20180508Koen ReijndersHFML 02.2020180531Radboud UniversityCaustics in graphene: construction of a uniform semiclassical
approximation and comparison with tight-binding results
We study above-barrier scattering of Dirac electrons by a smooth
electrostatic potential combined with a coordinate-dependent mass in
graphene. We assume that the potential and mass are sufficiently smooth,
so that we can define a small dimensionless semiclassical parameter h.
This setup naturally leads to focusing and the formation of a so-called
cusp caustic. We construct an asymptotic solution for the wavefunction
near this caustic in the form of the Maslov canonical operator. This
construction is greatly simplified by introducing so-called eikonal
coordinates on the Lagrangian manifold and by using a recently proposed
new representation [1]. The matrix character of the Dirac equation gives
rise to a nontrivial semiclassical phase in the wavefunction, which
makes this problem different from a scalar wave equation. Because of
this semiclassical phase, the leading-order approximation is no longer
sufficient to describe the wavefunction near the cusp, and one has to
use the uniform approximation [2]. In this talk, I will place particular
emphasis on the numerical implementation of this uniform approximation
and compare it with tight-binding calculations of real graphene samples.
We show that the semiclassical phase can have a significant effect on
the position of the intensity maximum. The observed effects are opposite
for graphene's two valleys and can be very well captured within the
uniform approximation. Additional information about the system can be
obtained by incorporating the semiclassical phase into the equations of
motion.
[1] S. Yu. Dobrokhotov, G. N. Makrakis, V. E. Nazaikinskii, T. Ya.
Tudorovskii; Theor. Math. Phys. 177, 1579 (2013)
[2] J. N. L. Connor and D. Farrelly, J. Chem. Phys. 75, 2831 (1981)
Andrey BagrovHFML 02.2020180503Radboud UniversityHolographic local quench and effective complexity
Complexity of a system or a process is one of the most intuitively
clear yet very elusive concepts in our perception of the reality. It
has recently attracted a lot of attention within the AdS/CFT
correspondence due to its relations to a wide class of problems
concerning holographic entanglement entropy, the black hole
information paradox, quantum chaos, and scrambling. While holography
deal with the computational complexity of quantum states, we make an
attempt to employ the holographic intuition to gain a better
understanding of complexity of a different kind - the effective (or
physical) complexity, that maximizes at the border between order and
chaos. Using the simplest possible model of a local quench in
$AdS_3$ as a tool to create a regular pattern coexistent with
homogeneous "randomness", we argue that the holographic complexity
has certain traits of what one would expect to be the physical
complexity, and suggest possible ways to investigate these
connections further.
Timothy BuddHFML 02.2020180426Radboud UniversityHidden rotational symmetry on the honeycomb lattice
The tight-binding model on the honeycomb lattice has a continuous
symmetry that can be interpreted as a continuous rotation, in the sense
that it interpolates the discrete rotational symmetry of the lattice.
Exploiting this symmetry the time-homogeneous solutions of the
one-particle excitations are seen to admit an exact polar decomposition
into angular and radial coordinates. I'll discuss how such a
decomposition may help to find exact solutions in the presence of
certain isolated defects in graphene, like conical defects, corners,
single flux lines or wedge-shaped potential steps.
Hylke DonkerHFML 02.2020180412Radboud UniversityQuantum dynamics of a (small) symmetry breaking measurement device
Despite the fact that the discussions relating to the foundations of quantum mechanics go back almost a century ago, some of its controversial elements seem far from settled. Quantum measurement is arguably the most prominent example, and this will be the subject of the talk.
First, a small overview is given of measurement, as introduced by von Neumann. Next, a brief survey is given of the most important features of a quantum measurement device, e.g., signal amplification, irreversible registration etc. And finally, simulation results are presented of a small quantum measurement apparatus that utilizes symmetry breaking to enhance a microscopic signal.
Giuseppe CarleoHFML 02.2020180322ETH Zurich and Flatiron institute New YorkCANCELLED!Askar IliasovHFML 02.2020180315Radboud UniversityBroadband electrostatic noise and inhomogeneous-energy-density-driven instability
Alfvenic turbulence and it’s low-frequency part, which is called broadband electrostatic noise, are repeatedly observed in the auroral region of ionosphere. There are plenty ideas explaining the origin of the broadband electrostatic noise. One of them is the IEDDI (inhomogeneous-energy-density-driven instability). This instability can generate ion-cyclotron and oblique ion-acoustic waves with broadband spectrum that can be identified as broadband electrostatic noise. In the group seminar, I will tell about this topic and other possible applications of the IEDDI to ionosphere physics.
