arXiv subjects

Jeroen van den Brink

Publications and source records attributed to Jeroen van den Brink.

At least 289 records · Page 16Linked to original sources

arXiv2011

Ab Initio determination of Cu 3d orbital energies in layered copper oxides

It has long been argued that the minimal model to describe the low-energy physics of the high-Tc superconducting cuprates must include copper states of other symmetries besides the canonical x2-y2 one, in particular the z2 orbital. Experimental and theoretical estimates of the energy splitting of these states vary widely. With a novel ab initio quantum chemical computational scheme we determine these energies for a range of copper-oxides and -oxyclorides, determine trends with the apical Cu-ligand distances and find excellent agreement with recent Resonant Inelastic X-ray Scattering measurements, available for La2CuO4, Sr2CuO2Cl2, and CaCuO2.

cond-mat.str-el↗

arXiv2011

Ab initio computation of d-d excitation energies in low-dimensional Ti and V oxychlorides

Using a quantum chemical cluster-in-solid computational scheme, we calculate the local d-d excitation energies for two strongly correlated Mott insulators, the oxychlorides TiOCl and VOCl. TiOCl harbors quasi-one-dimensional spin chains made out of S = 1/2 Ti3+ ions while the electronic structure of VOCl displays a more two-dimensional character. We find in both cases that the lowest-energy d-d excitations are within the t2g subshell, starting at 0.34 eV and indicating that orbital degeneracies are significantly lifted. In the vanadium oxychloride, spin triplet to singlet excitations are calculated to be 1 eV higher in energy. For TiOCl, the computed d-level electronic structure and the symmetries of the wavefunctions are in very good agreement with resonant inelastic x-ray scattering results and optical absorption data. For VOCl, future resonant inelastic x-ray scattering experiments will constitute a direct test of the symmetry and energy of about a dozen of different d-d excitations that we predict here.

cond-mat.str-el↗

arXiv2011

Buckling induced Zener polaron instability in half-doped manganites

By calculating the electronic, orbital, magnetic and lattice structure of half-doped manganites we establish the central role of oxygen buckling caused by the tilting of the MnO$_6$ octahedra -- in essence a steric effect. The buckling itself does not change the Mn-Mn bonds. Instead it drives the system towards an instability where these bonds can dimerize. In presence of electron-electron interactions, this instability can fully develop and beyond a critical buckling a Zener polaron groundstate with dimerized spins, lattice and orbitals forms spontaneously, resulting in a multiferroic state.

cond-mat.str-el↗

arXiv2011

Intrinsic coupling of orbital excitations to spin fluctuations in Mott insulators

We show how the general and basic asymmetry between two fundamental degrees of freedom present in strongly correlated oxides, spin and orbital, has very profound repercussions on the elementary spin and orbital excitations. Whereas the magnons remain largely unaffected, orbitons become inherently coupled with spin fluctuations in spin-orbital models with antiferromagnetic and ferroorbital ordered ground states. The composite orbiton-magnon modes that emerge fractionalize again in one dimension, giving rise to spin-orbital separation in the peculiar regime where spinons are faster than orbitons.

cond-mat.str-el↗

arXiv2011

Narrowing of topological bands due to electronic orbital degrees of freedom

The Fractional Quantum Hall (FQH) effect has been predicted to occur in absence of magnetic fields and at high temperature in lattice systems that have flat bands with non-zero Chern number. We demonstrate that the presence of orbital degrees of freedom in frustrated lattice systems leads to a narrowing of topologically nontrivial bands. This robust effect does not rely on a fine-tuning of long-range hopping parameters as do other recent proposals to realize lattice FQH states and is directly relevant to a wide class of transition metal compounds.

cond-mat.str-el↗

arXiv2011

Macroscopic Degeneracy and Emergent Frustration in a Honeycomb Lattice Magnet

Using a hybrid method based on fermionic diagonalization and classical Monte Carlo, we investigate the interplay between itinerant and localized spins, with competing double- and super-exchange interactions, on a honeycomb lattice. For moderate superexchange, a geometrically frustrated triangular lattice of hexagons forms spontaneously. For slightly larger superexchange a dimerized groundstate is stable that has macroscopic degeneracy. The presence of these states on a non-frustrated honeycomb lattice highlights a novel phenomenon in this itinerant electron system: emergent geometrical frustration and degeneracy.

cond-mat.str-el↗

arXiv2011

The Effect of Charge Order on the Plasmon Dispersion in Transition-Metal Dichalcogenides

