Found 28 papers in cond-mat
Date of feed: Mon, 20 Nov 2023 01:30:00 GMT

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Quantum Hall Effect on Dirac electrons in modulated graphene. (arXiv:2311.10106v1 [cond-mat.mes-hall])
M Arsalan Ali

Theoretical investigation of Dirac electrons in electrically modulated graphene under perpendicular magnetic field B is presented. We have carried out a detailed study of modulation effect on Dirac electrons, which determine its electrical transport properties. The periodic potential broadens the Landau levels (LL), which oscillate with magnetic field B and a comparison made with two-dimensional electron gas system (2DEGS). We have found the effect of Hall conductivity on electronic conduction in this system. In addition, we find that Hall conductivity exhibits Weiss oscillations and Shubnikov de Haas (SdH) oscillations. The effect of temperature and the period of periodic potential on these oscillations are studied in this work. Furthermore, an integral quantum Hall effect in graphene is also discussed.


Itinerant Magnetism in the Triangular Lattice Hubbard Model at Half-doping: Application to Twisted Transition-Metal Dichalcogenides. (arXiv:2311.10146v1 [cond-mat.str-el])
Yuchi He, Roman Rausch, Matthias Peschke, Christoph Karrasch, Philippe Corboz, Nick Bultinck, S.A. Parameswaran

We use unrestricted Hartree-Fock, density matrix renormalization group, and variational projected entangled pair state calculations to investigate the ground state phase diagram of the triangular lattice Hubbard model at "half doping" relative to single occupancy, i.e. at a filling of $(1\pm \frac{1}{2})$ electrons per site. The electron-doped case has a nested Fermi surface in the non-interacting limit, and hence a weak-coupling instability towards density-wave orders whose wavevectors are determined by Fermi surface nesting conditions. We find that at moderate to strong interaction strengths other spatially-modulated orders arise, with wavevectors distinct from the nesting vectors. In particular, we identify a series closely-competing itinerant long-wavelength magnetically ordered states, yielding to uniform ferromagnetic order at the largest interaction strengths. For half-hole doping and a similar range of interaction strengths, our data indicate that magnetic orders are most likely absent.


Time- and spectrum-resolved quantum-path interferometry reveals exciton dephasing in MoS$_2$ under strong-field conditions. (arXiv:2311.10286v1 [physics.optics])
Yaxin Liu, Bingbing Zhu, Shicheng Jiang, Shenyang Huang, Mingyan Luo, Sheng Zhang, Hugen Yan, Yuanbo Zhang, Ruifeng Lu, Zhensheng Tao

Floquet engineering, the nonthermal manipulation of material properties on ultrafast timescales using strong and time-periodic laser fields, has led to many intriguing observations in quantum materials. However, recent studies on high-order harmonic generation from solids reveal exceptionally short dephasing times for field-dressed quantum states, thereby raising questions about the feasibility of Floquet engineering under strong-field conditions. In this study, we employ time- and spectrum-resolved quantum-path interferometry to investigate the dephasing mechanism of excitons driven by intense terahertz fields in bulk MoS$_2$. By driving with a photon energy far below the material bandgap, we observe strong hybridization of exciton excited states, with resonant transitions to these states leading to phase and amplitude modulations in interferograms. Our results reveal a field-strength-dependent dephasing rate of dressed excitons, with exciton dissociation identified as the primary cause of exciton dephasing under high driving fields. Importantly, we demonstrate that strong-field-driven excitons can exhibit long dephasing times, supporting the feasibility of Floquet engineering in strong-field environments. Our study sheds light on the underlying physics of strong-field-driven exciton decoherence and underscores the potential for nonthermal manipulation of quantum materials.


Tunable Inter-Moir\'e Physics in Consecutively-Twisted Trilayer Graphene. (arXiv:2311.10313v1 [cond-mat.mes-hall])
Wei Ren, Konstantin Davydov, Ziyan Zhu, Jaden Ma, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Mitchell Luskin, Ke Wang

We fabricate a twisted trilayer graphene device with consecutive twist angles of 1.33 and 1.64 degrees, in which we electrostatically tune the electronic states from each of the two co-existing moir\'e superlattices and the interactions between them. When both moir\'e superlattices contribute equally to electrical transport, we report a new type of inter-moir\'e Hofstadter butterfly. Its Brown-Zak oscillation corresponds to one of the intermediate quasicrystal length scales of the reconstructed moir\'e of moir\'e (MoM) superlattice, shedding new light on emergent physics from competing atomic orders.