Erik van LoonHFML 02.2020180308Radboud UniversityCompetition of strong charge and spin fluctuations in monolayer NbS2
(Short DPG practice talk)
In this group seminar, I will present our recent work on the electronic structure of monolayer NbS2. This talk is a trial version of my presentation at the DPG Spring Meeting in Berlin, which will be held the week afterwards. The conference talk is 12+3 minutes, this version will probably be a bit but not much longer. It would be nice if some of you could give some suggestions for improving the talk afterwards.
Single-layers of transition metal dichalcogenides have rich phase diagrams featuring metallic, insulating and charge/spin density wave phases. Competing interactions lie beneath these competing phases. Theoretical descriptions have so far focused on the electron-phonon interactions in these materials, whereas the electron-electron interaction has mostly been ignored. In this talk, we show that in NbS2 the local Coulomb interaction is by itself strong enough to turn the material insulating. Screening by the electron-phonon and the non-local Coulomb interaction restores the metallic phase, leads to a broadening of the electronic spectral function and to a coexistence of strong charge and spin fluctuations. These results are obtained by combining an ab-initio determination of the band structure and Coulomb interaction with the Dual Boson approach for the extended Hubbard model.
Dmitri BykovHFML 02.2020180222Ludwig-Maximilians-Universität (München, Germany) and Steklov Mathematical Institute (Moscow, Russia)Haldane limits for SU(N) spin chains
Using methods from the theory of geometric quantization, I will describe the generalization of Haldane’s continuum limit around anti-ferromagnetic configurations of spin chains with SU(N) symmetry. The continuum theory is a sigma-model with flag manifold target space and a (generically) non-zero topological term.
Alexander KrikunHFML 02.2020180212Leiden UniversityDoping the holographic Mott insulator
Mott insulators are central to the physics of strongly correlated electron systems. If the normal state of the system is a Fermi liquid, characterized by the well-defined quasiparticles, the Mott insulator formed after introducing strong enough background lattice can be qualitatively understood as a "traffic jam" of these repulsive quasi-electrons.
But what if the normal state is instead the densely entangled strange metal, where quasiparticles are not identifiable anymore? Then one gets the most notorious Mott insulator, observed in under-doped cuprate high-Tc superconductors. This state violates some of the simple logic which follows from classical "traffic jam" picture.
The holographic duality, discovered in string theory, describes generic properties of certain classes of such densely entangled quantum matter. Using holography, we build the "strange metal" version of the Mott insulator as a commensurate state between spontaneous intertwined charge density wave and a lattice. I will show that this state shares many properties with the conventional Mott insulator but is different in several aspects. Crucially, these differences can shed light on the unconventional features of the Mott state in the under-doped cuprate.
Based on: arXiv:1710.05791
Koen ReijndersHFML 02.2020180118Radboud UniversityTheory of electronic Veselago lenses in graphene: caustics, aberrations and valley effects
(Veldhoven conference practice talk)
A graphene n-p junction has the ability to focus electrons and can therefore act as a (Veselago) lens. For the Dirac Hamiltonian, this focusing is ideal. Using semiclassical methods, we study how Veselago lensing is affected by two sources of aberrations.
The first of these is the next order term in graphene's Hamiltonian. We find that this trigonal warping term leads to disintegration of the ideal focus. The position of the focus becomes dependent on the valley, with a splitting that depends on energy and lattice orientation. The second source of aberrations is an initial pseudospin polarization. This leads to a lateral shift of the focus, depending on the valley and the polarization. These aberrations, confirmed by numerical simulations, are relevant for the recently proposed Dirac Fermion Microscope.
Our results can easily be applied to other Dirac materials, where the effects are likely to be stronger.
Andrey BagrovHFML 02.2020171130Radboud UniversityHolographic thermalization and optical pump-probe spectroscopy
Using holography, we model experiments in which a 2+1D strange metal is pumped by a laser pulse into a highly excited state, after which the time evolution of the optical conductivity is probed. We consider a finite-density state with mildly broken translation invariance and excite it by oscillating electric field pulses. At zero density, the optical conductivity would assume its thermalized value immediately after the pumping has ended. At finite density, pulses with significant DC components give rise to slow exponential relaxation, governed by a vector quasinormal mode. In contrast, for high-frequency pulses the amplitude of the quasinormal mode is strongly suppressed, so that the optical conductivity assumes its thermalized value effectively instantaneously.
