arXiv subjects

Gabriel Kotliar

Publications and source records attributed to Gabriel Kotliar.

At least 127 records · Page 7Linked to original sources

arXiv2010

Incommensurate Magnetism in FeAs Strips: Neutron Scattering from CaFe$_4$As$_3$

Magnetism in the orthorhombic metal CaFe$_4$As$_3$ was examined through neutron diffraction for powder and single crystalline samples. Incommensurate (${\bm q}_{\rm m}\approx (0.37-0.39)\times{\bm b}^{\ast}$) and predominantly longitudinally ($\parallel b$) modulated order develops through a 2nd order phase transition at $T_{\rm N}=89.63(6)$ K with a 3D Heisenberg-like critical exponent $β=0.365(6)$. A 1st order transition at $T_2=25.6(9)$ K is associated with the development of a transverse component, locking ${\bm q}_{\rm m}$ to $0.375(2){\bm b}^{\ast}$, and increasing the moments from 2.1(1) to 2.2(3) $μ_{\rm B}$ for Fe$^{2+}$ and from 1.3(3) to 2.4(4) $μ_{\rm B}$ for Fe$^+$. The {\it ab-initio} Fermi surface is consistent with a nesting instability in cross-linked FeAs strips.

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arXiv2010

Local suppression of the hidden order phase by impurities in URu2Si2

We consider the effects of impurities on the enigmatic hidden order (HO) state of the heavy-fermion material URu2Si2. In particular, we focus on local effects of Rh impurities as a tool to probe the suppression of the HO state. To study local properties we introduce a lattice free energy, where the time invariant HO order parameter "psi" and local antiferromagnetic (AFM) order parameter M are competing orders. Near each Rh atom the HO order parameter is suppressed, creating a hole in which local AFM order emerges as a result of competition. These local holes are created in the fabric of the HO state like in a Swiss cheese and "filled" with droplets of AFM order. We compare our analysis with recent NMR results on URu2Si2 doped with Rh and find good agreement with the data.

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arXiv2010

Magnetization, Maxwell's Relations and the Local Physics of the Dilute System Th_{1-x}U_{x}Ru_{2}Si_{2}

The dilute Kondo compound, Th_{1-x}U_{x}Ru_{2}Si_{2} (TURS), displays non-Fermi liquid behavior but no zero-point entropy; it thus appears to elude description by known single-ion models. It may also provide a clue to the underlying local degrees of freedom in its dense counterpart, URu_{2}Si_{2}. Here we use high-resolution magnetization studies to cross-check the thermodynamic consistency of previous experiments. Measurements of the field-dependence of the temperature-scale, T_F(H), associated with Fermi liquid behavior probes the nature of the underlying impurity fixed point. We find that T_F(H) grows linearly with applied field, in contrast to the quadratic form expected for the two-channel Kondo model. We use a scaling argument to show that the observed behavior of T_F(H) is consistent with the absence of zero-point entropy, suggesting novel impurity behavior in this material. More generally, we suggest the field-magnetization as a probe of single-ion physics and make predictions for its behavior in other actinide compounds.

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arXiv2010

Extended Hubbard model: Charge Ordering and Wigner-Mott transition

Strong correlation effects, which are often associated to the approach to a Mott insulating state, in some cases may be observed even far from half-filling. This typically happens whenever the inter-site Coulomb repulsion induces a tendency towards charge ordering, an effect that confines the electrons, and in turn favors local moment formation, i.e. Mott localization. A distinct intermediate regime then emerges as a precursor of such a Wigner-Mott transition, which is characterized by both charge and spin correlations, displaying large mass enhancements and strong renormalizations of other Fermi liquid parameters. Here we present a careful study of a quarter filled extended Hubbard model - a simple example where such physics can be studied in detail, and discuss its relevance for the understanding of the phenomenology of low-density two dimensional electron gases.

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arXiv2010

Strength of Correlations in electron and hole doped cuprates

High temperature superconductivity was achieved by introducing holes in a parent compound consisting of copper oxide layers separated by spacer layers. It is possible to dope some of the parent compounds with electrons, and their physical properties are bearing some similarities but also significant differences from the hole doped counterparts. Here, we use a recently developed first principles method, to study the electron doped cuprates and elucidate the deep physical reasons why their behavior is so different than the hole doped materials. We find that electron doped compounds are Slater insulators, e.g. a material where the insulating behavior is the result of the presence of magnetic long range order. This is in sharp contrast with the hole doped materials, where the parent compound is a Mott charge transfer insulator, namely a material which is insulating due to the strong electronic correlations but not due to the magnetic order.

