arXiv subjects

O. S. Barisic

Publications and source records attributed to O. S. Barisic.

10 recordsLinked to original sources

arXiv2015

Polaronic metal state at the LaAlO3/SrTiO3 interface

Interplay of spin, charge, orbital and lattice degrees of freedom in oxide heterostructures results in a plethora of fascinating properties, which can be exploited in new generations of electronic devices with enhanced functionalities. The paradigm example is the interface between the two band insulators LaAlO3 and SrTiO3 (LAO/STO) that hosts two-dimensional electron system (2DES). Apart from the mobile charge carriers, this system exhibits a range of intriguing properties such as field effect, superconductivity and ferromagnetism, whose fundamental origins are still debated. Here, we use soft-X-ray angle-resolved photoelectron spectroscopy to penetrate through the LAO overlayer and access charge carriers at the buried interface. The experimental spectral function directly identifies the interface charge carriers as large polarons, emerging from coupling of charge and lattice degrees of freedom, and involving two phonons of different energy and thermal activity. This phenomenon fundamentally limits the carrier mobility and explains its puzzling drop at high temperatures.

cond-mat.mtrl-sci↗

arXiv2013

Tunable polaronic conduction in anatase TiO2

Oxygen vacancies created in anatase TiO2 by UV photons (80 - 130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission (ARPES) reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.

cond-mat.mtrl-sci↗

arXiv2009

Incoherent transport induced by a single static impurity in a Heisenberg chain

The effect of a single static impurity on the many-body states and on the spin and thermal transport in the one-dimensional anisotropic Heisenberg chain at finite temperatures is studied. Whereas the pure Heisenberg model reveals Poisson level statistics and dissipationless transport due to integrability, we show using the numerical approach that a single impurity induces Wigner-Dyson level statistics and at high enough temperature incoherent transport within the chain, whereby the relaxation time and d.c. conductivity scale linearly with length.

cond-mat.str-el↗

arXiv2008

Slave fermion approach to the metallic phase in large $U_d$ high-T$_c$ cuprates

Atomic repulsion $U_d$ on the Cu site in high T$_c$ cuprates is large but, surprisingly, some important properties are consistent with moderate couplings. The time dependent perturbation theory with slave particles is therefore formulated in the $U_d\to\infty$ limit for the metallic phase in the physically relevant regime of the three-band Emery model. The basic theory possesses the local gauge invariance asymptotically but its convergence is fast when the average occupation of the Cu-site is small. The leading orders exhibit the band narrowing and the dynamic Cu/O$_2$ charge transfer disorder. The effective local repulsion between particles on oxygen sites is shown to be moderate in the physical regime under consideration. It enhances the coherent incommensurate SDW correlations. The latter compete with the Cu/O$_2$ charge transfer disorder, in agreement with basic observations in high T$_c$ cuprates.

cond-mat.str-el↗

arXiv2008

Phase diagram of the Holstein polaron in one dimension

The behavior of the 1D Holstein polaron is described, with emphasis on lattice coarsening effects, by distinguishing between adiabatic and nonadiabatic contributions to the local correlations and dispersion properties. The original and unifying systematization of the crossovers between the different polaron behaviors, usually considered in the literature, is obtained in terms of quantum to classical, weak coupling to strong coupling, adiabatic to nonadiabatic, itinerant to self-trapped polarons and large to small polarons. It is argued that the relationship between various aspects of polaron states can be specified by five regimes: the weak-coupling regime, the regime of large adiabatic polarons, the regime of small adiabatic polarons, the regime of small nonadiabatic (Lang-Firsov) polarons, and the transitory regime of small pinned polarons for which the adiabatic and nonadiabatic contributions are inextricably mixed in the polaron dispersion properties. The crossovers between these five regimes are positioned in the parameter space of the Holstein Hamiltonian.

cond-mat.str-el↗

arXiv2007

Diagrammatic content of the DMFT for the Holstein polaron problem in finite dimensions