Bertarnd DupeHFML 02.2020171116Johannes Gutenberg University Mainz, GermanyMultiscale study of skyrmions in ultra-thin films
Due to their unique topological and dynamic properties, skyrmions in magnetic materials offer attractive perspectives for future spintronic applications [1]. Recently, it has been discovered that magnetic skyrmions of the Néel-type can occur at interfaces [2-4] due to strong Dzyaloshinskii-Moriya (DMI) interactions. We carried out first-principles calculations to study the stabilization mechanism of skyrmions in ultra-thin-film and multilayers [3,5]. We showed that the competition between the Heisenberg exchange beyond first nearest neighbor, the DM, the anisotropy and the Zeeman interactions are crucial to describe equilibrium properties of skyrmions at interfaces. Especially, such competitions may stabilize higher order skyrmionic states [6].
Here, we focus on the effects of these competing interactions on topologically protected excited states. As a test case, we use the simulation parameters corresponding to the Pd(fcc)/Fe/Ir(111) ultrathin film [2,3]. We simulate thermally activated excited states, e.g. skyrmions and antiskyrmions by exploring the B-T phase diagram [7] as well as their respective energy barriers with respect to the ferromagnetic state [8]. We show that competing magnetic interactions may enhance the stability of skyrmionic states. We also study the motion of these states under spin transfer torque [9].
[1] A. Fert, et al., Nature Nano. 8, 152 (2013).
[2] N. Romming, et al., Science 341, 636 (2013).
[3] B. Dupé, et al., Nature Comm. 5, 4030 (2014).
[4] C. Moreau-Luchaire, et al., Nature Nano. 11, 444 (2016).
[5] B. Dupé, et al., Nature Comm. 7, 11779 (2016).
[6] B. Dupé, et al., New Journal of Physics 18, 055015 (2016).
[7] M. Böttcher, et al. submitted
[8] S. von Malottki, et al. Scientific Reports 7, 12299 (2017).
[9] U. Ritzmann, et al. in preparation
Robert JeffersonHG00.10820171103Amsterdam University and Max Planck Institute for Gravitational Physics, Potsdam, GermanyCircuit complexity in quantum field theory
We examine the question of circuit complexity in quantum field theory. We provide a quantum circuit model for the preparation of Gaussian states, in particular the ground state, in a free scalar field theory for general dimensions. Applying the geometric approach of Nielsen to this quantum circuit model, the complexity of the state becomes the length of the shortest geodesic in the space of circuits. We compare the complexity of the ground state of the free scalar field to the analogous results from holographic complexity, and find some surprising similarities.
Hylke DonkerHFML 02.2020171019Radboud UniversityDecoherence in antiferromagnets
I will try to give a pedagogical presentation, explaining the key
concepts of decoherence theory to a diverse audience. Emphasis is on
concepts rather than equations. The abstract that I originally
submitted is:
Present day experiments in magnetism are approaching the borderline
between classical and quantum physics. The decoherence program
aims to explain this crossover within the quantum mechanical
formalism, by immersing a closed system into a quantum environment. It
is therefore of interest to study in detail the impact of such a
quantum environment on small magnets.
To this end, a brief account is given of several aspects of
decoherence, specifically geared towards small antiferromagnets. In
particular, the concept of pointer states is scrutinised,
dynamical consequences such as the decoherence wave is discussed,
and a physical origin of the staggered (conjugate) field is suggested.
Achille MauriHFML 02.2020171005Pisa University, ItalyDensity response of doped Luttinger semimetals
Luttinger semimetals are materials hosting a parabolic crossing in their band structure. Conduction
and valence bands touch each other at the center of the Brillouin zone and disperse quadratically in
its proximity. The effective description of the low-energy single particle states in the neighborhood
of the nodal point is enforced by the symmetry properties of the crystal. The single-particle states
are characterized by a spin 3/2 and described by the Luttinger Hamiltonian, which is quadratic in
the spin and momentum operators. Several semimetals of the Luttinger type are known. Examples
include α-Sn, HgTe and the pyrochlore iridate Pr2Ir2O7. In ideally pure samples, the Fermi energy
lies at the crossing point. In this case the inclusion of the electron-electron interactions is known to
drive the system either into a strongly coupled non-Fermi liquid state or into broken-symmetry
states. In the presence of a sufficiently large electron/hole concentration, instead, the system
behaves as a normal Fermi liquid. Some of the most important properties of the system are
connected to the dynamical dielectric screening function, which is essential to predict plasmons and
screening. In this seminar I will present the dielectric properties of doped Luttinger semimetals in
the normal phase, calculated within the Random Phase Approximation.