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arXiv2010

Apical oxygens and correlation strength in electron and hole doped copper oxides

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to carry out a comparative investigation of a typical electron doped and a typical hole doped copper oxide, NCCO and LSCO respectively. The parent compounds of both materials are strongly correlated electron systems in the vicinity of the metal to charge transfer insulator transition. In NCCO the magnetic long range order is essential to open a charge transfer gap, while Mott physics is responsible for the gap in LSCO. We highlights the role of the apical oxygens in determining the strength of the correlations and obtaining overall good agreement between theory and several experimentally determined quantities. Results for optical conductivity, polarized X-ray absorption and angle resolved photoemission are presented and compared with experiments.

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arXiv2010

Effects of strain on the electronic structure of VO_2

We present cluster-DMFT (CTQMC) calculations based on a downfolded tight-binding model in order to study the electronic structure of vanadium dioxide (VO_2) both in the low-temperature (M_1) and high-temperature (rutile) phases. Motivated by the recent efforts directed towards tuning the physical properties of VO_2 by depositing films on different supporting surfaces of different orientations we performed calculations for different geometries for both phases. In order to investigate the effects of the different growing geometries we applied both contraction and expansion for the lattice parameter along the rutile c-axis in the 3-dimensional translationally invariant systems miming the real situation. Our main focus is to identify the mechanisms governing the formation of the gap characterizing the M_1 phase and its dependence on strain. We found that the increase of the band-width with compression along the axis corresponding to the rutile c-axis is more important than the Peierls bonding-antibonding splitting.

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arXiv2010

Bulk magnetic order in a two dimensional Ni1+/Ni2+ (d9/d8) nickelate, isoelectronic with superconducting cuprates

Bulk magnetic order in two dimensional La4Ni3O8 nickelate with Ni1+/Ni2+ (d9/d8), isoelectronic with superconducting cuprates is demonstrated experimentally and theoretically. Magnetization, specific heat and 139La NMR evidence a transition at 105 K to an antiferromagnetic state. Theoretical calculations by DFT relate the transition to a nesting instability of the Fermi surface with ordering wave-vector Q = [1/3, 1/3, 0].

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arXiv2009

Complex Landau Ginzburg Theory of the Hidden Order in URu_2Si_2

We develop a Landau Ginzburg theory of the hidden order phase and the local moment antiferromagnetic phase of URu_2Si_2. We unify the two broken symmetries in a common complex order parameter and derive many experimentally relevant consequences such as the topology of the phase diagram in magnetic field and pressure. The theory accounts for the appearance of a moment under application of stress and the thermal expansion anomaly across the phase transitions. It identifies the low energy mode which is seen in the hidden order phase near the conmensurate wavector (0,0, 1) as the pseudo-Goldstone mode of the approximate U(1) symmetry.

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arXiv2009

Valence Fluctuations and Quasiparticle Multiplets in Pu Chalcogenides and Pnictides

The spectra of Pu chalcogenides and pnictides are computed with LDA+DMFT and interpreted with the aid of valence histograms and slave-boson calculations. We find the chalcogenides are mixed-valent ($n_f = 5.2$) materials with a strongly $T$-dependent low-energy density of states and a triplet of quasiparticle peaks below the Fermi level. Furthermore, we predict a doublet of reflected peaks above the Fermi level. In the pnictides, the raising of $f^6$ states relative to $f^5$ suppresses valence fluctuations, resulting in integral-valent ($n_f = 5.0$) local moment metals.

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arXiv2009

Arrested Kondo effect and hidden order in URu_2Si_2

Complex electronic matter exhibit subtle forms of self organization which are almost invisible to the available experimental tools, but which have dramatic physical consequences. One prominent example is provided by the actinide based heavy fermion material URu_2Si_2. At high temperature, the U-5f electrons in URu_2Si_2 carry a very large entropy. This entropy is released at 17.5K via a second order phase transition to a state which remains shrouded in mystery, and which was termed a "hidden order" state. Here we develop a first principles theoretical method to analyze the electronic spectrum of correlated materials as a function of the position inside the unit cell of the crystal, and use it to identify the low energy excitations of the URu_2Si_2. We identify the order parameter of the hidden order state, and show that it is intimately connected with magnetism. We present first principles results for the temperature evolution of the electronic states of the material. At temperature below 70K U-5f electrons undergo a multichannel Kondo effect, which is arrested at low temperature by the crystal field splitting. At lower temperatures, two broken symmetry states emerge, characterized by a complex order parameter ψ. A real $ψ$ describes the hidden order phase, and an imaginary ψcorresponds to the large moment antiferromagnetic phase, thus providing a unified picture of the two broken symmetry phases, which are realized in this material.

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arXiv2009

Thermoelectrics Near the Mott Localization-Delocalization Transition

We give an overview on current status of the theoretical research on Thermoelectricity for correlated materials. We derive the theoretical formulas which become exact at low and high temperature and discuss the intermediate temperature results. In particular, we show that within Dynamical Mean Field Theory the low temperature sign of the thermopower is not necessary the same as in LDA, and that significant non-universality is expected due to strong correlations.

