28 results
Search Results
Now showing 1 - 10 of 28
- ItemSixfold fermion near the Fermi level in cubic PtBi2(Amsterdam : SciPost Foundation, 2021) Thirupathaiah, Setti; Kushnirenko, Yevhen; Koepernik, Klaus; Piening, Boy Roman; Büchner, Bernd; Aswartham, Saicharan; van den Brink, Jeroen; Borisenko, Sergey; Fulga, Ion CosmaWe show that the cubic compound PtBi2, is a topological semimetal hosting a sixfold band touching point in close proximity to the Fermi level. Using angle-resolved photoemission spectroscopy, we map the bandstructure of the system, which is in good agreement with results from density functional theory. Further, by employing a low energy effective Hamiltonian valid close to the crossing point, we study the effect of a magnetic field on the sixfold fermion. The latter splits into a total of twenty Weyl cones for a Zeeman field oriented in the diagonal, [111] direction. Our results mark cubic PtBi2, as an ideal candidate to study the transport properties of gapless topological systems beyond Dirac and Weyl semimetals.
- ItemEngineering spectral properties of non-interacting lattice Hamiltonians(Amsterdam : SciPost Foundation, 2021) Moghaddam, Ali G.; Chernyavsky, Dmitry; Morice, Corentin; van Wezel, Jasper; van den Brink, JeroenWe investigate the spectral properties of one-dimensional lattices with position-dependent hopping amplitudes and on-site potentials that are smooth bounded functions of the position. We find an exact integral form for the density of states (DOS) in the limit of an infinite number of sites, which we derive using a mixed Bloch-Wannier basis consisting of piecewise Wannier functions. Next, we provide an exact solution for the inverse problem of constructing the position-dependence of hopping in a lattice model yielding a given DOS. We confirm analytic results by comparing them to numerics obtained by exact diagonalization for various incarnations of position-dependent hoppings and on-site potentials. Finally, we generalize the DOS integral form to multi-orbital tight-binding models with longer-range hoppings and in higher dimensions.
- ItemEngineering a pure Dirac regime in ZrTe5(Amsterdam : SciPost Foundation, 2023) Facio, Jorge I.; Nocerino, Elisabetta; Fulga, Ion Cosma; Wawrzynczak, Rafal; Brown, Joanna; Gu, Genda; Li, Qiang; Mansson, Martin; Sassa, Yasmine; Ivashko, Oleh; von Zimmermann, Martin; Mende, Felix; Gooth, Johannes; Galeski, Stanislaw; van den Brink, Jeroen; Meng, TobiasReal-world topological semimetals typically exhibit Dirac and Weyl nodes that coexist with trivial Fermi pockets. This tends to mask the physics of the relativistic quasiparticles. Using the example of ZrTe5, we show that strain provides a powerful tool for in-situ tuning of the band structure such that all trivial pockets are pushed far away from the Fermi energy, but only for a certain range of Van der Waals gaps. Our results naturally reconcile contradicting reports on the presence or absence of additional pockets in ZrTe5, and provide a clear map of where to find a pure three-dimensional Dirac semimetallic phase in the structural parameter space of the material.
- ItemAxion Mie theory of electron energy loss spectroscopy in topological insulators(Amsterdam : SciPost Foundation, 2021) Schultz, Johannes; Nogueira, Flavio S.; Büchner, Bernd; van den Brink, Jeroen; Lubk, AxelElectronic topological states of matter exhibit novel types of responses to electromagnetic fields. The response of strong topological insulators, for instance, is characterized by a so-called axion term in the electromagnetic Lagrangian which is ultimately due to the presence of topological surface states. Here we develop the axion Mie theory for the electromagnetic response of spherical particles including arbitrary sources of fields, i.e., charge and current distributions. We derive an axion induced mixing of transverse magnetic and transverse electric modes which are experimentally detectable through small induced rotations of the field vectors. Our results extend upon previous analyses of the problem. Our main focus is on the experimentally relevant problem of electron energy loss spectroscopy in topological insulators, a technique that has so far not yet been used to detect the axion electromagnetic response in these materials.