Louk RademakerHG00.62220170918Perimeter Institute for Theoretical Physics, Waterloo, CanadaThermalization in Quantum Systems - and its breakdown
How does thermalization in quantum systems work? Naively, the unitary time evolution prevents thermalization, but one can easily show that in general quantum systems thermalize when brought into contact with a thermal bath. In noninteracting systems, the approach to the thermal value can be either ballistic or diffusive depending on particle statistics and bath temperature. However, many systems cannot be thermalized when placed in a bath: glasses.
I will discuss a disorder free model of an organic electronic glass that is formed through rapid supercooling. Geometric frustration and long-range interactions cause the Arrhenius-type freezing.
Quenched disorder can also lead to glassiness, a phenomenon known as many-body localization. In this case, thermalization is prevented by the existence of extensively many local integrals of motion. I will show how to compute these integrals of motion and their properties.
Andrei LugovskoiHFML 02.2020170615Radboud UniversityTransition metals at extreme conditions: mechanical and structural stability at high pressures and temperatures
I will cover a number of studies, dedicated to the properties and structural stability of metallic systems, subjected to high pressure, temperature and deformation. These investigations are based on DFT calculations of the elastic moduli, including higher order elastic constants, lattice dynamics, and electronic structure for a range of transition metals under extreme compressions. The discussed phenomena have diverse character and include phase transitions to the phases with lower symmetry than of initial phase, electronic topological transitions, soft mode driven phase transitions. I will also demonstrate the results of ab initio molecular dynamics modeling of equiatomic Nb-Ru alloy, which demonstrates shape memory effect at high temperatures.
Koenraad SchalmHFML 02.2020170608Leiden UniversityRandom potentials, string theory and condensed matter: Disorder and conductivity in strongly correlated metals
In electronic matter the dominant contribution to the resistivity is due to momentum relaxation controlled by translational symmetry breaking in the material. I discuss the various ways this translational symmetry breaking is encoded in holographic duals of strongly correlated electron systems. This exhibits how the physics underlying the momentum relaxation can be very different from conventional weakly coupled systems. Partially driven by recent experiments in high Tc superconductors and graphene, I will focus in particular on disorder-driven resistivity in quantum critical systems.
Andrey AntipovHG00.06220170509University of California, Santa BarbaraModeling superconductor-semiconductor heterostructures in the presence
of gate-induced electric fields
We study the effect of gate-induced electric fields on the properties
of semiconductor-superconductor heterostructures.
Using a model that describes the semiconductor and the superconductor
on the same footing we are able to describe
the changes of the heterostructure states induced by external electric
fields and quantify the effect that these changes
have on the effective parameters of the heterostructures. The
effective g-factor of the heterostructure is a key parameter
for the realization and observation of Majorana modes in these
systems. We show that the changes of the heterostructure's
wavefunctions
induced by external electric fields can significantly modify the
effective g-factor of superconductor-semiconductor heterostructures.
Anatoly NeishtadtHFML 02.2020170425Loughborough University, UKOn long-term behaviour of systems with passages through resonances
Small perturbations imposed on an integrable nonlinear multifrequency oscillatory system cause a slow evolution. During this evolution the system may pass through resonant states. There are important phenomena related to such passages: capture into resonance and scattering on resonance. We will discuss the dynamics on long time intervals on which many passages through resonances occur.
Effects of passages through resonances can be considered as random events. Such effects separated by long time intervals can be treated as statistically independent. In this talk we describe model examples from charged particles dynamics that demonstrate these quasi-random effects. In particular, we present an analog of kinetic equation for description of such kind of dynamics.
Viatcheslav DobrovitskiHFML 02.2020170406QuTech and Kavli Institute of Nanoscience, TU DelftSpins in diamond for nanoscale sensing and quantum information processing
Understanding and controlling quantum spins in solids is an exciting scientific endeavor. Besides fundamental interest in non-equilibrium many-spin dynamics, this research is needed for applications in nanomagnetism, spintronics, quantum information, and advanced sensing at nanoscale. The nitrogen-vacancy (NV) centers in diamond constitute a particularly promising platform for future solid-state quantum technologies. I will present our work on quantum spin dynamics and control of individual electronic and nuclear spins in diamond. I will discuss how controlling and protecting the coherent dynamics of coupled spins enables accurate quantum gates on spin qubits in diamond, and how such gates allow development of the quantum registers with solid-state spin qubits. I will talk about extending this approach into the area of nanoscience, which results in very sensitive nanoscale tomography with single-spin resolution, and about using these advances for the small-scale quantum information processing in diamond. Finally, I will discuss how the same ideas can be modified and used to control light emission from the NV centers, required for high-fidelity quantum communication.