We investigate the dispersion of the charge carrier plasmon in the three prototypical charge-density wave bearing transition-metal dichalcogenides 2H-TaSe2, 2H-TaS2 and 2H-NbSe2 employing electron energy-loss spectroscopy. For all three compounds the plasmon dispersion is found to be negative for small momentum transfers. This is in contrast to the generic behavior observed in simple metals as well as the related system 2H-NbS2, which does not exhibit charge order. We present a semiclassical Ginzburg-Landau model which accounts for these observations, and argue that the vicinity to a charge ordered state is thus reflected in the properties of the collective excitations.

cond-mat.str-el↗

arXiv2011

As-vacancies, local moments, and Pauli limiting in LaO_0.9F_0.1FeAs_(1-delta) superconductors

We report magnetization measurements of As-deficient LaO_0.9F_0.1FeAs_1-delta (delta about 0.06) samples with improved superconducting properties as compared with As-stoichiometric optimally doped La-1111 samples. In this As-deficient system with almost homogeneously distributed As-vacancies (AV), as suggested by the (75)As-nuclear quadrupole resonance (NQR) measurements,we observe a strong enhancement of the spin-susceptibility by a factor of 3-7. This observation is attributed to the presence of an electronically localized state around each AV, carrying a magnetic moment of about 3.2 mu_Bohr per AV or 0.8 mu_Bohr/Fe atom. From theoretical considerations we find that the formation of a local moment on neighboring iron sites of an AV sets in when the local Coulomb interaction exceeds a critical value of about 1.0 eV in the dilute limit. Its estimated value amounts to ~ 2.5 eV and implies an upper bound of ~ 2 eV for the Coulomb repulsion at Fe sites beyond the first neighbor-shell of an AV. Electronic correlations are thus moderate/weak in doped La-1111. The strongly enhanced spin susceptibility is responsible for the Pauli limiting behavior of the superconductivity that we observe in As-deficient LaO_0.9F_0.1FeAs_1-delta. In contrast, no Pauli limiting behavior is found for the optimally doped, As-stoichiometric LaO_0.9F_0.1FeAs superconductor in accord with its low spin susceptibility.

cond-mat.supr-con↗

arXiv2011

Metacinnabar (β-HgS): a strong 3D topological insulator with highly anisotropic surface states

We establish the presence of topologically protected edge states on the (001) surface of HgS in the zinc-blende structure using density-functional electronic structure calculations. The Dirac point of the edge state cone is very close to the bulk valence band maximum. The Dirac cone is extremely anisotropic with a very large electron velocity along one diagonal of the surface elementary cell x' and a nearly flat dispersion in the perpendicular direction y'. The strong anisotropy originates from a broken fourfold rotoinversion symmetry at the surface.

cond-mat.mtrl-sci↗

arXiv2011

Calculation of magnetic exchange couplings in S=3/2 honeycomb system Bi$_3$Mn$_4$O$_{12}$(NO$_3$)} from first principles

Absence of magnetic ordering in Bi$_3$Mn$_4$O$_{12}$(NO$_3$), (BMNO) which has a magnetic subsystem that consists of honeycomb bi-layers of Mn$^{4+}$ ions with spin S=3/2, has raised the expectation that its ground state is strongly frustrated due to longer-range antiferromagnetic interactions. We calculate the magnetic exchange coupling constants of the BMNO complex within a density functional approach and find that the dominating interactions between Mn spins are the antiferromagnetic nearest-neighbor $J_1$ and interlayer interaction $J_c$. The largest interaction is $J_c$, which substantially exceeds $J_1$. Longer-range interactions are antiferromagnetic, but only weakly frustrating.

cond-mat.mtrl-sci↗

arXiv2011

Defect Formation Preempts Dynamical Symmetry Breaking in Closed Quantum Systems

We show that no matter how slowly a quantum-to-classical symmetry breaking process is driven, the adiabatic limit can never be reached in a macroscopic body. Massive defect formation preempts an adiabatic quantum-classical crossover and triggers the appearance of a symmetric non-equilibrium state that recursively collapses into the classical state, breaking the symmetry at punctured times. The presence of this state allows the quantum-classical transition to be investigated and controlled in mesoscopic devices by supplying externally the proper dynamical symmetry breaking perturbation.