- ItemSymmetry‐Induced Selective Excitation of Topological States in Su–Schrieffer–Heeger Waveguide Arrays(Weinheim : Wiley-VCH, 2023) Tang, Min; Wang, Jiawei; Valligatla, Sreeramulu; Saggau, Christian N.; Dong, Haiyun; Saei Ghareh Naz, Ehsan; Klembt, Sebastian; Lee, Ching Hua; Thomale, Ronny; van den Brink, Jeroen; Fulga, Ion Cosma; Schmidt, Oliver G.; Ma, LiboThe investigation of topological state transition in carefully designed photonic lattices is of high interest for fundamental research, as well as for applied studies such as manipulating light flow in on-chip photonic systems. Herein, the topological phase transition between symmetric topological zero modes (TZM) and antisymmetric TZMs in Su–Schrieffer–Heeger mirror symmetric waveguides is reported. The transition of TZMs is realized by adjusting the coupling ratio between neighboring waveguide pairs, which is enabled by selective modulation of the refractive index in the waveguide gaps. Bidirectional topological transitions between symmetric and antisymmetric TZMs can be achieved with proposed switching strategy. Selective excitation of topological edge mode is demonstrated owing to the symmetry characteristics of the TZMs. The flexible manipulation of topological states is promising for on-chip light flow control and may spark further investigations on symmetric/antisymmetric TZM transitions in other photonic topological frameworks.
- ItemLong-range magnetic order in the ~S=1/2 triangular lattice antiferromagnet KCeS2(Amsterdam : SciPost Foundation, 2020) Bastien, Gaël; Rubrecht, Bastian; Haeussler, Ellen; Schlender, Philipp; Zangeneh, Ziba; Avdoshenko, Stanislav; Sarkar, Rajib; Alfonsov, Alexey; Luther, Sven; Onykiienko, Yevhen A.; Walker, Helen C.; Kühne, Hannes; Grinenko, Vadim; Guguchia, Zurab; Kataev, Vladislav; Klauss, Hans-Henning; Hozoi, Liviu; van den Brink, Jeroen; Inosov, Dmytro S.; Büchner, Bernd; Wolter, Anja U.B.; Doert, ThomasRecently, several putative quantum spin liquid (QSL) states were discovered in ~S=1/2 rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. A way to clarify the origin of the QSL state in these systems is to identify ways to tune them from the putative QSL state towards long-range magnetic order. Here, we introduce the Ce-based TLAF KCeS2 and show via low-temperature specific heat and μSR investigations that it yields magnetic order below TN=0.38 K despite the same delafossite structure. We identify a well separated ~S=1/2 ground state for KCeS2 from inelastic neutron scattering and embedded-cluster quantum chemical calculations. Magnetization and electron spin resonance measurements on single crystals indicate a strong easy-plane g~factor anisotropy, in agreement with the ab initio calculations. Finally, our specific-heat studies reveal an in-plane anisotropy of the magnetic field-temperature phase diagram which may indicate anisotropic magnetic interactions in KCeS2.
- ItemQuantum dynamics in 1D lattice models with synthetic horizons(Amsterdam : SciPost Foundation, 2022) Morice, Corentin; Chernyavsky, Dmitry; van Wezel, Jasper; van den Brink, Jeroen; Moghaddam, AliWe investigate the wave packet dynamics and eigenstate localization in recently proposed generalized lattice models whose low-energy dynamics mimics a quantum field theory in (1+1)D curved spacetime with the aim of creating systems analogous to black holes. We identify a critical slowdown of zero-energy wave packets in a family of 1D tight-binding models with power-law variation of the hopping parameter, indicating the presence of a horizon. Remarkably, wave packets with non-zero energies bounce back and reverse direction before reaching the horizon. We additionally observe a power-law localization of all eigenstates, each bordering a region of exponential suppression. These forbidden regions dictate the closest possible approach to the horizon of states with any given energy. These numerical findings are supported by a semiclassical description of the wave packet trajectories, which are shown to coincide with the geodesics expected for the effective metric emerging from the considered lattice models in the continuum limit.