Evgeny StepanovHFML 02.2020170316Radboud UniversityNonlocal screening effects in strongly correlated materials
Strongly correlated electron systems remain one of the interesting subjects in modern condensed matter physics. It is hard to treat such systems analytically due to the large local and nonlocal electron-electron interaction. One of the most popular approaches is the dynamical mean-field theory (DMFT), which provides an approximate solution of the Hubbard model by mapping it to a local impurity problem. Later, an extended dynamical mean-field theory (EDMFT) was introduced to include collective (bosonic) degrees of freedom, such as charge or spin fluctuations, into DMFT. Unfortunately, these collective excitations have a strongly nonlocal nature, so a dynamical mean-field approach is insufficient and it was necessary to develop some extensions to treat non-local correlations. To go beyond EDMFT, one needs to determine the corrections to the electronic self-energy and polarization operator that describe excitations that
were not taken into account at the impurity level. Therefore, the great care should be taken to avoid double counting of correlation effects when merging EDMFT with the extension part, which is necessary for a correct construction of the theory and is the subject of hot discussions nowadays. EDMFT+GW approach is a well-known example of the EDMFT extension, where the GW diagrams for the self-energy and polarization operator are introduced beyond the impurity level. In an attempt to avoid double counting, all local contributions of the GW diagrams are subtracted and only the purely nonlocal
part is used to describe nonlocal correlations.
Here we introduce the Dual Boson (DB) approach and show that existing up to now approaches, such as EDMFT+GW, can be easily derived from the exact dual transformations and should be corrected in order to obtain better physical description of strongly correlated systems. The DB theory is free from double counting problem by construction, therefore, for the same computational complexity as the standard EDMFT+GW approach, the Dual Boson formalism significantly improves physical results and solves the double counting problem, which is illustrated in the phase boundary between the charge-ordered phase and the Fermi liquid.
Artem IvashkoHFML 02.2020170309Leiden UniversityChiral Magnetic Effect: the interplay of the bulk and the boundary
Weyl semimetals provide us an outstanding platform to probe the properties of three-dimensional relativistic massless electrons in condensed-matter setup. One of the theoretical predictions for such excitations that goes back to 1980 is the so-called Chiral Magnetic Effect. This effect lies in appearance of electric current proportional to the external magnetic field, but is known to vanish in thermal equilibrium. A simple and yet experimentally accessible way to probe the effect is to drive the system out of equilibrium by applying magnetic field that oscillates with time. We attempt to resolve the controversy present in the literature regarding the value of the coefficient entering the expression for the current, and to clarify the contributions of the bulk and the boundary states therein. We confirm a recent surprising finding of [1] that the boundary states may dominate the current, and that their contribution survives in the limit of large sample sizes.
[1] Baireuther, Hutasoit, Tworzydło, and Beenakker, New J. Phys. (2016)
Bektur MurzalievHFML 02.2020170302Radboud UniversityMagnon activation by hot electrons in magnetic semiconductors and half-metallic ferromagnets: the role of non-quasiparticle states
We consider the situation when a femtosecond laser pulse creates a hot electron state in half- metallic ferromagnet (e.g., ferromagnetic semiconductor) on a picosecond timescale but does not act directly on localized spin system. We investigate the energy and magnetic moment transfer from hot itinerant electrons to localized spins. In a half-metal such a transfer is facilitated by the so-called non-quasiparticle states, which are the scattering states of a magnon and spin-majority electron. We predict that in a typical ferromagnetic semiconductor such as EuO magnons remain essentially in non-equilibrium on a scale of the order of microsecond after the laser pulse. In the framework of the s-d exchange model the evolution of the magnon distribution is described by a quantum kinetic equation that we derive using the non-equilibrium Keldysh diagram technique. At short time scales we obtain approximately linear-in-time growth of the number of magnons with a distribution that is, however, essentially different from the Bose-Einstein one. We argue that such a non-equilibrium magnon state must be typical for experiments on ultra-fast magnetization dynamics in many half- metallic materials and magnetic semiconductors.