Design and performance of an ultrahigh vacuum spectroscopic-imaging scanning tunneling microscope with a hybrid vibration isolation system. (arXiv:2311.10451v1 [cond-mat.mtrl-sci])
Pei-Fang Chung, Balaji Venkatesan, Chih-Chuan Su, Jen-Te Chang, Hsu-Kai Cheng, Che-An Liu, Shan-An Yu, Syu-You Guan, Tien-Ming Chuang

A spectroscopic imaging-scanning tunneling microscope (SI-STM) allows the atomic scale visualization of surface electronic and magnetic structure of novel quantum materials with high energy resolution. To achieve the optimal performance, low vibration facility is required. Here, we describe the design and the performance of an ultrahigh vacuum STM system supported by a hybrid vibration isolation system that consists of a pneumatic passive and a piezoelectric active vibration isolation stages. The STM system is equipped with a 1K pot cryogenic insert and a 9 Tesla superconducting magnet, capable of continuous SI-STM measurements for 7 days. A field ion microscopy system is installed for in situ STM tip treatment. We present the detailed vibrational noise analysis of the hybrid vibration isolation system and demonstrate the performance of our STM system by taking high resolution spectroscopic maps and topographic images on several quantum materials. Our results establish a new strategy to achieve an effective vibration isolation system for high-resolution STM and other scanning probe microscopy to investigate the nanoscale quantum phenomena.


Enhancement of spin to charge conversion efficiency at the topological surface state by inserting normal metal spacer layer in the topological insulator based heterostructure. (arXiv:2311.10460v1 [cond-mat.mes-hall])
Sayani Pal, Anuvab Nandi, Shambhu G. Nath, Pratap Kumar Pal, Kanav Sharma, Subhadip Manna, Anjan Barman, Chiranjib Mitra

We report efficient spin to charge conversion (SCC) in the topological insulator (TI) based heterostructure ($BiSbTe_{1.5}Se_{1.5}/Cu/Ni_{80}Fe_{20}$) by using spin-pumping technique where $BiSbTe_{1.5}Se_{1.5}$ is the TI and $Ni_{80}Fe_{20}$ is the ferromagnetic layer. The SCC, characterized by inverse Edelstein effect length ($\lambda_{IEE}$) in the TI material gets altered with an intervening Copper (Cu) layer and it depends on the interlayer thickness. The introduction of Cu layer at the interface of TI and ferromagnetic metal (FM) provides a new degree of freedom for tuning the SCC efficiency of the topological surface states. The significant enhancement of the measured spin-pumping voltage and the linewidth of ferromagnetic resonance (FMR) absorption spectra due to the insertion of Cu layer at the interface indicates a reduction in spin memory loss at the interface that resulted from the presence of exchange coupling between the surface states of TI and the local moments of ferromagnetic metal. The temperature dependence (from 8K to 300K) of the evaluated $\lambda_{IEE}$ data for all the trilayer systems, TI/Cu/FM with different Cu thickness confirms the effect of exchange coupling between the TI and FM layer on the spin to charge conversion efficiency of the topological surface state.


Testing the Renormalization of the von Klitzing Constant by Cavity Vacuum Fields. (arXiv:2311.10462v1 [cond-mat.mes-hall])
Josefine Enkner, Lorenzo Graziotto, Felice Appugliese, Vasil Rokaj, Jie Wang, Michael Ruggenthaler, Christian Reichl, Werner Wegscheider, Angel Rubio, Jérôme Faist

In light of recent developments demonstrating the impact of cavity vacuum fields inducing the breakdown of topological protection in the integer quantum Hall effect, a compelling question arises: what effects might cavity vacuum fields have on fundamental constants in solid-state systems? In this work we present an experiment that assesses the possibility of the von Klitzing constant itself being modified. By employing a Wheatstone bridge, we precisely measure the difference between the quantized Hall resistance of a cavity-embedded Hall bar and the resistance standard, achieving an accuracy down to 1 part in 105 for the lowest Landau level. While our results do not suggest any deviation that could imply a modified Hall resistance, our work represents pioneering efforts in exploring the fundamental implications of vacuum fields in solid-state systems.


