arXiv subjects

Kristjan Haule

Publications and source records attributed to Kristjan Haule.

At least 73 records · Page 4Linked to original sources

arXiv2016

Spin excitations in optimally P-doped BaFe2(As0.7P0.3)2superconductor

We use inelastic neutron scattering to study temperature and energy dependence of spin excitations in optimally P-doped BaFe2(As0.7P0.3)2 superconductor (Tc = 30 K) throughout the Brillouin zone. In the undoped state, spin waves and paramagnetic spin excitations of BaFe2As2 stem from antiferromagnetic (AF) ordering wave vector QAF= (1/-1,0) and peaks near zone boundary at (1/-1,1/-1) around 180 meV. Replacing 30% As by smaller P to induce superconductivity, low-energy spin excitations of BaFe2(As0.7P0.3)2form a resonance in the superconducting state and high-energy spin excitations now peaks around 220 meV near (1/-1,1/-1). These results are consistent with calculations from a combined density functional theory and dynamical mean field theory, and suggest that the decreased average pnictogen height in BaFe2(As0.7P0.3)2 reduces the strength of electron correlations and increases the effective bandwidth of magnetic excitations.

cond-mat.supr-con↗

arXiv2016

First-principles treatment of Mott insulators: linearized QSGW+DMFT approach

The theoretical understanding of emergent phenomena in quantum materials is one of the greatest challenges in condensed matter physics. In contrast to simple materials such as noble metals and semiconductors, macroscopic properties of quantum materials cannot be predicted by the properties of individual electrons. One of the examples of scientific importance is strongly correlated electron system. Neither localized nor itinerant behaviors of electrons in partially filled 3d, 4f, and 5f orbitals give rise to rich physics such as Mott insulators, high-temperature superconductors, and superior thermoelectricity, but hinder quantitative understanding of low-lying excitation spectrum. Here we present a new first-principles approach to strongly correlated solids. It is Q4 based on a combination of the quasiparticle self-consistent GW approximation and the dynamical mean-field theory. The sole input in this method is the projector to the set of correlated orbitals for which all local Feynman graphs are being evaluated. For that purpose, we choose very localized quasiatomic orbitals spanning large energy window, which contains most strongly hybridized bands, as well as upper and lower Hubbard bands. The self-consistency is carried out on the Matsubara axis. This method enables the first-principles study of Mott insulators in both their paramagnetic and antiferromagnetic phases. We illustrate the method on the archetypical charge transfer correlated insulators La2CuO4 and NiO, and obtain spectral properties and magnetic moments in good agreement with experiments.

cond-mat.str-el↗

arXiv2016

Validity of the Local Approximation in Iron- Pnictides and Chalcogenides

We introduce a cluster DMFT (Dynamical Mean Field Theory) approach to study the normal state of the iron pnictides and chalcogenides. In the regime of moderate mass renormalizations, the self-energy is very local, justifying the success of single site DMFT for these materials and for other Hunds metals. We solve the corresponding impurity model with CTQMC (Continuous Time Quantum Monte-Carlo) and find that the minus sign problem is not severe in regimes of moderate mass renormalization.

cond-mat.str-el↗

arXiv2016

Orbital selective spin excitations and their impact on superconductivity of LiFe1-xCoxAs

We use neutron scattering to study spin excitations in single crystals of LiFe$_{0.88}$Co$_{0.12}$As, which is located near the boundary of the superconducting phase of LiFe$_{1-x}$Co$_{x}$As and exhibits non-Fermi-liquid behavior indicative of a quantum critical point. By comparing spin excitations of LiFe$_{0.88}$Co$_{0.12}$As with a combined density functional theory (DFT) and dynamical mean field theory (DMFT) calculation, we conclude that wave-vector correlated low energy spin excitations are mostly from the $d_{xy}$ orbitals, while high-energy spin excitations arise from the $d_{yz}$ and $d_{xz}$ orbitals. Unlike most iron pnictides, the strong orbital selective spin excitations in LiFeAs family cannot be described by anisotropic Heisenberg Hamiltonian. While the evolution of low-energy spin excitations of LiFe$_{1-x}$Co$_x$As are consistent with electron-hole Fermi surface nesting condition for the $d_{xy}$ orbital, the reduced superconductivity in LiFe$_{0.88}$Co$_{0.12}$As suggests that Fermi surface nesting conditions for the $d_{yz}$ and $d_{xz}$ orbitals are also important for superconductivity in iron pnictides.