In the context of the Holstein polaron problem it is shown that the dynamical mean field theory (DMFT) corresponds to the summation of a special class of local diagrams in the skeleton expansion of the self-energy. In the real space representation, these local diagrams are characterized by the absence of vertex corrections involving phonons at different lattice sites. Such corrections vanish in the limit of infinite dimensions, for which the DMFT provides the exact solution of the Holstein polaron problem. However, for finite dimensional systems the accuracy of the DMFT is limited. In particular, it cannot describe correctly the adiabatic spreading of the polaron over multiple lattice sites. Arguments are given that the DMFT limitations on vertex corrections found for the Holstein polaron problem persist for finite electron densities and arbitrary phonon dispersion.

cond-mat.str-el↗

arXiv2006

Relevant coherent states method for the quantum adiabatic dynamics of lattice-coupled charge carriers

A new numerical method is proposed for determining the low-frequency dynamics of the charge carrier coupled to the deformable quantum lattice. As an example, the polaron band structure is calculated for the one-dimensional Holstein model. The adiabatic limit on the lattice, which cannot be reached by other approaches, is investigated. In particular, an accurate description is obtained of the crossover between quantum small adiabatic polarons, pinned by the lattice, and large adiabatic polarons, moving along the continuum as classical particles. It is shown how the adiabatic contributions to the polaron dispersion, involving spatial correlations over multiple lattice sites, can be treated easily in terms of coherent states.

cond-mat.str-el↗

arXiv2006

Quantum adiabatic polarons by translationally invariant perturbation theory

The translationally invariant diagrammatic quantum perturbation theory (TPT) is applied to the polaron problem on the 1D lattice, modeled through the Holstein Hamiltonian with the phonon frequency omega0, the electron hopping t and the electron-phonon coupling constant g. The self-energy diagrams of the fourth-order in g are calculated exactly for an intermittently added electron, in addition to the previously known second-order term. The corresponding quadratic and quartic corrections to the polaron ground state energy become comparable at t/omega0>1 for g/omega0~(t/omega0)^{1/4} when the electron self-trapping and translation become adiabatic. The corresponding non adiabatic/adiabatic crossover occurs while the polaron width is large, i.e. the lattice coarsening negligible. This result is extended to the range (t/omega0)^{1/2}>g/omega0>(t/omega0)^{1/4}>1 by considering the scaling properties of the high-order self-energy diagrams. It is shown that the polaron ground state energy, its width and the effective mass agree with the results found traditionally from the broken symmetry side, kinematic corrections included. The Landau self trapping of the electron in the classic self-consistent, localized displacement potential, the restoration of the translational symmetry by the classic translational Goldstone mode and the quantization of the polaronic translational coordinate are thus all encompassed by a quantum theory which is translationally invariant from the outset.

cond-mat.str-el↗

arXiv2006

Holstein light quantum polarons

The polaron formation is investigated in the intermediate regime of the Holstein model by using an exact diagonalization technique for the one-dimensional infinite lattice. The numerical results for the electron and phonon propagators are compared with the nonadiabatic weak- and strong-coupling perturbation theories, as well as with the harmonic adiabatic approximation. A qualitative explanation of the crossover regime between the self-trapped and free-particle-like behaviors, not well-understood previously, is proposed. It is shown that a fine balance of nonadiabatic and adiabatic contributions determines the motion of small polarons, making them light. A comprehensive analysis of spatially and temporally resolved low-frequency lattice correlations that characterize the translationally invariant polaron states is derived. Various behaviors of the polaronic deformation field, ranging from classical adiabatic for strong couplings to quantum nonadiabatic for weak couplings, are discussed.

cond-mat.str-el↗

arXiv2002

Variational study of the Holstein polaron

The paper deals with the ground and the first excited state of the polaron in the one dimensional Holstein model. Various variational methods are used to investigate both the weak coupling and strong coupling case, as well as the crossover regime between them. Two of the methods, which are presented here for the first time, introduce interesting elements to the understanding of the nature of the polaron. Reliable numerical evidence is found that, in the strong coupling regime, the ground and the first excited state of the self-trapped polaron are well described within the adiabatic limit. The lattice vibration modes associated with the self-trapped polarons are analyzed in detail, and the frequency softening of the vibration mode at the central site of the small polaron is estimated. It is shown that the first excited state of the system in the strong coupling regime corresponds to the excitation of the soft phonon mode within the polaron. In the crossover regime, the ground and the first excited state of the system can be approximated by the anticrossing of the self-trapped and the delocalized polaron state. In this way, the connection between the behavior of the ground and the first excited state is qualitatively explained.

cond-mat↗