Petter SaterskogHFML 02.2020170216Leiden UniversityThe exact spectrum of a d=2 quantum critical metal in the limit kF→∞, Nf→0 with NfkF fixed
We show that the fermionic and bosonic spectrum of d=2 fermions at finite density coupled to a critical boson can be determined exactly in the combined limit kF→∞, Nf→0 with NfkF fixed. In this double scaling limit, the boson two-point function is corrected, but only at one-loop. This double scaling limit therefore incorporates the leading effect of Landau damping. The exact fermion two-point function is determined analytically in real space and numerically in (Euclidean) momentum space. The resulting spectrum is discontinuously connected to the quenched Nf→0 result. For ω→0 with k fixed it exhibits the distinct non-Fermi-liquid behavior previously surmised from the RPA approximation. However, the exact answer obtained here shows that the RPA result does not fully capture the IR of the theory.
Jasper van WezelHG00.62220170127Amsterdam UniversityAn Inherent Instability of Unitary Quantum Dynamics
Is it possible to observe experimental consequences of the inherent instabilities of Quantum Dynamics?
In this seminar, I will argue that the answer in principle should be "yes". I will show that quantum dynamics as described by Schrodinger's equation is inherently unstable. In close analogy to the well-known description of spontaneous symmetry breaking, the dynamics of quantum systems in the thermodynamic limit is infinitely sensitive to even infinitesimally small perturbations of its unitarity time evolution. For finite-sized quantum objects, this results in qualitative changes of the dynamics if even exceedingly small perturbations of the unitary dynamics are allowed. I will argue that such small perturbations may indeed be expected to exist, and I will indicate how this affects model descriptions of microscopic and macroscopic objects, as well as systems on the nano-scale. The question that then remains, is whether such effects are observable in practice as well as in principle. I will give some arguments for why and how instabilities might show up in existing and recently proposed nano-scale experiments, and I will indicate the main problems that are to be overcome in order for the instability to be distinguishable from generic effects in the quantum dynamics of open systems.
Rob OuwerslootHFML 02.2020161215Radboud UniversityThe effect of grain boundaries on the elastic properties of graphene and nanotubes
In this talk I will present the main results of the research I did in the TCM group for my master's internship and thesis.
Grain boundaries arise there where two grains meet. We have studied the influence of [0001] tilt grain boundaries, which are caused by a mismatch in the orientation of neighboring grains, on the elastic properties of graphene and carbon nanotubes. Building on earlier research [1] we took three model systems and determined several of their elastic properties, using classical atomistic Monte Carlo simulations with the empirical bond-order potential LCBOPII. The bending rigidity was calculated through analysis of the normal-normal correlation function at finite temperatures, while the in-plane elastic coefficients were probed by stretching and compressing the 2D samples at 0 K. We also determined the bending rigidity for carbon nanotubes, both with and without grain boundaries.
Along the way, we found many interesting and unexpected effects. As an example, one of the grain boundary systems shows a structural transition in the 400-600 K range which destroys its characteristic zig-zag shape and alters its bending rigidity. A second surprise came from the simulations on carbon nanotubes, which predict a different temperature behavior of the bending rigidity for carbon nanotubes than for 2D graphene. On the basis of these and other cases, combined with the results for the grain boundaries in general, I will show that when it comes to the elastic properties of graphene, shape matters.
[1] J.M. Carlsson, L.M. Ghiringhelli, and A. Fasolino. Theory and hierarchical calculations of the structure and energetics of [0001] tilt grain boundaries in graphene. Phys. Rev. B, 84(16), 2011.
Stefan VandorenHG00.30320161122Utrecht UniversityEntanglement Entropy for Periodic Sublattices
We study the entanglement entropy (EE) of Gaussian systems on a lattice with periodic boundary conditions, both in the vacuum and at nonzero temperatures. By restricting the reduced subsystem to periodic sublattices, we can compute the entanglement spectrum and EE exactly. We illustrate this for a free (1+1)-dimensional massive scalar field at a fixed temperature. Consistent with previous literature, we demonstrate that for a sufficiently large periodic sublattice the EE grows extensively, even in the vacuum. Furthermore, the analytic expression for the EE allows us probe its behavior both in the massless limit and in the continuum limit at any temperature.
Philippe CorbozHG00.06220161118Amsterdam UniversitySimulation of strongly correlated systems with 2D tensor network methods
Tensor networks are a class of variational wave functions enabling an efficient representation of quantum many-body states, where the accuracy can be systematically controlled by the so-called bond dimension. A well-known example are matrix product states (MPS), the underlying tensor network of the density matrix renormalization group (DMRG) method, which has become the state-of-the-art tool to study (quasi-) one dimensional systems. Progress in quantum information theory, in particular a better understanding of entanglement in quantum many-body systems, has led to the development of tensor networks for two-dimensional systems, including e.g. projected entangled-pair states (PEPS) or the 2D multi-scale entanglement renormalization ansatz (MERA). These methods provide one of the most promising routes for the simulation of strongly correlated systems in 2D, in particular models where Quantum Monte Carlo fails due to the negative sign problem.
