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Jeroen van den Brink

Publications and source records attributed to Jeroen van den Brink.

At least 325 records · Page 18Linked to original sources
arXiv2007

Multiferroicity induced by dislocated spin-density waves

We uncover a new pathway towards multiferroicity, showing how magnetism can drive ferroelectricity without relying on inversion symmetry breaking of the magnetic ordering. Our free-energy analysis demonstrates that any commensurate spin-density-wave ordering with a phase dislocation, even if it is collinear, gives rise to an electric polarization. Due to the dislocation the electronic and magnetic and inversion centers do not coincide, which turns out to be a sufficient condition for multiferroic coupling. The novel mechanism explains the formation of multiferroic phases at the magnetic commensurability transitions, such as the ones observed in YMn$_{2}$O$_{5}$ and related compounds. We predict that in these multiferroics an oscillating electrical polarization is concomitant with the uniform polarization. On the basis of our theory we put forward new types of magnetic materials that are potentially ferroelectric.

cond-mat.mtrl-sci↗
arXiv2007

Thin Spectrum States in Superconductors

We show that finite size superconductors have a spectrum of states at extremely low energy, i.e. inside the superconducting gap. The presence of this {\it thin spectrum} is a generic feature and related to the fact that in a superconductor the global phase rotation symmetry is spontaneously broken. For a strong coupling superconductor we find the spectrum by exactly solving the Lieb-Mattis type Hamiltonian onto which the problem maps. One of the physical consequences of the presence of thin states is that they cause quantum decoherence in superconducting qubits of finite extent.

cond-mat.supr-con↗
arXiv2007

New light on magnetic excitations: indirect resonant inelastic X-ray scattering on magnons

Recent experiments show that indirect resonant inelastic X-ray scattering (RIXS) is a new probe of spin dynamics. Here I derive the cross-section for magnetic RIXS and determine the momentum dependent four-spin correlation function that it measures. These results show that this technique offers information on spin dynamics that is complementary to e.g. neutron scattering. The RIXS spectrum of Heisenberg antiferromagnets is calculated. It turns out that only scattering processes that involve at least two magnons are allowed. Other selection rules imply that the scattering intensity vanishes for specific transferred momenta ${\bf q}$, in particular for ${\bf q}=0$. The calculated spectra agree very well with the experimental data.

cond-mat.str-el↗
arXiv2006

Ultrashort Lifetime Expansion for Indirect Resonant Inelastic X-ray Scattering

In indirect resonant inelastic X-ray scattering (RIXS) an intermediate state is created with a core-hole that has a ultrashort lifetime. The core-hole potential therefore acts as a femtosecond pulse on the valence electrons. We show that this fact can be exploited to integrate out the intermediate states from the expressions for the scattering cross section. By this we obtain an effective scattering cross section that only contains the initial and final scattering states. We derive in detail the effective cross section which turns out to be a resonant scattering factor times a linear combination of the charge response function $S({\bf q},ω)$ and the dynamic longitudinal spin density correlation function. This result is asymptotically exact for both strong and weak local core-hole potentials and ultrashort lifetimes. The resonant scattering pre-factor is shown to be weakly temperature dependent. We also derive a sum-rule for the total scattering intensity and generalize the results to multi-band systems. One of the remarkable outcomes is that one can change the relative charge and spin contribution to the inelastic spectral weight by varying the incident photon energy.

cond-mat.str-el↗
arXiv2006

Spontaneous Symmetry Breaking in Quantum Mechanics

We present a clear and mathematically simple procedure explaining spontaneous symmetry breaking in quantum mechanical systems. The procedure is applicable to a wide range of models and can be easily used to explain the existence of a symmetry broken state in crystals, antiferromagnets and even superconductors. It has the advantage that it automatically brings to the fore the main players in spontaneous symmetry breaking: the symmetry breaking field, the thermodynamic limit, and the global excitations of the thin spectrum.

physics.class-ph↗
arXiv2006

Electronic structure and correlations in pristine and potassium doped Cu-Phthalocyanine molecular crystals

