated quasi-particle weight of the mid-gap state, and discuss the effects of local correlation in the TI region. The optical conductivity and the Drude weight are also computed with changing the electron tunneling near the interface.

cond-mat.str-el↗

arXiv2013

The electronic transport of top subband and disordered sea in InAs nanowire in presence of a mobile gate

We performed measurements at helium temperatures of the electronic transport in an InAs quantum wire ($R_{wire} \sim 30$\,k$Ω$) in the presence of a charged tip of an atomic force microscope serving as a mobile gate. The period and the amplitude of the observed quasiperiodic oscillations are investigated in detail as a function of electron concentration in the linear and non-linear regime. We demonstrate the influence of the tip-to-sample distance on the ability to locally affect the top subband electrons as well as the electrons in the disordered sea. Furthermore, we introduce a new method of detection of the subband occupation in an InAs wire, which allows us to evaluate the number of the electrons in the conductive band of the wire.

cond-mat.mes-hall↗

arXiv2013

CaMn$_{2}$Sb$_{2}:$ a fully frustrated classical magnetic system

We show by means of density functional calculations that CaMn$_{2}$Sb$_{2}$ is very close to a mean-field critical point known for the classical Heisenberg model on the honeycomb lattice. Three entirely different long range ordered magnetic phases become degenerate at this point: the Neel phase and two different spiral phases. We speculate that the unusual physical properties of this compound, observed in recent experiments, in particular the enigmatic intermediate temperature phase, are due to this proximity.

cond-mat.str-el↗

arXiv2013

Entanglement Entropies in One-Dimensional Systems

Entanglement entropies have revealed, in the last years, to be a powerful tool to extract information about the physics of condensed-matter systems. In the first part of this thesis, we show how to extract essential details about the quasi-long-range order of a one-dimensional critical systems by means of entanglement entropies. In the second part we show how to derive analytically the scaling of such quantities for critical systems, whose low-energy physics is described by a conformal field theory, in presence of general open boundary conditions that preserve the conformal invariance.

cond-mat.str-el↗

arXiv2013

Ground state of the competing spin chain Cs2Cu2Mo3O12

The high-field ground state of the competing-spin-chain compound Cs2Cu2Mo3O12 with the ferromagnetic first-nearest-neighbor J1=-93 K and the antiferromagnetic second-nearest-neighbor J2 = +33 K was investigated by 133Cs-NMR. A divergence of T1-1 and a peak-splitting in spectra were observed at TN = 1.85 K, indicating the existence of a field-induced long range magnetic order. In the paramagnetic region above 4 K, T1-1 showed a power-law temperature dependence T2K-1. The exponent K was strongly field-dependent, suggesting the possibility of the spin-nematic Tomonaga Luttinger Liquid state.

cond-mat.stat-mech↗

arXiv2013

Magnetic Orders of Correlated Topological Insulators at Finite Temperature

In this paper, we study the magnetic orders of two dimensional correlated topological insulators including the correlated Chern insulator and the correlated Z2 topological insulator at finite temperature. For the 2D correlated Chern insulator, we found that thermal-fluctuation-induced magnetic order appears in the intermediate interaction region of the correlated Chern insulator. On the contrary, for the correlated Z2 topological insulator there doesn't exist the thermal-fluctuation-induced magnetic order. In the end, we give an explanation on the difference.