cond-mat.supr-con↗

arXiv2016

Forces for Structural Optimizations in Correlated Materials within DFT+Embedded DMFT Functional Approach

We implemented the derivative of the free energy functional with respect to the atom displacements, so called force, within the combination of Density Functional Theory and the Embedded Dynamical Mean Field Theory. We show that in combination with the numerically exact quantum Monte Carlo (MC) impurity solver, the MC noise cancels to a great extend, so that the method can be used very efficiently for structural optimization of correlated electron materials. As an application of the method, we show how strengthening of the fluctuating moment in FeSe superconductor leads to a substantial increase of the anion height, and consequently to a very large effective mass, and also strong orbital differentiation.

cond-mat.str-el↗

arXiv2015

Unveiling hidden ferrimagnetism and giant magnetoelectricity in polar magnet Fe2Mo3O8

Magnetoelectric (ME) effect is recognized for its utility for low-power electronic devices.Largest ME coefficients are often associated with phase transitions in which ferroelectricity is induced by magnetic order. Unfortunately, in these systems, large ME response is revealed only upon elaborate poling procedures. These procedures may become unnecessary in single-polar-domain crystals of polar magnets. Here we report giant ME effects in a polar magnet Fe2Mo3O8 at temperatures as high as 60 K.Polarization jumps of 0.3 μC/cm2, and repeated mutual control of ferroelectric and magnetic moments with differential ME coefficients on the order of 10$^4$ ps/m are achieved.Importantly, no electric or magnetic poling is needed, as necessary for applications.The sign of the ME coefficients can be switched by changing the applied "bias" magnetic field. The observed effects are associated with a hidden ferrimagnetic order unveiled by application of a magnetic field.

cond-mat.mtrl-sci↗

arXiv2015

Dynamical Mean Field Theory for Diatomic Molecules and the Exact Double Counting

Dynamical mean field theory (DMFT) combined with the local density approximation (LDA) is widely used in solids to predict properties of correlated systems. In this paper, its application to one of the simplest strongly correlated systems, the hydrogen molecule H$_2$, is demonstrated to develop a parameter-free LDA+DMFT framework. We propose a method to calculate the exact intersection of LDA and DMFT that leads to highly accurate subtraction of the doubly counted correlation in both methods. When the exact double-counting treatment and a good projector to the correlated subspace are used, LDA+DMFT yields very accurate total energy and excitation spectrum of the H$_2$ molecule. We also discuss how this double-counting scheme can be extended to solid state calculations.

cond-mat.str-el↗

arXiv2015

Metal-insulator transition and topological properties of pyrochlore iridates

Combining density functional theory (DFT) and embedded dynamical mean-field theory (DMFT) methods, we study the metal-insulator transition in $R_2$Ir$_2$O$_7$ ($R$=Y, Eu, Sm, Nd, Pr, and Bi) and the topological nature of the insulating compounds. Accurate free energies evaluated using the charge self-consistent DFT+DMFT method reveal that the metal-insulator transition occurs for an A-cation radius between that of Nd and Pr, in agreement with experiments. The all-in-all-out magnetic phase, which is stable in the Nd compound but not the Pr one, gives rise to a small Ir$^{4+}$ magnetic moment of $\approx 0.5μ_B$ and opens a sizable correlated gap. We demonstrate that within this state-of-the-art theoretical method, the insulating bulk pyrochlore iridates are topologically trivial.