We investigate the changes in the electronic structure of copper phthalocyanine (CuPc) crystals that is caused by intercalation with potassium. This is done by means of {\it ab initio} LSDA and LSDA+U calculations of the electronic structure of these molecular crystals. Pristine CuPc is found to be an insulator with local magnetic moments and a Pc-derived valence band with a width of 0.32 eV. In the intercalated compound $\rm K_2CuPc$ the additional electrons that are introduced by potassium are fully transferred to the $e_g$ states of the Pc-ring. A molecular low spin state results, preserving, however, the local magnetic moment on the copper ions. The degeneracy of the $e_g$ levels is split by a crystal field that quenches the orbital degeneracy and gives rise to a band splitting of 110 meV. Molecular electronic Coulomb interactions enhance this splitting in $\rm K_2CuPc$ to a charge gap of 1.4 eV. The bandwidth of the conduction band is 0.56 eV, which is surprisingly large for a molecular solid. This is line with the experimentally observation that the system with additional potassium doping, $\rm K_{2.75}{CuPc}$, is a metal as the unusually large bandwidth combined with the substantial carrier concentration acts against localization and polaron formation, while strongly promoting the delocalization of the charge carriers.

cond-mat.str-el↗
arXiv2006

On the relation between decoherence and spontaneous symmetry breaking

We have recently shown that there is a limit to quantum coherence in many-particle spin qubits due to spontaneous symmetry breaking. These results were derived for the Lieb-Mattis spin model. Here we will show that the underlying mechanism of decoherence in systems with spontaneous symmetry breaking is in fact more general. We present here a generic route to finding the decoherence time associated with spontaneous symmetry breaking in many particle qubits, and subsequently we apply this approach to two model systems, indicating how the continuous symmetries in these models are spontaneously broken and discussing the relation of this symmetry breaking to the thin spectrum. We then present in detail the calculations that lead to the limit to quantum coherence, which is due to energy shifts in the thin spectrum.

cond-mat.str-el↗
arXiv2005

Orbital-Peierls State in NaTiSi2O6

Does the quasi one-dimensional titanium pyroxene NaTiSi2O6 exhibit the novel {\it orbital-Peierls} state? We calculate its groundstate properties by three methods: Monte Carlo simulations, a spin-orbital decoupling scheme and a mapping onto a classical model. The results show univocally that for the spin and orbital ordering to occur at the same temperature --an experimental observation-- the crystal field needs to be small and the orbitals are active. We also find that quantum fluctuations in the spin-orbital sector drive the transition, explaining why canonical bandstructure methods fail to find it. The conclusion that NaTiSi2O6 shows an orbital-Peierls transition is therefore inevitable.

cond-mat.str-el↗
arXiv2005

An intrinsic limit to quantum coherence due to spontaneous symmetry breaking

We investigate the influence of spontaneous symmetry breaking on the decoherence of a many-particle quantum system. This decoherence process is analyzed in an exactly solvable model system that is known to be representative of symmetry broken macroscopic systems in equilibrium. It is shown that spontaneous symmetry breaking imposes a fundamental limit to the time that a system can stay quantum coherent. This universal timescale is $t_{spon} \simeq 2πN \hbar / (k_B T)$, given in terms of the number of microscopic degrees of freedom $N$, temperature $T$, and the constants of Planck ($\hbar$) and Boltzmann ($k_B$).

cond-mat.str-el↗
arXiv2004

Electronic Correlations in Oligo-acene and -thiophene Organic Molecular Crystals

From first principles calculations we determine the Coulomb interaction between two holes on oligo-acene and -thiophene molecules in a crystal, as a function of the oligomer length. The relaxation of the molecular geometry in the presence of holes is found to be small. In contrast, the electronic polarization of the molecules that surround the charged oligomer, reduces the bare Coulomb repulsion between the holes by approximately a factor of two. In all cases the effective hole-hole repulsion is much larger than the calculated valence bandwidth, which implies that at high doping levels the properties of these organic semiconductors are determined by electron-electron correlations.