In this seminar I give a short introduction to tensor network methods and report on recent progress with infinite projected entangled-pair states (iPEPS) which is a tensor network ansatz for 2D wave functions in the thermodynamic limit. I present iPEPS results for the t-J model and the 2D Hubbard model which reveal an extremely close competition between a uniform d-wave superconducting state and different types of stripe states, with lower variational energies than in previous state-of-the-art studies for large 2D systems. Finally, I show how iPEPS simulations of the Shastry-Sutherland model helped to shed new light on the magnetization process in SrCu2(BO3)2, which has been an intriguing puzzle for more than a decade.
Hylke DonkerHFML 02.2020161027Radboud UniversityOn the validity of pointer states: few-atom antiferromagnets as a benchmark test
Maintaining coherence in quantum circuits is one of the primary hurdles which prevent rapid development of the field. It is therefore of interest to gain fundamental insight in the process which destroys coherence. We study decoherence by considering small antiferromagnetic systems that interact with environmental degrees of freedom. Particular emphasis is on pointer states: these special states are least prone to environmental deterioration and are effectively classical. By performing simulations we test conjectures [2] concerning the pointer basis beyond the perturbative limit.
What is found is that contrary to popular believe [1,3], when the system-environment interaction is dominant, generically the pointer basis is not given by the interaction Hamiltonian. To substantiate this claim we construct a continuous family of counter-examples. Conversely, in the opposite limit when the system-environment interaction is weak and the environment evolves adiabatically, we find that the preferred basis coincides with the energy eigen states of the central-system, in accordance with Ref. [2]. It appears that the intrinsic dynamics of the bath plays an important (although not crucial) role. Finally, I will say a few words about the quantum-to-classical crossover in small antiferromagnets and conclude by recapitulating the main findings.
[1] Zurek, Rev. Mod. Phys. 75 ('03)
[2] Paz et al. Phys. Rev. Lett. 82 ('99)
[3] Joos et al. Decoherence and the Appearance of a Classical World in Quantum Theory, Springer ('03)
Piotr ChudzinskiHFML 02.2020161020Utrecht UniversityTomonaga-Luttinger liquid and beyond: the intriguing case of lithium molybdenum purple bronze
We study the low energy physics of a quasi-1D material - lithium molybdenum purple bronze (LMO), Li$_0.9$Mo$_{6}$O$_{17}$, which undergoes a mysterious phase transition at T$^*=28K$ to later become superconductor at 1.9K. Based on band structure results we derive an effective low energy theory within the Tomonaga-Luttinger liquid framework. We estimate the TLL parameters and strength of possible instabilities. Our aim here is to understand these experimental findings that are certainly lying within the 1D regime. In the second part we move beyond the standard 1D theory, we investigate the role of inter-orbital fluctuations. We make a conjecture that the physics around T$^*$ is dominated by multi-orbital excitons. They couple with 1D fermions and properties of such system can be captured using a polaronic picture. Using this model we compute fermionic Green's function to find that the spectral function is broadened with a Gaussian and its temperature dependence acquires an extra T$^1$ factor. Both effects are in perfect agreement with experimental findings. We also compute the resistivity for temperatures above and below the critical temperature T$^*$ which allows us to explain an upturn of the resistivity and interpret the suppression of this extra component when a magnetic field is applied along the conducting axis.
Yaroslav KvashninHFML 02.2020161013Uppsala UniversityTransition metals under high pressure: combined theoretical and experimental study
Thanks to the development of the diamond anvil cells (DACs), it is now possible to reach the pressures exceeding that in the Earth's core.
In our work, we investigate the behaviour of transition metals under such extreme conditions and look at the pressure-driven phase transitions.
We use synchrotron-radiation-based techniques such as x-ray diffraction and K-edge x-ray magnetic circular dichroism (K-XMCD) in order to obtain both structural and magnetic information about the systems under consideration.
First-principles calculations, based on density functional theory (DFT) are employed to interpret the experimental data.
In this talk I will show a couple of examples of how such a combined theoretical and experimental approach works.
In particular, we have demonstrated that nickel remains magnetic even under pressures exceeding 200 GPa [1].
DFT calculations predict a different pressure dependence of orbital and spin moments, which is also reflected in K-XMCD data.
We argue that this behaviour is general for transition metals and explain its origin.
