arXiv subjects

Kristjan Haule

Publications and source records attributed to Kristjan Haule.

104 records · Page 6Linked to original sources

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.

cond-mat.str-el↗

arXiv2010

Dynamical Mean-Field Theory within the Full-Potential Methods: Electronic structure of Ce-115 materials

We implemented the charge self-consistent combination of Density Functional Theory and Dynamical Mean Field Theory (DMFT) in two full-potential methods, the Augmented Plane Wave and the Linear Muffin-Tin Orbital methods. We categorize the commonly used projection methods in terms of the causality of the resulting DMFT equations and the amount of partial spectral weight retained. The detailed flow of the Dynamical Mean Field algorithm is described, including the computation of response functions such as transport coefficients. We discuss the implementation of the impurity solvers based on hybridization expansion and an analytic continuation method for self-energy. We also derive the formalism for the bold continuous time quantum Monte Carlo method. We test our method on a classic problem in strongly correlated physics, the isostructural transition in Ce metal. We apply our method to the class of heavy fermion materials CeIrIn_5, CeCoIn_5 and CeRhIn_5 and show that the Ce 4f electrons are more localized in CeRhIn_5 than in the other two, a result corroborated by experiment. We show that CeIrIn_5 is the most itinerant and has a very anisotropic hybridization, pointing mostly towards the out-of-plane In atoms. In CeRhIn_5 we stabilized the antiferromagnetic DMFT solution below 3K, in close agreement with the experimental Néel temperature.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

arXiv2006

Quantum Monte Carlo Impurity Solver for Cluster DMFT and Electronic Structure Calculations in Adjustable Base

We generalized the recently introduced new impurity solver based on the diagrammatic expansion around the atomic limit and Quantum Monte Carlo summation of the diagrams. We present generalization to the cluster of impurities, which is at the heart of the cluster Dynamical Mean-Field methods, and to realistic multiplet structure of a correlated atom, which will allow a high precision study of actinide and lanthanide based compounds with the combination of the Dynamical Mean-Field theory and band structure methods. The approach is applied to both, the two dimensional Hubbard and t-J model within Cellular Dynamical Mean Field method. The efficient implementation of the new algorithm, which we describe in detail, allows us to study coherence of the system at low temperature from the underdoped to overdoped regime. We show that the point of maximal superconducting transition temperature coincides with the point of maximum scattering rate although this optimal doped point appears at different electron densities in the two models. The power of the method is further demonstrated on the example of the Kondo volume collapse transition in Cerium. The valence histogram of the DMFT solution is presented showing the importance of the multiplet splitting of the atomic states.

cond-mat.str-el↗

arXiv2006

Avoided Quantum Criticality near Optimally Doped High Temperature Superconductors

We study the crossover from the underdoped to the overdoped regime in the t-J model.The underdoped regime is dominated by the superexchange interaction, locking the spins into singlets which weakly perturbe coherent charge carriers. In the overdoped, large carrier concentration regime, the Kondo effect dominates resulting in spin-charge composite quasiparticles which are also coherent. Separating these two Fermi liquid regimes, there is a critical doping where superexchange and Kondo interaction balance each other, bringing the system close to a local quantum critical point near the point of maximal superconducting transition temperature. At this point, particle hole symmetry is dynamically restored and physical quantities such as the optical conductivity, exhibit power law behaviour at intermediate frequencies as observed experimentally. Quantum criticality is avoided by the onset of superconductivity.

cond-mat.str-el↗

arXiv2006

Optical Conductivity of the t-J model within Cluster Dynamical Mean Field Theory

We study the evolution of the optical conductivity in the t-J model with temperature and doping using the Extended Dynamical Cluster Approximation. The cluster approach results in an optical mass which is doping independent near half filling. The transition to the superconducting state in the overdoped regime is characterized by a decrease in the hole kinetic energy, in contrast to the underdoped side where kinetic energy of holes increases upon superfluid condensation. In both regimes, the optical conductivity displays anomalous transfers of spectral weight over a broad frequency region.

cond-mat.str-el↗

arXiv2005

Many-Body Electronic Structure of Americium metal

We report computer based simulations of energetics, spectroscopy and electron-phonon interaction of americium using a novel spectral density functional method. This approach gives rise to a new concept of a many-body electronic structure and reveals the unexpected mixed valence regime of Am 5f6 electrons which under pressure acquire the 5f7 valence state. This explains unique properties of Am and addresses the fundamental issue of how the localization delocalization edge is approached from the localized side in a closed shell system.

cond-mat.str-el↗

arXiv2005

Strong Coupling Solver for the Quantum Impurity Model

We propose a fast impurity solver for the general quantum impurity model based on the perturbation theory around the atomic limit, which can be used in combination with the local density approximation (LDA) and the dynamical mean field theory (DMFT). We benchmark the solver in the two band Hubbard model within DMFT against quantum Monte Carlo (QMC) and numerical renormalization group (NRG) results. We find that the solver works very well in the paramagnetic Mott insulator phase. We also apply this impurity solver to the DMFT study of the anti-ferromagnetic phase transition in the unfrustrated Bethe lattice. The Neel temperature obtained by the fast impurity solver agrees very well with the QMC results in the large Hubbard U limit. The method is a promising tool to be used in combination with the LDA+DMFT to study Mott insulators starting from first principles.

cond-mat.str-el↗

arXiv2004

The alpha to gamma transition in Ce: a theoretical view from optical spectroscopy

Using a novel approach to calculate optical properties of strongly correlated systems, we address the old question of the physical origin of the alpha to gamma transition in Ce. We find that the Kondo collapse model describes the optical data better than the Mott transition picture. Our results compare well with existing thin--film experiments. We predict the full temperature dependence of the optical spectra and find the development of a pseudogap around 0.6 eV in the vicinity of the alpha to gamma phase transition.

cond-mat.str-el↗