- ItemTwo-Dimensional Discommensurations: An Extension to McMillan’s Ginzburg–Landau Theory(Basel : MDPI, 2023) Mertens, Lotte; van den Brink, Jeroen; Wezel, Jasper vanCharge density waves (CDWs) profoundly affect the electronic properties of materials and have an intricate interplay with other collective states, like superconductivity and magnetism. The well-known macroscopic Ginzburg–Landau theory stands out as a theoretical method for describing CDW phenomenology without requiring a microscopic description. In particular, it has been instrumental in understanding the emergence of domain structures in several CDW compounds, as well as the influence of critical fluctuations and the evolution towards or across lock-in transitions. In this context, McMillan’s foundational work introduced discommensurations as the objects mediating the transition from commensurate to incommensurate CDWs, through an intermediate nearly commensurate phase characterised by an ordered array of phase slips. Here, we extended the simplified, effectively one-dimensional, setting of the original model to a fully two-dimensional analysis. We found exact and numerical solutions for several types of discommensuration patterns and provide a framework for consistently describing multi-component CDWs embedded in quasi-two-dimensional atomic lattices.
- ItemMachine learning for additive manufacturing: Predicting materials characteristics and their uncertainty(Amsterdam [u.a.] : Elsevier Science, 2023) Chernyavsky, Dmitry; Kononenko, Denys Y.; Han, Jun Hee; Kim, Hwi Jun; van den Brink, Jeroen; Kosiba, KonradAdditive manufacturing (AM) is known for versatile fabrication of complex parts, while also allowing the synthesis of materials with desired microstructures and resulting properties. These benefits come at a cost: process control to manufacture parts within given specifications is very challenging due to the relevance of a large number of processing parameters. Efficient predictive machine learning (ML) models trained on small datasets, can minimize this cost. They also allow to assess the quality of the dataset inclusive of uncertainty. This is important in order for additively manufactured parts to meet property specifications not only on average, but also within a given variance or uncertainty. Here, we demonstrate this strategy by developing a heteroscedastic Gaussian process (HGP) model, from a dataset based on laser powder bed fusion of a glass-forming alloy at varying processing parameters. Using amorphicity as the microstructural descriptor, we train the model on our Zr52.5Cu17.9Ni14.6Al10Ti5 (at.%) alloy dataset. The HGP model not only accurately predicts the mean value of amorphicity, but also provides the respective uncertainty. The quantification of the aleatoric and epistemic uncertainty contributions allows to assess intrinsic inaccuracies of the dataset, as well as identify underlying physical phenomena. This HGP model approach enables to systematically improve ML-driven AM processes.
- ItemMomentum space entanglement from the Wilsonian effective action(Woodbury, NY : Inst., 2022) Martins Costa, Matheus H.; van den Brink, Jeroen; Nogueira, Flavio S.; Krein, Gastão I.The entanglement between momentum modes of a quantum field theory at different scales is not as well studied as its counterpart in real space, despite the natural connection with the Wilsonian idea of integrating out the high-momentum degrees of freedom. Here, we push such a connection further by developing a novel method to calculate the Rényi and entanglement entropies between slow and fast modes, which is based on the Wilsonian effective action at a given scale. This procedure is applied to the perturbative regime of some scalar theories, comparing the lowest-order results with those from the literature and interpreting them in terms of Feynman diagrams. This method is easily generalized to higher-order or nonperturbative calculations. It has the advantage of avoiding matrix diagonalizations of other techniques.
- «
- 1 (current)
- 2
- 3
- »