cond-mat.str-el↗

arXiv2013

Tunable single particle and many body physics in graphene

This thesis studies how the rudimentary attributes of graphene's charge carriers, and local moments on its surface, can be directly manipulated and controlled with electrostatic potentials. We first consider bilayer graphene subject to a spatially varying electrostatic potential that forms two neighbouring regions with opposite interlayer bias. Along the boundary, 1D chiral "kink" states emerge. We find that these 1D modes behave as a strongly interacting Luttinger liquid whose properties can be tuned via an external gate. Next, we consider superlattices in bilayer graphene. Superlattices are seen to have a more dramatic effect on bilayer graphene than monolayer graphene because the quasi-particles are changed in a fundamental way; the dispersion goes from a quadratic band touching point to linearly dispersing Dirac cones. We illustrate that a 1D superlattice of either the chemical potential or an interlayer bias generates multiple anisotropic Dirac cones. General arguments delineate how certain symmetries protect the Dirac points. We then map the Hamiltonian of an interlayer bias superlattice onto a chain model comprised of "topological" modes. This is followed by another study that considers the effect of a magnetic field on graphene superlattices. We show that magnetotransport measurements in a weak perpendicular magnetic field probe the number of emergent Dirac points and reveal details about the dispersion. In the case of bilayer graphene, we also discuss the properties of kink states in an applied magnetic field. Finally, we investigate local moment formation of adatoms on bilayer graphene using an Anderson impurity model. We identify regions where the local moments can be turned on or off by applying a external electric fields. In addition, we compute the RKKY interaction between local moments and show how it too can be controlled with electric fields.

cond-mat.str-el↗

arXiv2013

Effect of Strain disorder on the magnetic glassy state in La5/8-yPryCa3/8MnO3 (y = 0.45) thin films

Present study reveals that the free energy landscape of the La5/8-yPryCa3/8MnO3 (LPCMO) system could be modified by elastic strain interaction in the epitaxial thin films. Epitaxial LPCMO thin films of various thicknesses are grown on LaAlO3 substrate by pulsed laser deposition. With increasing thickness, by virtue of island growth morphology, strain disorder is invoked in thin films during strain relaxation process. The length-scale of phase separation is found to be highly correlated with strain disorder. Magneto-transport measurements demonstrate that coherent strain stabilizes charge ordered insulating phase while strain disorder stabilizes metallic phase. Resistivity under cooling and heating in unequal field (CHUF) protocol exhibits lower value of freezing temperature for strain disordered films compared to bulk system. Raman spectroscopy reveals that the charge ordered insulating and ferromagnetic metallic phases are structurally dissimilar and possess monoclinic and rhombohedral like symmetries respectively. Interfaces between two phases strongly influence low temperature glassy metastable state resulting in different phase separation states in the LPCMO thin films.

cond-mat.str-el↗

arXiv2013

Exact holographic mapping and emergent space-time geometry

In this paper, we propose an {\it exact holographic mapping} which is a unitary mapping from the Hilbert space of a lattice system in flat space (boundary) to that of another lattice system in one higher dimension (bulk). By defining the distance in the bulk system from two-point correlation functions, we obtain an emergent bulk space-time geometry that is determined by the boundary state and the mapping. As a specific example, we study the exact holographic mapping for $(1+1)$-dimensional lattice Dirac fermions and explore the emergent bulk geometry corresponding to different boundary states including massless and massive states at zero temperature, and the massless system at finite temperature. We also study two entangled one-dimensional chains and show that the corresponding bulk geometry consists of two asymptotic regions connected by a worm-hole. The quantum quench of the coupled chains is mapped to dynamics of the worm-hole. In the end we discuss the general procedure of applying this approach to interacting systems, and other open questions.

hep-th↗

arXiv2013

Linear temperature dependence of electron spin resonance linewidths in La0.7Ca0.3MnO3 and YBaMn2O6

We analyze recent electron spin resonance (ESR) experiments in La0.7Ca0.3MnO3 and YBaMn2O6 focusing on the behavior of the linewidth at high temperatures where it is a linear function of the temperature. Noting that the g-factors of the resonances are characteristic of the Mn4+ ion in a cubic environment, we make the assumption that the linewidth involves the static susceptibility of the Mn4+ spins which we analyze in the molecular field approximation. We conclude that the linear dependence on temperature is associated with the susceptibility having a Curie or Curie-Weiss form while the temperature-dependent relaxation mechanism has a microscopic rate proportional to the temperature. In La0.7Ca0.3MnO3, the Mn4+ susceptibility has the ferromagnetic Curie-Weiss form, and the static contribution to the linewidth arising from distortions of the oxygen octahedra is absent due to motional narrowing brought on by the rapid hopping of the eg polarons. In YBaMn2O6 either of two scenarios is possible. The Mn4+ susceptibility above 520 K is Curie-like and the static term is present, or the susceptibility has the antiferromagnetic Curie-Weiss form and the static term is absent due to motional narrowing. It is concluded that the Curie model, with offsetting double exchange and and superexchange Curie-Weiss parameters, is the more likely scenario. It is suggested that the linear-T variation of the linewidth in both materials arises from either a Korringa-like mechanism involving interactions with mobile carriers or from a spin-phonon process coming from interactions between the Mn4+ ions and the lattice vibrations.