cond-mat.str-el↗
arXiv2004

Orbital order in classical models of transition-metal compounds

We study the classical 120-degree and related orbital models. These are the classical limits of quantum models which describe the interactions among orbitals of transition-metal compounds. We demonstrate that at low temperatures these models exhibit a long-range order which arises via an "order by disorder" mechanism. This strongly indicates that there is orbital ordering in the quantum version of these models, notwithstanding recent rigorous results on the absence of spin order in these systems.

cond-mat.stat-mech↗
arXiv2003

Theory of indirect resonant inelastic X-ray scattering

We express the cross section for indirect resonant inelastic X-ray scattering in terms of an intrinsic dynamic correlation function of the system that is studied with this technique. The cross section is a linear combination of the charge response function and the dynamic longitudinal spin density correlation function. This result is asymptotically exact for both strong and weak local core-hole potentials. We show that one can change the relative charge and spin contribution to the inelastic spectral weight by varying the incident photon energy.

cond-mat.str-el↗
arXiv2003

Charge order versus Zener polarons: ferroelectricity in manganites

We show that in manganites close to half-doping novel non-bipartite magnetic phases appear due to the interplay between double exchange, superexchange and orbital ordering. In considerable part of the phase diagram the groundstate has a magnetic order that is intermediate between the canonical magnetic CE-phase and a state that we identify as the recently observed Zener polaron state. The intermediate phase shows a type of charge ordering that breaks inversion symmetry and is therefore predicted to be ferroelectric.

cond-mat.str-el↗
arXiv2002

Orbital effects in manganites

We review some aspects related to orbital degrees of freedom in manganites. The Mn$^{3+}$ ions in these compounds have double orbital degeneracy and are strong Jahn-Teller ions, causing structural distortions and orbital ordering. We discuss ordering mechanisms and the consequences of orbital order. The additional degeneracy of low-energy states and the extreme sensitivity of the chemical bonds to the spatial orientation of the orbitals result in a variety of competing interactions. This quite often leads to frustration of classical ordered states and to the enhancement of quantum effects. Quantum fluctuations and related theoretical models are briefly discussed, including the occurence of resonating orbital bonds in the metallic phase of the colossal magnetoresistance manganites.

cond-mat.str-el↗
arXiv2001

Orbital excitations in LaMnO$_3$

We study the recently observed orbital excitations, orbitons, and treat electron-electron correlations and lattice dynamics on equal footing. It is shown that the orbiton energy and dispersion are determined by both correlations and lattice-vibrations. The electron-phonon coupling causes satellite structures in the orbiton spectral function and the elementary excitations of the system are mixed modes with both orbital and phonon character. It is proposed that the satellite structures observed in recent Raman-scattering experiments on LaMnO$_3$ are actually orbiton derived satellites in the phonon spectral function, caused by the phonon-orbiton interaction.

cond-mat.str-el↗
arXiv2001

Coulomb interaction and stability of CE-type structure in half-doped manganites, reply

In his Comment (cond-mat/0104353), Shen points out that the on-site Coulomb interaction, that can cause charge order in half-doped manganites, also destabilizes the magnetic CE-phase observed in these systems. This is a valid observation, but it is not a priori clear whether in the relevant parameter regime the C-phase is indeed lower in energy then the CE-phase within our model. We conclude that the proposed model, which correctly captures the interplay of spin, charge and orbital degrees of freedom in the half-doped manganites and gives a reasonable description of their electronic structure, is by itself not sufficient for the precise determination of the regions of stabilities of different phases. For this several other factors should be taken into account.

cond-mat.str-el↗
arXiv2001

Photoemission spectra of LaMnO3 controlled by orbital excitations

We investigate the spectral function of a hole moving in the orbital-ordered ferromagnetic planes of LaMnO$_3$, and show that it depends critically on the type of orbital ordering. While the hole does not couple to the spin excitations, it interacts strongly with the excitations of $e_g$ orbitals (orbitons), leading to new type of quasiparticles with a dispersion on the orbiton energy scale and with strongly enhanced mass and reduced weight. Therefore we predict a large redistribution of spectral weight with respect to the bands found in local density approximation (LDA) or in LDA+U.

cond-mat.str-el↗