Interplay of phase segregation and chemical reaction: Crossover and effect on growth laws. (arXiv:2311.10464v1 [cond-mat.stat-mech])
Shubham Thwal, Suman Majumder

By combining the nonconserved spin-flip dynamics driving ferromagnetic ordering with the conserved Kawasaki-exchange dynamics driving phase segregation, we perform Monte Carlo simulations of the nearest neighbor Ising model. Such a set up mimics a system consisting of a binary mixture of \emph{isomers} which is simultaneously undergoing a segregation and an \emph{interconversion} reaction among themselves . Here, we study such a system following a quench from the high-temperature homogeneous phase to a temperature below the demixing transition. We monitor the growth of domains of both the \emph{winner}, the \emph{isomer} which survives as the majority and the \emph{loser}, the \emph{isomer} that perishes. Our results show a strong interplay of the two dynamics at early times leading to a growth of the average domain size of both the \emph{winner} and \emph{loser} as $\sim t^{1/7}$, slower than a purely phase-segregating system. At later times, eventually the dynamics becomes reaction dominated, and the \emph{winner} exhibits a $\sim t^{1/2}$ growth, expected for a system with purely nonconserved dynamics. On the other hand, the \emph{loser} at first show a faster growth, albeit, slower than the \emph{winner}, and then starts to decay before it almost vanishes. Further, we estimate the time $\tau_s$ marking the crossover from the early-time slow growth to the late-time reaction dominated faster growth. As a function of the reaction probability $p_r$, we observe a power-law scaling $\tau_s \sim p_r^{-x}$, where $x\approx 1.05$, irrespective of temperature. For a fixed value of $p_r$ too, $\tau_s$ appears to be independent of temperature.


Graphene-based thermopneumatic generator for on-board pressure supply of soft robots. (arXiv:2311.10488v1 [cond-mat.mtrl-sci])
Armin Reimers, Jannik Rank, Erik Greve, Morten Möller, Sören Kaps, Jörg Bahr, Rainer Adelung, Fabian Schütt

Various fields, including medical and human interaction robots, gain advantages from the development of bioinspired soft actuators. Many recently developed grippers are pneumatics that require external pressure supply systems, thereby limiting the autonomy of these robots. This necessitates the development of scalable and efficient on-board pressure generation systems. While conventional air compression systems are hard to miniaturize, thermopneumatic systems that joule-heat a transducer material to generate pressure present a promising alternative. However, the transducer materials of previously reported thermopneumatic systems demonstrate high heat capacities and limited surface area resulting in long response times and low operation frequencies. This study presents a thermopneumatic pressure generator using aerographene, a highly porous (>99.99 %) network of interconnected graphene microtubes, as lightweight and low heat capacity transducer material. An aerographene pressurizer module (AGPM) can pressurize a reservoir of 4.2 cm3 to about ~140 mbar in 50 ms. Periodic operation of the AGPM for 10 s at 0.66 Hz can further increase the pressure in the reservoir to ~360 mbar. It is demonstrated that multiple AGPMs can be operated parallelly or in series for improved performance. For example, three parallelly operated AGPMs can generate pressure pulses of ~215 mbar. Connecting AGPMs in series increases the maximum pressure achievable by the system. It is shown that three AGPMs working in series can pressurize the reservoir to ~2000 mbar in about 2.5 min. The AGPM's minimalistic design can be easily adapted to circuit boards, making the concept a promising fit for the on-board pressure supply of soft robots.


Phase Coexistence of Mn Trimer Clusters and Antiferromagnetic Mn Islands on Ir(111). (arXiv:2311.10506v1 [cond-mat.mtrl-sci])
Arturo Rodríguez-Sota, Vishesh Saxena, Jonas Spethmann, Roland Wiesendanger, Roberto Lo Conte, André Kubetzka, Kirsten von Bergmann

Clusters supported by solid substrates are prime candidates for heterogeneous catalysis and can be prepared in various ways. While mass-selected soft-landing methods are often used for the generation of monodisperse particles, self-assembly typically leads to a range of different cluster sizes. Here we show by scanning tunneling microscopy measurements that in the initial stages of growth Mn forms trimers on a close-packed hexagonal Ir surface, providing a route for self-organized monodisperse cluster formation on an isotropic metallic surface. For an increasing amount of Mn, first a phase with reconstructed monolayer islands is formed, until at full coverage a pseudomorphic Mn phase evolves which is the most densely packed one of the three different observed Mn phases on Ir(111). The magnetic state of both the reconstructed islands and the pseudomorphic film is found to be the prototypical antiferromagnetic N\'eel state with 120{\deg} spin rotation between all nearest neighbors in the hexagonal layer.