cond-mat.str-el↗

arXiv2015

Transport properties of Metallic Ruthenates: a DFT+DMFT investigation

We present a systematical theoretical study on the transport properties of an archetypal family of Hund's metals, Sr$_2$RuO$_4$,Sr$_3$Ru$_2$O$_7$, SrRuO$_3$ and CaRuO$_3$, within the combination of first principles density functional theory and dynamical mean field theory. The agreement between theory and experiments for optical conductivity and resistivity is good, which indicates that electron-electron scattering dominates the transport of ruthenates. We demonstrate that in the single-site dynamical mean field approach the transport properties of Hund's metals fall into the scenario of "resilient quasiparticles". We explains why the single layered compound Sr$_2$RuO$_4$ has a relative weak correlation with respect to its siblings, which corroborates its good metallicity.

cond-mat.str-el↗

arXiv2015

$J_{eff}=1/2$ Mott Insulating State in Rh and Ir Fluorides

Discovery of new transition metal compounds with large spin orbit coupling (SOC) coexisting with strong electron-electron correlation among the $d$ electrons is essential for understanding the physics that emerges from the interplay of these two effects. In this study, we predict a novel class of $J_{eff}=1/2$ Mott insulators in a family of fluoride compounds that are previously synthesized, but not characterized extensively. First principles calculations in the level of all electron Density Functional Theory + Dynamical Mean Field Theory (DFT+DMFT) indicate that these compounds have large Mott gaps and some of them exhibit unprecedented proximity to the ideal, $SU(2)$ symmetric $J_{eff}=1/2$ limit.

cond-mat.str-el↗

arXiv2015

Free energy from stationary implementation of the DFT+DMFT functional

The stationary functional of the all-electron density functional plus dynamical mean field theory (DFT+DMFT) formalism to perform free energy calculations and structural relaxations is implemented for the first time. Here, the first order error in the density leads to a much smaller, second order error in the free energy. The method is applied to several well known correlated materials; metallic SrVO$_3$, Mott insulating FeO, and elemental Cerium, to show that it predicts the lattice constants with very high accuracy. In Cerium, we show that our method predicts the iso-structural transition between the $α$ and $γ$ phases, and resolve the long standing controversy in the driving mechanism of this transition.

cond-mat.str-el↗

arXiv2015

Exact double-counting in combining the Dynamical Mean Field Theory and the Density Functional Theory

We propose a continuum representation of the Dynamical Mean Field Theory, in which we were able to derive an exact overlap between the Dynamical Mean Field Theory and band structure methods, such as the Density Functional Theory. The implementation of this exact double-counting shows improved agreement between theory and experiment in several correlated solids, such as the transition metal oxides and lanthanides. Previously introduced nominal double-counting is in much better agreement with the exact double-counting than most widely used fully localized limit formula.

cond-mat.str-el↗

arXiv2014

Hierarchical stripe phases in IrTe2 driven by competition between Ir dimerization and Te bonding

Layered 5d transition metal dichalcogenide (TMD) IrTe2 is distinguished from the traditional TMDs (such as NbSe2) by the existence of multiple CDW-like stripe phases and superconductivity at low temperatures. Despite of intensive studies, there is still no consensus on the physical origin of the stripe phases or even the ground state modulation for this 5d material. Here, we present atomic-scale evidence from scanning tunneling microscopy and spectroscopy (STM/STS), that the ground state of IrTe2 is a q=1/6 stripe phase, identical to that of the Se-doped compound. Furthermore, our data suggest that the multiple transitions and stripe phases are driven by the intralayer Ir-Ir dimerization that competes against the interlayer Te-Te bonding. The competition results in a unified phase diagram with a series of hierarchical modulated stripe phases, strikingly similar to the renowned "devil's staircase" phenomena.

cond-mat.str-el↗

arXiv2014

Lazy skip lists, a new algorithm for fast hybridization-expansion quantum Monte Carlo

The solution of a generalized impurity model lies at the heart of electronic structure calculations with dynamical mean-field theory (DMFT). In the strongly-correlated regime, the method of choice for solving the impurity model is the hybridization expansion continuous time quantum Monte Carlo(CT-HYB). Enhancements to the CT-HYB algorithm are critical for bringing new physical regimes within reach of current computational power. Taking advantage of the fact that the bottleneck in the algorithm is a product of hundreds of matrices, we present optimizations based on the introduction and combination of two concepts of more general applicability: a) skip lists and b) fast rejection of proposed configurations based on matrix bounds. Considering two very different test cases with $d$ electrons, we find speedups of $\sim 25$ up to $\sim 500$ compared to the direct evaluation of the matrix product. Even larger speedups are likely with $f$ electron systems and with clusters of correlated atoms.