cond-mat.str-el↗

arXiv2013

Spin Gap in Heavy Fermion UBe$_{13}$

Muon spin rotation ($μ^+$SR) experiments have been carried out in the heavy fermion compound UBe$_{13}$ in the temperature range from 0.025 K to 300 K and in magnetic fields up to 7 T, supported by resistivity and magnetization measurements. The $μ^+$SR spectra are characteristic of formation of a spin polaron which persists to the lowest temperature. A gap in the spin excitation spectrum of f-electrons opens up at about 180 K, consistent with anomalies discovered in resistivity, heat capacity, NMR and optical conductivity measurements of UBe$_{13}$.

cond-mat.str-el↗

arXiv2013

On the BCS-BEC crossover in the Spin-Polarized Attractive Hubbard Model at T=0

The influence of the Zeeman magnetic field ($h$) on the ground state evolution of superfluid properties from the weak coupling (BCS like) to the strong coupling limit of tightly bound local pairs (LP) with increasing attraction has been studied. The analysis was carried out within the spin-polarized attractive Hubbard model, for a fixed number of particles and a $d=3$ simple cubic lattice. The broken symmetry Hartree approximation was used. For strong attraction and in the dilute limit, the homogeneous magnetized superconducting phase (SC$_M$) was found in the $(h-|U|)$ and ($P-|U|$) diagrams.

cond-mat.str-el↗

arXiv2013

On the BCS-BEC crossover in the 2D Asymmetric Attractive Hubbard Model

We analyze the evolution from the weak coupling (BCS-like limit) to the strong coupling limit of tightly bound local pairs (LP's) in the 2D asymmetric attractive Hubbard model, in the presence of the Zeeman magnetic field ($h$). The broken symmetry Hartree approximation is used. We also apply the Kosterlitz-Thouless (KT) scenario to determine the phase coherence temperatures. We obtain that for the spin dependent hopping integrals ($t^{\uparrow}\neq t^{\downarrow}$) the homogeneous polarized superfluid (SC$_M$) phase in the ground state for the strong attraction and lower filling can be stabilized. We find a topological quantum phase transition (Lifshitz type) from the unpolarized superfluid phase (SC$_0$) to SC$_M$ and tricritical point in the ($h-μ$) and spin polarization ($P$) vs. attraction ($U<0$) ground state phase diagrams. The finite temperatures phase diagrams for $t^{\uparrow}\neq t^{\downarrow}$ are constructed.

cond-mat.str-el↗

arXiv2013

On the Imbalanced d-wave Superfluids within the Spin Polarized Extended Hubbard Model: Weak Coupling Limit

We investigate the superfluid properties of d-wave pairing symmetry within the Extended Hubbard Model (EHM) in a magnetic field. We analyze the temperature and magnetic field dependencies of the order parameter. We find that in the two-dimensional case, the spatially homogeneous spin polarized superfluidity ($SC(P\neq0)$) is stable in the weak coupling limit, at T=0, as opposed to the s-wave pairing symmetry case in 2D. We construct the ground state phase diagrams both for fixed chemical potential ($μ$) and electron concentration ($n$). Furthermore, we obtain the temperature vs. magnetic field and temperature vs. spin polarization phase diagrams.

cond-mat.str-el↗

arXiv2013

The influence of magnetic field on the superconducting properties and the BCS-BEC crossover in systems with local fermion pairing