Machine Learning Assisted Characterization of Labyrinthine Pattern Transitions. (arXiv:2311.10558v1 [cond-mat.soft])
Kotaro Shimizu, Vinicius Yu Okubo, Rose Knight, Ziyuan Wang, Joseph Burton, Hae Yong Kim, Gia-Wei Chern, B. S. Shivaram

We present a comprehensive approach to characterizing labyrinthine structures that often emerge as a final steady state in pattern forming systems. We employ machine learning based pattern recognition techniques to identify the types and locations of topological defects of the local stripe ordering to augment conventional Fourier analysis. A pair distribution function analysis of the topological defects reveals subtle differences between labyrinthine structures which are beyond the conventional characterization methods. We utilize our approach to highlight a clear morphological transition between two zero-field labyrinthine structures in single crystal Bi substituted Yttrium Iron Garnet films. An energy landscape picture is proposed to understand the athermal dynamics that governs the observed morphological transition. Our work demonstrates that machine learning based recognition techniques enable novel studies of rich and complex labyrinthine type structures universal to many pattern formation systems.


Fluctuation-induced currents in suspended graphene nanoribbons: Adiabatic quantum pumping approach. (arXiv:2311.10560v1 [cond-mat.mes-hall])
Federico D. Ribetto, Silvina A. Elaskar, Hernán L. Calvo, Raúl A. Bustos-Marún

Graphene nanoribbons (GNRs) are thin strips of graphene with unique properties due to their structure and nanometric dimensions. They stand out as basic components for the construction of different types of nanoelectromechanical systems (NEMS), including some very promising sensors and pumps. However, various phenomena, such as unintended mechanical vibrations, can induce undesired electrical currents in these devices. Here, we take a quantum mechanical approach to analyze how currents induced by fluctuations (either thermal or of some other kind) in suspended GNRs contribute to the electric current. In particular, we study the pumping current induced by the adiabatic variation of the Hamiltonian of the system when a transverse vibration (flexural mode) of a GNR suspended over a gate is excited. Our theoretical approach and results provide useful tools and rules of thumb to understand and control the charge current induced by fluctuations in GNR-based NEMS, which is important for their applications in nanoscale sensors, pumps, and energy harvesting devices.


Revealing the Charge Density Wave Proximity Effect in Graphene on 1T-TaS$_2$. (arXiv:2311.10606v1 [cond-mat.mes-hall])
Nikhil Tilak, Michael Altvater, Sheng-Hsiung Hung, Choong-Jae Won, Guohong Li, Taha Kaleem, Sang-Wook Cheong, Chung-Hou Chung, Horng-Tay Jeng, Eva Y. Andrei

The proximity-effect, a phenomenon whereby materials in close contact appropriate each others electronic-properties, is widely used in nano-scale devices to induce electron-correlations at heterostructure interfaces. Layered group-V transition metal dichalcogenides host charge density waves and are expected to induce CDWs in a thin proximal 2D metal such as graphene. Thus far, however, the extremely large density of states of the TMDs compared to graphene have precluded efforts to unambiguously prove such proximity induced charge density waves (CDW). Here, using scanning tunneling microscopy (STM) and spectroscopy (STS), we report the first conclusive evidence of a CDW proximity effect between graphene and the commensurate CDW in 1T-TaS$_2$ (TaS$_2$ for brevity). We exploit the Mott gap of 1T-TaS$_2$ to scan the sample at bias voltages wherein only the graphene layer contributes to the STM topography scans. Furthermore, we observe that graphene modifies the band structure at the surface of TaS$_2$, by providing mid-gap carriers and reducing the strength of electron correlations there. We show that the mechanism underlying the proximity induced CDW is well-described by short-range exchange interactions that are distinctly different from previously observed proximity effects.