cond-mat.str-el↗

arXiv2014

Covalency in transition metal oxides within all-electron Dynamical Mean Field Theory

A combination of dynamical mean field theory and density functional theory, as implemented in Phys. Rev. B 81, 195107 (2010), is applied to both the early and late transition metal oxides. For fixed value of the local Coulomb repulsion, without fine tuning, we obtain the main features of these series, such as the metallic character of SrVO$_3$ and the the insulating gaps of LaVO$_3$, LaTiO$_3$ and La$_2$CO$_4$ which are in good agreement with experiment. The study highlights the importance of local physics and high energy hybridization in the screening of the Hubbard interaction and how different low energy behaviors can emerge from the a unified treatment of the transition metal series.

cond-mat.str-el↗

arXiv2014

Quantum anomalous Hall phase in (001) double-perovskite monolayers via intersite spin-orbit coupling

Using tight-binding models and first-principles calculations, we demonstrate the possibility to achieve a quantum anomalous Hall (QAH) phase on a two-dimensional square lattice, which can be realized in monolayers of double perovskites. We show that effective intersite spin-orbit coupling between eg orbitals can be induced perturbatively, giving rise to a QAH state. Moreover, the effective spin-orbit coupling can be enhanced by octahedral rotations. Based on first-principles calculations, we propose that this type of QAH state could be realized in La2MnIrO6 monolayers, with the size of the gap as large as 26 meV in the ideal case. We observe that the electronic structure is sensitive to structural distortions, and that an enhanced Hubbard U tends to stabilize the nontrivial gap.

cond-mat.mtrl-sci↗

arXiv2014

Shining light on transition metal oxides: unveiling the hidden Fermi Liquid

We use low energy optical spectroscopy and first principles LDA+DMFT calculations to test the hypothesis that the anomalous transport properties of strongly correlated metals originate in the strong temperature dependence of their underlying resilient quasiparticles. We express the resistivity in terms of an effective plasma frequency $ω_p^*$ and an effective scattering rate $ 1/τ^*_{tr}$. We show that in the archetypal correlated material V2O3, $ω_p^*$ increases with increasing temperature, while the plasma frequency from partial sum rule exhibits the opposite trend . $ 1/τ^*_{tr}$ has a more pronounced temperature dependence than the scattering rate obtained from the extended Drude analysis. The theoretical calculations of these quantities are in quantitative agreement with experiment. We conjecture that these are robust properties of all strongly correlated metals, and test it by carrying out a similar analysis on thin film NdNiO3 on LaAlO3 substrate.

cond-mat.str-el↗

arXiv2013

Effective J=1/2 insulating state in Ruddlesden-Popper iridates: An LDA+DMFT study

Using ab-initio methods for correlated electrons in solids, we investigate the metal-insulator transition across the Ruddlesden-Popper (RP) series of iridates and explore the robustness of the J_eff=1/2 state against band effects due to itineracy, tetragonal distortion, octahedral rotation and Coulomb interaction. We predict the effects of epitaxial strain on the optical conductivity, magnetic moments, and J_eff=1/2 ground-state wave functions in the RP series. To describe the solution of the many-body problem in an intuitive picture, we introduce a concept of energy-dependent atomic states, which strongly resemble the atomic J_eff=1/2 states but with coefficients that are energy/time-dependent. We demonstrate that the deviation from the ideal J_eff=1/2 state is negligible at short time scales for both single- and double-layer iridates, while it becomes quite significant for Sr3Ir2O7 at long times and low energy. Interestingly, Sr2IrO4 is positioned very close to the SU(2) limit, with only 3% deviation from the ideal J_eff=1/2 situation.

cond-mat.str-el↗