quant-ph↗

arXiv2011

The Spin-Exchange Dynamical Structure Factor of the S=1/2 Heisenberg Chain

We determine the spin-exchange dynamical structure factor of the Heisenberg spin chain, as is measured by indirect Resonant Inelastic X-ray Scattering (RIXS). We find that two-spin RIXS excitations nearly entirely fractionalize into *two spinon* states. These share the same continuum lower bound as single-spin neutron scattering excitations, even if the relevant final states belong to orthogonal symmetry sectors. The RIXS spectral weight is mainly carried by higher-energy excitations, and is beyond the reach of the low-energy effective theories of Luttinger liquid type.

cond-mat.str-el↗

arXiv2010

Absence of anomalous interactions in the quantum theory of constrained charged particles in presence of electrical currents

The experimental progress in synthesizing low-dimensional nanostructures where carriers are confined to bent surfaces has boosted the interest in the theory of quantum mechanics on curved two-dimensional manifolds. It was recently asserted that constrained electrically charged particles couple to a term linear in A_3 M, where A_3 is the transversal component of the electromagnetic vector potential and M the surface mean curvature, thereby making a dimensional reduction procedure impracticable in the presence of fields. Here we resolve this apparent paradox by providing a consistent general framework of the thin-wall quantization procedure. We also show that the separability of the equation of motions is not endangered by the particular choice of the constraint imposed on the transversal fluctuations of the wavefunction, which renders the thin-wall quantization procedure well-founded. It can be applied without restrictions.

cond-mat.mes-hall↗

arXiv2010

Frustration-induced insulating chiral spin state in itinerant triangular-lattice magnets

We study the double-exchange model at half-filling with competing superexchange interactions on a triangular lattice, combining exact diagonalization and Monte-Carlo methods. We find that in between the expected itinerant ferromagnetic and $120^{\circ}$ Yafet-Kittel phases a robust scalar-chiral, insulating spin state emerges. At finite temperatures the ferromagnet - scalar-chiral quantum critical point is characterized by anomalous bad-metal behavior in charge transport as observed in frustrated itinerant magnets R$_2$Mo$_{2}$O$_7$.

cond-mat.str-el↗

arXiv2010

Determining the Electron-Phonon Coupling Strength in Correlated Electron Systems from Resonant Inelastic X-ray Scattering

We show that high resolution Resonant Inelastic X-ray Scattering (RIXS) provides direct, element-specific and momentum-resolved information on the electron-phonon (e-p) coupling strength. Our theoretical analysis demonstrates that the e-p coupling can be extracted from RIXS spectra by determining the differential phonon scattering cross section. An alternative, very direct manner to extract the coupling is to use the one and two-phonon loss ratio, which is governed by the e-p coupling strength and the core-hole life-time. This allows measurement of the e-p coupling on an absolute energy scale.

cond-mat.str-el↗

arXiv2010

Resonant Inelastic X-ray Scattering Studies of Elementary Excitations

In the past decade, Resonant Inelastic X-ray Scattering (RIXS) has made remarkable progress as a spectroscopic technique. This is a direct result of the availability of high-brilliance synchrotron X-ray radiation sources and of advanced photon detection instrumentation. The technique's unique capability to probe elementary excitations in complex materials by measuring their energy-, momentum-, and polarization-dependence has brought RIXS to the forefront of experimental photon science. We review both the experimental and theoretical RIXS investigations of the past decade, focusing on those determining the low-energy charge, spin, orbital and lattice excitations of solids. We present the fundamentals of RIXS as an experimental method and then review the theoretical state of affairs, its recent developments and discuss the different (approximate) methods to compute the dynamical RIXS response. The last decade's body of experimental RIXS data and its interpretation is surveyed, with an emphasis on RIXS studies of correlated electron systems, especially transition metal compounds. Finally, we discuss the promise that RIXS holds for the near future, particularly in view of the advent of x-ray laser photon sources.

cond-mat.str-el↗

arXiv2010

Existence, character and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe_{2-x}Co_{x}As_{2}

Low energy electron diffraction (LEED) experiments, LEED simulations and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe_{2-x}Co_{x}As_{2} (x = 0.1, 0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe_2-As block. As a result, surface related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe_{2-x}Co_{x}As_{2} system for accurate determination of self-energies using ARPES.

cond-mat.supr-con↗

arXiv2010

The absence of surface states for LiFeAs

We investigate the cleaving behavior of LiFeAs and determine its surface electronic structure by detailed density functional calculations. We show that due to the neutral surface of LiFeAs after cleaving, barely any influence of the surface on the electronic states is present. Therefore the data of surface sensitive probes such as angle resolved photoemission spectroscopy (ARPES) represent to a high degree the bulk electronic structure. This we highlighted by a direct comparison of the calculations to ARPES spectra.

cond-mat.supr-con↗