Discrete step walks reveal unconventional anomalous topology in synthetic photonic lattices. (arXiv:2311.10619v1 [cond-mat.mes-hall])
Rabih El Sokhen, Álvaro Gómez-León, Albert F. Adiyatullin, Stéphane Randoux, Pierre Delplace, Alberto Amo

Anomalous topological phases, where edge states coexist with topologically trivial Chern bands, can only appear in periodically driven lattices. When the driving is smooth and continuous, the bulk-edge correspondence is guaranteed by the existence of a bulk invariant known as the winding number. However, in lattices subject to periodic time-step walks the existence of edge states does not only depend on bulk invariants but also on the geometry of the boundary. This is a consequence of the absence of an intrinsic time-dependence or micromotion in discrete-step walks. We report the observation of edge states and a simultaneous measurement of the bulk invariants in anomalous topological phases in a two-dimensional discrete-step walk in a synthetic photonic lattice made of two coupled fibre rings. The presence of edge states is inherent to the periodic driving and depends on the geometry of the boundary in the implemented two-band model with zero Chern number. We provide a suitable expression for the topological invariants whose calculation does not rely on micromotion dynamics.


Magnon topological transition in skyrmion crystal. (arXiv:2311.10622v1 [cond-mat.mes-hall])
V. E. Timofeev, Yu. V. Baramygina, D. N. Aristov

We study the magnon spectrum in skyrmion crystal formed in thin ferromagnetic films with Dzyalosinskii-Moria interaction in presence of magnetic field. Focusing on two low-lying observable magnon modes and employing stereographic projection method, we develop a theory demonstrating a topological transition in the spectrum. Upon the increase of magnetic field, the gap between two magnon bands closes, with the ensuing change in the topological character of both bands. This phenomenon of gap closing, if confirmed in magnetic resonance experiments, may deserve further investigation by thermal Hall conductivity experiments.


Critical ultrasonic propagation in magnetic fields. (arXiv:2311.10654v1 [cond-mat.stat-mech])
A. Pawlak

Effect of an external magnetic field on the critical sound attenuation and velocity of the longitudinal wave is studied in ferromagnets. We derive a parametric model that incorporates a crossover from the asymptotic critical behavior to the Landau-Ginzburg regular behavior far away from the critical point. The dynamics is based on the time dependent Ginzburg-Landau model with non conserved order parameter (model A). The variations of the sound attenuation coefficient and velocity have been obtained for arbitrary values of the magnetic field and reduced temperature. The scaling functions are given within the renormalization group formalism at one-loop order. Using MnP as an example, we show that such parametric crossover model yields an accurate description of ultrasonic data in a large region of temperatures and magnetic fields around the critical point.


Phononic dynamical axion in magnetic Dirac insulators. (arXiv:2311.10674v1 [cond-mat.mes-hall])
M. Nabil Y. Lhachemi, Ion Garate

In cosmology, the axion is a hypothetical particle that is currently considered as candidate for dark matter. In condensed matter, a counterpart of the axion (the "axion quasiparticle") has been predicted to emerge in magnetoelectric insulators with fluctuating magnetic order and in charge-ordered Weyl semimetals. To date, both the cosmological and condensed-matter axions remain experimentally elusive or unconfirmed. Here, we show theoretically that ordinary lattice vibrations can form an axion quasiparticle in Dirac insulators with broken time- and space-inversion symmetries, even in the absence of magnetic fluctuations. The physical manifestation of the phononic axion is a magnetic-field-induced phonon effective charge, which can be probed in optical spectroscopy. By replacing magnetic fluctuations with lattice vibrations, our theory widens the scope for the observability of the axion quasiparticle in condensed matter.


TODD-Graphene: A Novel Porous 2D Carbon Allotrope for High-Performance Lithium-Ion Batteries. (arXiv:2311.10704v1 [cond-mat.mtrl-sci])
E. J. A. Santos, K. A. L. Lima, L. A. Ribeiro Junior

The class of 2D carbon allotropes has garnered significant attention due to its exceptional optoelectronic and mechanical properties, crucial for diverse device applications, such as energy storage. This study employs density functional theory calculations, ab initio molecular dynamics (AIMD), and classical reactive (ReaxFF) molecular dynamics (MD) simulations to introduce TODD-Graphene, a novel 2D planar carbon allotrope with a porous structure composed of 3-8-10-12 carbon rings. TODD-G exhibits intrinsic metallic properties with low formation energy and demonstrates exceptional dynamic, thermal, and mechanical stability. Calculations reveal a high theoretical capacity for adsorbing Li atoms by showing a low average diffusion barrier of 0.83 eV and a metallic framework boasting excellent conductivity, emerging as a promising anode material for lithium-ion batteries. We also calculated the charge carrier mobility for electrons and holes in TOOD-G, and the values surpassed the graphene ones. Classical reactive MD simulation results suggested its structural integrity with no bond reconstructions at 1800 K.