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arXiv2009

Ground State Properties of Simple Elements from GW Calculations

A novel self-consistent implementation of Hedin's GW perturbation theory is introduced. This finite-temperature method uses Hartree-Fock wave functions to represent Green's function. GW equations are solved with full potential linear augmented plane wave (FLAPW) method at each iteration of a self-consistent cycle. With our approach we are able to calculate total energy as a function of the lattice parameter. Ground state properties calculated for Na, Al, and Si compare well with experimental data.

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arXiv2009

Cellular Dynamical Mean Field Theory of the Periodic Anderson Model

We develop a cluster dynamical mean field theory of the periodic Anderson model in three dimensions, taking a cluster of two sites as a basic reference frame. The mean field theory displays the basic features of the Doniach phase diagram: a paramagnetic Fermi liquid state, an antiferromagnetic state and a transition between them. In contrast with spin density wave theories, the transition is accompanied by a large increase of the effective mass everywhere on the Fermi surface and a substantial change of the Fermi surface shape across the transition. To understand the nature and the origin of the phases near the transition, we investigate the paramagnetic solution underlying the antiferromagnetic state, and identify the transition as a point where the $f$ electrons decouple from the conduction electrons undergoing an orbitally selective Mott transition. This point turns out to be intimately related to the two impurity Kondo model quantum critical point. In this regime, non local correlations become important and result in significant changes in the photoemission spectra and the de Haas-van Alphen frequencies. The transition involves considerable $f$ spectral weight transfer from the Fermi level to its immediate vicinity, rather than to the Hubbard bands as in single site DMFT.

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arXiv2009

T=0 heavy fermion quantum critical point as an orbital selective Mott transition

We describe the T=0 quantum phase transition in heavy fermion systems as an orbital selective Mott transition (OSMT) using a cluster extension of dynamical mean field theory. This transition is characterized by the emergence of a new intermediate energy scale corresponding to the opening of a pseudogap and the vanishing of the low-energy hybridization between light and heavy electrons. We identify the fingerprint of Mott physics in heavy electron systems with the appearance of surfaces in momentum space where the self-energy diverges and we derive experimental consequences of this scenario for photoemission, compressibility, optical conductivity, susceptibility and specific heat.

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arXiv2008

Parent Phases of Doped Mott Insulators: A Cluster DMFT Study

We investigate the insulating phases of a frustrated Hubbard model in its strong coupling regime at half-filling. We pay special attention to all the symmetry breaking instabilities that can be described by Dynamical Mean Field Theory (DMFT) on a square plaquette. We identify several mean-field solutions, two Neel states breaking the SU(2) symmetry, a dimer phase and a Mott insulator which doesn't break any symmetry. The singlet to singlet fluctuations soften dramatically in the latter phase, giving rise to dimerization fluctuations as well as chiral fluctuations that are both low-lying. We present a simple picture of the different DMFT states and their evolution with frustration.

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arXiv2008

Optical Weights and Waterfalls in Doped Charge Transfer Insulators: an LDA+DMFTStudy of LSCO

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to investigate intermediate energy properties of the copper oxides. We identify coherent and incoherent spectral features that results from doping a charge transfer insulator, namely quasiparticles, Zhang-Rice singlet band, and the upper and lower Hubbard bands. Angle resolving these features, we identify a \emphasize{waterfall} like feature, between the quasiparticle part and the incoherent part of the Zhang-Rice band. We investigate the assymetry between particle and hole doping. On the hole doped side, there is a very rapid transfer of spectral weight upon doping in the one particle spectra. The optical spectral weight increases superlinearly on the hole doped side in agreement with experiments.

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arXiv2008

Strongly Correlated Superconductivity: a plaquette Dynamical mean field theory study

We use cluster Dynamical Mean Field Theory to study the simplest models of correlated electrons, the Hubbard model and the t-J model. We use a plaquette embedded in a medium as a reference frame to compute and interpret the physical properties of these models. We study various observables such as electronic lifetimes, one electron spectra, optical conductivities, superconducting stiffness, and the spin response in both the normal and the superconducting state in terms of correlation functions of the embedded cluster. We find that the shortest electron lifetime occurs near optimal doping where the superconducting critical temperature is maximal. A second critical doping connected to the change of topology of the Fermi surface is also identified. The mean field theory provides a simple physical picture of three doping regimes, the underdoped, the overdoped and the optimally doped regime in terms of the physics of the quantum plaquette impurity model. We compare the plaquette Dynamical Mean Field Theory results with earlier resonating valence bond mean field theories, noting the improved description of the momentum space anisotropy of the normal state properties and the doping dependence of the coefficient of the linear temperature dependence of the superfluid density in the superconducting state.

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