The aim of this Ph.D. thesis was to investigate superconducting properties in the presence of Zeeman magnetic field in systems with local fermion pairing on the lattice. The study also concerned the evolution from the weak coupling (BCS-like) limit to the strong coupling limit of tightly bound local pairs (BEC) with increasing attraction, both in the ground state and at finite temperatures, within the spin-polarized extended Hubbard model. The analysis was also extended to the case of spin dependent hopping integrals (mass imbalance), with special attention paid to the BCS-BEC crossover physics in the ground state. The methods used included: the mean field approximation (BCS-Stoner type) and the estimation of the phase coherence temperature within the Kosterlitz-Thouless (KT) scenario in two dimensions. The BCS-BEC crossover was also analyzed in three dimensions, at finite temperatures, within the spin polarized Attractive Hubbard Model (AHM), going beyond the mean field approximation. In this case, the critical temperatures of the superconducting transition were determined within the self-consistent T-matrix method. The strong coupling expansion was applied to map the spin polarized AHM onto the model of hard-core bosons on the lattice.

cond-mat.str-el↗

arXiv2013

The Transition to the Metallic State in Low Density Hydrogen

Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work we use diffusion quantum Monte Carlo to benchmark the transition between paramagnetic and anti-ferromagnetic body centered cubic atomic hydrogen in its ground state. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of $r_s=2.27(3) a_0$. We compare our results to previously reported density functional theory, Hedin's GW approximation, and dynamical mean field theory results.

cond-mat.str-el↗

arXiv2013

Band filling effect on polaron localization in La$_{1-x}($Ca$_y$Sr$_{1-y}$)$_{x}$MnO$_3$ manganites

We report on an a $μ$SR and $^{55}$Mn NMR investigation of the magnetic order parameter as a function of temperature in the optimally doped La$_{5/8}$(Ca$_y$Sr$_{1-y}$)$_{3/8}$MnO$_3$ and in the underdoped La$_{1-x}$Sr$_{x}$MnO$_3$ and La$_{1-x}$Ca$_{x}$MnO$_3$ metallic manganite families. The study is aimed at unraveling the effect of lattice distortions, implicitly controlled by the Ca-Sr isoelectronic substitution, from that of hole doping $x$ on the Curie temperature $T_c$ and the order of the magnetic transition. At optimal doping, the transitions are second order at all $y$ values, including the $y=1$ (La$_{5/8}$Ca$_{3/8}$MnO$_3$) end member. In contrast, they are first order in the underdoped samples, which show a finite (truncated) order parameter at the Curie point, including La$_{0.75}$Sr$_{0.25}$MnO$_3$ whose $T_c$ is much higher than that of La$_{5/8}$Ca$_{3/8}$MnO$_3$. The order parameter curves, on the other hand, exhibit a very minor dependence on $x$, if truncation is excepted. This suggests that the effective exchange interaction between Mn ions is essentially governed by local distortions, in agreement with the original double-exchange model, while truncation is primarily, if not entirely, an effect of under- or overdoping. A phase diagram, separating in the $x-y$ plane polaron-driven first order transitions from regular second order transitions governed by critical fluctuations, is proposed for the La$_{1-x}($Ca$_y$Sr$_{1-y}$)$_{x}$MnO$_3$ system.

cond-mat.str-el↗

arXiv2013

Collapsing Schrödinger Cats in the Density Matrix Renormalization Group

In this paper, we propose a modified Density Matrix Renormalization Group (DMRG) algorithm to preferentially select minimum entropy states (minimally entangled states) in finite systems with asymptotic ground state degeneracy. The algorithm adds a "quench" process to the conventional DMRG method, which mimics the decoherence of physical systems, and collapses non-locally entangled states such as Schrödinger cats. We show that the method works for representative models with ground state degeneracy arising from either topological order or spontaneous discrete symmetry breaking. In the minimal entropy states thus obtained, properties associated with thermodynamic limit, such as topological entanglement entropy and magnetic order parameters, can be obtained directly and efficiently.

cond-mat.str-el↗