A topological mechanism for robust and efficient global oscillations in biological networks. (arXiv:2302.11503v3 [physics.bio-ph] UPDATED)
Chongbin Zheng, Evelyn Tang

Long and stable timescales are often observed in complex biochemical networks, such as in emergent oscillations. How these robust dynamics persist remains unclear, given the many stochastic reactions and shorter time scales demonstrated by underlying components. We propose a topological model with parsimonious parameters that produces long oscillations around the network boundary, effectively reducing the system dynamics to a lower-dimensional current. Using this to model KaiC, which regulates the circadian rhythm in cyanobacteria, we compare the coherence of oscillations to that in other KaiC models. Our topological model localizes currents on the system edge for an efficient regime with simultaneously increased precision and decreased cost. Further, we introduce a new predictor of coherence from the analysis of spectral gaps, and show that our model saturates a global thermodynamic bound. Our work presents a new mechanism for emergent oscillations in complex biological networks utilizing dissipative cycles to achieve robustness and efficient performance.


Transport and localization properties of excitations in one-dimensional lattices with diagonal disordered mosaic modulations. (arXiv:2303.13736v2 [cond-mat.dis-nn] UPDATED)
Ba Phi Nguyen, Kihong Kim

We present a numerical study of the transport and localization properties of excitations in one-dimensional lattices with diagonal disordered mosaic modulations. The model is characterized by the modulation period $\kappa$ and the disorder strength $W$. We calculate the disorder averages $\langle T\rangle$, $\langle \ln T\rangle$, and $\langle P\rangle$, where $T$ is the transmittance and $P$ is the participation ratio, as a function of energy $E$ and system size $L$, for different values of $\kappa$ and $W$. For excitations at quasiresonance energies determined by $\kappa$, we find power-law scaling behaviors of the form $\langle T \rangle \propto L^{-\gamma_{a}}$, $\langle \ln T \rangle \approx -\gamma_g \ln L$, and $\langle P \rangle \propto L^{\beta}$, as $L$ increases to a large value. This behavior is in contrast to the exponential localization behavior occurring at all other energies. The appearance of sharp peaks in the participation ratio spectrum at quasiresonance energies provides additional evidence for the existence of an anomalous power-law localization phenomenon. The corresponding eigenstates demonstrate multifractal behavior and exhibit unique node structures. In addition, we investigate the time-dependent wave packet dynamics and calculate the mean square displacement $\langle m^2(t) \rangle$, spatial probability distribution, participation number, and return probability. When the wave packet's initial momentum satisfies the quasiresonance condition, we observe a subdiffusive spreading of the wave packet, characterized by $\langle m^2(t) \rangle\propto t^{\eta}$ where $\eta$ is always less than 1. We also note the occurrence of partial localization at quasiresonance energies, as indicated by the saturation of the participation number and a nonzero value for the return probability at long times.


3D Ising CFT and Exact Diagonalization on Icosahedron: The Power of Conformal Perturbation Theory. (arXiv:2307.02540v3 [hep-th] UPDATED)
Bing-Xin Lao, Slava Rychkov

We consider the transverse field Ising model in $(2+1)$D, putting 12 spins at the vertices of the regular icosahedron. The model is tiny by the exact diagonalization standards, and breaks rotation invariance. Yet we show that it allows a meaningful comparison to the 3D Ising CFT on $\mathbb{R}\times S^2$, by including effective perturbations of the CFT Hamiltonian with a handful of local operators. This extreme example shows the power of conformal perturbation theory in understanding finite $N$ effects in models on regularized $S^2$. Its ideal arena of application should be the recently proposed models of fuzzy sphere regularization.


Unifying temperature definition in atomistic and field representations of conservation laws. (arXiv:2308.10127v2 [cond-mat.stat-mech] UPDATED)
Youping Chen

This work presents a formalism to derive field quantities and conservation laws from the atomistic using the theory of distributions as the mathematical tool. By defining temperature as a derived quantity as that in molecular kinetic theory and atomistic simulations, a field representation of the conservation law of linear momentum is derived and expressed in terms of temperature field, leading to a unified atomistic and continuum description of temperature and a new conservation equation of linear momentum that, supplemented by an interatomic potential, completely governs thermal and mechanical processes across scales from the atomic to the continuum. The conservation equation can be used to solve atomistic trajectories for systems at finite temperatures, as well as the evolution of field quantities in space and time, with atomic or multiscale resolution. Four sets of numerical examples are presented to demonstrate the efficacy of the formulation in capturing the effect of temperature or thermal fluctuations, including phonon density of states, thermally activated dislocation motion, dislocation formation during epitaxial processes, and attenuation of longitudinal acoustic waves as a result of their interaction with thermal phonons.