Co undergoes a transition from the ferromagnetic hcp phase to the nonmagnetic fcc one around 100 GPa [2,3].
We suggest that suggest that there are few more transitions taking place at lower applied pressure (~80 GPa), which are of Lifshitz type [3].
It is found that these Lifshitz transitions are responsible for the anomalies in various elastic properties, observed experimentally, and also lead to the stabilisation of a noncollinear spin arrangement in highly compressed hcp phase.
Finally, we argue that the disappearance of K-XMCD signal in high-pressure phase of FeCo alloy is related with the emergence of antiferromagnetism among Fe moments [4].
[1] R.Torchio, et al., PRL 107, 237202 (2011)
[2] R.Torchio, et al., PRB 94, 024429 (2016)
[3] YK, W.Sun, I. Di Marco, O. Eriksson, PRB 92, 134422 (2015)
[4] R.Torchio, et al., PRB 88, 184412 (2013)
Eugene KoganHFML 02.2020161010Bar-Ilan UniversityElectronic band structure in graphene: non-tight-binding vs. tight-binding bands
We compare the classification of the electron bands in graphene, obtained by group theory algebra
in the framework of a tight-binding model (TBM), with that calculated in a density-functional-
theory (DFT) framework. Identification in the DFT band structure of all eight energy bands (four
valence and four conduction bands) corresponding to the TBM-derived energy bands is performed
and the corresponding analysis is presented. The four occupied (three
σ-like and one π-like) and three unoccupied (two σ-like and one π-like) bands given by the DFT closely correspond to those
predicted by the TBM, both by their symmetry and their dispersion law. However, the two lowest
lying at the Γ-point unoccupied bands (one of them of a σ-like type and the other of a π-like one),
are not of the TBM type. We suggest a minimalistic model for these two bands. This description
among other things predicts the symmetry of the bands.
Enrico RossiHFML 02.2020161006William and Mary collegeSpin-orbit coupling effects in 2D heterostructures and superconductors
The ability to realize 2D crystals and advances in fabrication techniques
allow the realization of new heterostructures with novel properties.
I will first discuss the case of heterostructures formed by one sheet of
graphene, or bilayer graphene, and a topological insulator.
I will show how the twist angle between the graphene layer and the TI can be
used to tune the electronic properties of such heterostructures and I will
discuss their spin-dependent transport properties.
I will then consider heterostructures formed by a 2D crystal
and a superconductor and show that in some of these heterostructures an anomalous
odd-frequency pairing term can be induced due to the interplay of
spin-orbit coupling and superconductivity.
Finally, I will discuss how the presence of spin-orbit
coupling affects the bound states induced by impurities in
superconductors and the conditions necessary to drive a chain of such impurities
into a topological phase exhibiting Majorana modes.
Andrey BagrovHFML 02.2020160915Radboud UniversityHolographic dual of a time machine
Assuming that the AdS/CFT prescription is valid in the case of noncausal backgrounds, we apply it to the simplest possible eternal time machine solution in AdS3 based on two conical defects moving around their center of mass along a circular orbit. Closed timelike curves in this space-time extend all the way to the boundary of AdS3, violating causality of the boundary field theory. By use of the geodesic approximation we address the issue of self-consistent dynamics of the dual 1+1 dimensional field theory when causality is violated, and calculate the two-point retarded Green function. It has a nontrivial analytical structure both at negative and positive times, providing us with an intuition on how an interacting quantum field could behave once causality is broken. I'll try to make the talk comprehensible for non-holographic audience.
Jaakko NissinenHFML 02.2020160901Leiden UniversityGeneralized nematic order and gauge theory
This talk discusses exotic or generalized nematic order that breaks the rotational symmetries of isotropic space to any three-dimensional point group. We show how a lattice gauge theory can efficiently incorporate the coarse-grained order parameter theory of such generalized nematics and allows them to be studied on a general footing. The lattice gauge theory consists of a nematic nearest-neighbor interaction between gauged three-dimensional rotors and a gauge field term that encodes for the nematic defects. We identify the generalized nematic phases at low temperatures and discuss the phase diagrams of the model obtained in simulations. In this common framework, we can quantify the fluctuations of the orientational order parameters of generalized nematics by comparing the lattice model with different point-group symmetries. For highly symmetric nematics, a vestigial chiral liquid phase is found in between the nematic and the isotropic liquid that arises via an order-out-of-disorder mechanism. The gauge theory also allows us to enlist the minimal order parameter tensors allowed by the symmetries for all point groups. Finally, we show how many phase transitions between different axial nematics can be accessed by tuning the parameters of the model.