Graph topological transformations in space-filling cell aggregates. (arXiv:2309.04818v2 [cond-mat.soft] UPDATED)
Tanmoy Sarkar, Matej Krajnc

Cell rearrangements are fundamental mechanisms driving large-scale deformations of living tissues. In three-dimensional (3D) space-filling cell aggregates, cells rearrange through local topological transitions of the network of cell-cell interfaces, which is most conveniently described by the vertex model. Since these transitions are not yet mathematically properly formulated, the 3D vertex model is generally difficult to implement. The few existing implementations rely on highly customized and complex software-engineering solutions, which cannot be transparently delineated and are thus mostly non-reproducible. To solve this outstanding problem, we propose a reformulation of the vertex model. Our approach, called Graph Vertex Model (GVM), is based on storing the topology of the cell network into a knowledge graph with a particular data structure that allows performing cell-rearrangement events by simple graph transformations. We find these transformations consinsting of transformation patterns corresponding to T1 transitions, thereby unifying topological transitions in 2D and 3D space-filling packings. This result suggests that the GVM's graph data structure may be the most natural representation of cell aggregates and tissues. We use GVM to characterize solid-fluid transition in 3D cell aggregates, driven by active noise and find aggregates undergoing efficient ordering close to the transition point. In all, our work showcases knowledge graphs as particularly suitable data models for structured storage, analysis, and manipulation of tissue data, which potentially has paradigm-shifting implications for the fields of tissue biophysics and biology.


Krylov Complexity of Fermionic and Bosonic Gaussian States. (arXiv:2309.10382v2 [quant-ph] UPDATED)
Kiran Adhikari, Adwait Rijal, Ashok Kumar Aryal, Mausam Ghimire, Rajeev Singh, Christian Deppe

The concept of \emph{complexity} has become pivotal in multiple disciplines, including quantum information, where it serves as an alternative metric for gauging the chaotic evolution of a quantum state. This paper focuses on \emph{Krylov complexity}, a specialized form of quantum complexity that offers an unambiguous and intrinsically meaningful assessment of the spread of a quantum state over all possible orthogonal bases. Our study is situated in the context of Gaussian quantum states, which are fundamental to both Bosonic and Fermionic systems and can be fully described by a covariance matrix. We show that while the covariance matrix is essential, it is insufficient alone for calculating Krylov complexity due to its lack of relative phase information. Our findings suggest that the relative covariance matrix can provide an upper bound for Krylov complexity for Gaussian quantum states. We also explore the implications of Krylov complexity for theories proposing complexity as a candidate for holographic duality by computing Krylov complexity for the thermofield double States (TFD) and Dirac field.


Electronic Properties and Interlayer Interactions in Antimony Oxide Homo- and Heterobilayers. (arXiv:2309.10653v2 [cond-mat.mtrl-sci] UPDATED)
Stefan Wolff, Roland Gillen, Janina Maultzsch

Antimony shows promise as a two-dimensional (2D) mono-elemental crystal, referred to as antimonene. When exposed to ambient conditions, antimonene layers react with oxygen, forming new crystal structures, leading significant changes in electronic properties. These changes are influenced by the degree of oxidation. Utilizing Density Functional Theory (DFT) calculations, stable configurations of bilayer antimony oxide and their corresponding electronic properties are studied. Additionally, different stacking arrangements and their effects on the physical properties of the materials are investigated. Furthermore, the analysis encompasses strain-free hetero-bilayers containing both pristine and oxidized antimonene layers, aiming to understand the interplay between these materials and their collective impact on the bilayer properties. Our results provide insight into how the properties of antimony-based bilayer structures can be modified by adjusting stoichiometry and stacking configurations.


Full Breit Hamiltonian in the Multiwavelets Framework. (arXiv:2309.16183v2 [physics.chem-ph] UPDATED)
Christian Tantardini, Roberto Di Remigio Eikås, Magnar Bjørgve, Stig Rune Jensen, Luca Frediani

New techniques in core-electron spectroscopy are necessary to resolve the structures of oxides of $f$-elements and other strongly correlated materials that are present only as powders and not as single crystals. Thus, accurate quantum chemical methods need to be developed to calculate core spectroscopic properties in such materials. In this contribution, we present an important development in this direction, extending our fully adaptive real-space multiwavelet basis framework to tackle the 4-component Dirac-Coulomb-Breit Hamiltonian. We show that Multiwavelets are able to reproduce one-dimensional grid-based approaches. They are however a fully three-dimensional approach which can later on be extended to molecules and materials. Our Multiwavelet implementation attained precise results irrespective of the chosen nuclear model, provided that the error threshold is tight enough and the chosen polynomial basis is sufficiently large. Furthermore, our results confirmed that in two-electron species, the magnetic and Gauge contributions from $s$-orbitals are identical in magnitude and can account for the experimental evidence from $K$ and $L$ edges.


Emergence of flat bands in the quasicrystal limit of boron nitride twisted bilayers. (arXiv:2310.02937v2 [cond-mat.mtrl-sci] UPDATED)
Lorenzo Sponza, Van Binh Vu, Elisa Serrano Richaud, Hakim Amara, Sylvain Latil

We investigate the electronic structure and the optical absorption onset of hexagonal boron nitride bilayers with twist angles in the vicinity of 30$^\circ$. Our study is carried out with a tight-binding model that we developed on purpose and validated against DFT simulations. We demonstrate that approaching 30$^\circ$ (quasicrystal limit), all bilayers sharing the same moir\'e supercell develop identical band structures, irrespective of their stacking sequence. This band structure features a bundle of flat bands laying slightly above the bottom conduction state which is responsible for an intense peak at the onset of independent-particle absorption spectra. These results reveal the presence of strong, stable and stacking-independent optical properties in boron nitride 30$^\circ$-twisted bilayers. By carefully analyzing the electronic spatial distribution, we elucidate the origin of these states as due to interlayer B-B coupling. We take advantage of the the physical transparency of the tight-binding parameters to derive a simple triangular model based on the B sublattice that accurately describes the emergence of the bundle. Being our conclusions very general, we predict that a similar bundle should emerge in other close-to-30$^\circ$ bilayers, like transition metal dichalcogenides, shedding new light on the unique potential of 2D materials.


Varying magnetism in the lattice distorted Y2NiIrO6 and La2NiIrO6. (arXiv:2310.18641v2 [cond-mat.mtrl-sci] UPDATED)
Lu Liu, Ke Yang, Di Lu, Yaozhenghang Ma, Yuxuan Zhou, Hua Wu

We investigate the electronic and magnetic properties of the newly synthesized double perovskites Y$_{2}$NiIrO$_{6}$ and La$_{2}$NiIrO$_{6}$, using density functional calculations, crystal field theory, superexchange pictures, and Monte Carlo simulations. We find that both systems are antiferromagnetic (AFM) Mott insulators, with the high-spin Ni$^{2+}$ $t_{2g}$$^{6}e_{g}$$^{2}$ ($S=1$) and the low-spin Ir$^{4+}$ $t_{2g}$$^{5}$ ($S=1/2$) configurations. We address that their lattice distortion induces $t_{2g}$-$e_{g}$ orbital mixing and thus enables the normal Ni$^{+}$-Ir$^{5+}$ charge excitation with the electron hopping from the Ir `$t_{2g}$' to Ni `$e_g$' orbitals, which promotes the AFM Ni$^{2+}$-Ir$^{4+}$ coupling. Therefore, the increasing $t_{2g}$-$e_{g}$ mixing accounts for the enhanced $T_{\rm N}$ from the less distorted La$_{2}$NiIrO$_{6}$ to the more distorted Y$_{2}$NiIrO$_{6}$. Moreover, our test calculations find that in the otherwise ideally cubic Y$_{2}$NiIrO$_{6}$, the Ni$^{+}$-Ir$^{5+}$ charge excitation is forbidden, and only the abnormal Ni$^{3+}$-Ir$^{3+}$ excitation gives a weakly ferromagnetic (FM) behavior. Furthermore, we find that owing to the crystal field splitting, Hund exchange, and broad band formation in the highly coordinated fcc sublattice, Ir$^{4+}$ ions are not in the $j_{\rm eff}=1/2$ state but in the $S=1/2$ state carrying a finite orbital moment by spin-orbit coupling (SOC). This work clarifies the varying magnetism in Y$_{2}$NiIrO$_{6}$ and La$_{2}$NiIrO$_{6}$ associated with the lattice distortions.