Showing 1–50 of 877 results for author: De, C

Perturbative Input-Output Theory of Floquet Cavity Magnonics and Magnon Energy Shifts

Authors: T. Aguiar, M. C. de Oliveira

Abstract: We develop a perturbative input-output formalism to compute the reflectance and transmittance spectra of cavity magnonics systems subject to a Floquet modulation. The method exploits the strong hierarchy between the magnetic-dipole couplings transverse (drive field) and parallel (modulation field) to the static bias field, which naturally introduces the small parameter $ε= (2Ns)^{-1/2}$ associated… ▽ More We develop a perturbative input-output formalism to compute the reflectance and transmittance spectra of cavity magnonics systems subject to a Floquet modulation. The method exploits the strong hierarchy between the magnetic-dipole couplings transverse (drive field) and parallel (modulation field) to the static bias field, which naturally introduces the small parameter $ε= (2Ns)^{-1/2}$ associated with the total spin $Ns$ of the ferromagnet. By organizing the cavity and magnon fields in a systematic expansion in $ε$, we obtain compact analytic expressions for the spectra up to second order. Using these results, we reproduce the characteristic sideband structure observed in recent Floquet cavity electromagnonics experiments. Furthermore, accounting for the Zeeman interaction between the modulation field and the fully polarized ground state - a contribution typically neglected in previous treatments - we predict an additional magnon detuning of approximately $0.8\,\mathrm{GHz}$, independent of both modulation frequency and sample size and determined solely by the spatial volume occupied by the modulation field. This identifies a measurable and previously overlooked shift relevant for the interpretation and design of cavity magnonics experiments. △ Less

Submitted 12 December, 2025; originally announced December 2025.

Electrical Stability of Cr2O3/\b{eta}-Ga2O3 and NiOx/\b{eta}-Ga2O3 Heterojunction Diodes

Authors: Yizheng Liu, Haochen Wang, Carl Peterson, Chinmoy Nath Saha, Chris G. Van de Walle, Sriram Krishnamoorthy

Abstract: This work reports the electrical characteristics comparison study between Cr2O3 and NiOx based heterojunction diodes (HJD) on halide vapor phase epitaxy (HVPE) grown \b{eta}-Ga2O3 epitaxial layers. Both as-fabricated Cr2O3 and NiOx HJDs exhibited forward current density in a range of 130-150 A/cm^2 at 5 V with rectifying ratios >10^10 and a reverse leakage current density at 10^-8 A/cm^2 at -5 V.… ▽ More This work reports the electrical characteristics comparison study between Cr2O3 and NiOx based heterojunction diodes (HJD) on halide vapor phase epitaxy (HVPE) grown \b{eta}-Ga2O3 epitaxial layers. Both as-fabricated Cr2O3 and NiOx HJDs exhibited forward current density in a range of 130-150 A/cm^2 at 5 V with rectifying ratios >10^10 and a reverse leakage current density at 10^-8 A/cm^2 at -5 V. The differential specific on-resistance of Cr2O3 and NiOx HJDs was 12.01 mΩ*cm^2 and 12.05 mΩ*cm^2, respectively. Breakdown voltages of Cr2O3 HJDs ranged from 1.4-1.9 kV and 1.5-2.3 kV for NiOx HJDs. Theoretical band alignment between Cr2O3 and \b{eta}-Ga2O3 was calculated from first principles. The ambient exposed NiOx/HVPE \b{eta}-Ga2O3 HJDs forward current density degraded after 10 days while that of Cr2O3/HVPE \b{eta}-Ga2O3 HJDs remained nearly unchanged after the same amount of time. It was later confirmed that the ambient exposed sputtered NiOx sheet resistance (Rsh) degradation gave rise to the reduction of the forward current density of the NiOx based HJDs, and water (H2O) was qualitatively determined to be the agent attributed to the forward conduction degradation by measuring the Rsh of NiOx-on-sapphire reference wafer after exposing it to different environments. The Cr2O3/HVPE \b{eta}-Ga2O3 HJD also exhibited enhanced thermal stability compared to the NiOx/\b{eta}-Ga2O3 heterostructures at elevated temperatures. Interfacial nickel gallate (Ga2NiO4) phase formation expected from phase diagrams can explain the reduced thermal stability of NiOx/\b{eta}-Ga2O3 HJDs. This study indicates that Cr2O3 is a stable p-type oxide for the realization of robust multi-kV \b{eta}-Ga2O3 HJDs. △ Less

Submitted 11 December, 2025; originally announced December 2025.

Anisotropic Response in Metamaterials with Elliptically Perforated Plates: Applications to Near-Field Radiative Heat Transfer

Authors: J. E. P'erez-Rodr'iguez, R. Esquivel-Sirvent, A. Camacho de la Rosa

Abstract: Metamaterials with tunable optical properties provide a versatile platform for controlling electromagnetic interactions at the nanoscale. This study explores the anisotropic thermal behavior of metamaterials composed of planar plates perforated with periodic arrays of cylinders possessing elliptical cross sections. In contrast to conventional circular perforations, elliptical geometries inherently… ▽ More Metamaterials with tunable optical properties provide a versatile platform for controlling electromagnetic interactions at the nanoscale. This study explores the anisotropic thermal behavior of metamaterials composed of planar plates perforated with periodic arrays of cylinders possessing elliptical cross sections. In contrast to conventional circular perforations, elliptical geometries inherently break rotational symmetry, introducing anisotropy in the effective electromagnetic and thermal response of the structure. Using a fluctuation electrodynamics framework combined with full-wave numerical simulations, we quantify the near-field radiative heat transfer between such elliptically perforated plates as a function of ellipse orientation, aspect ratio, and separation distance. The results reveal that elliptical perforations enable enhanced spectral and directional control of evanescent mode coupling and surface polariton excitation, leading to significant modulation of the near-field heat flux. These findings highlight the potential of geometrically engineered anisotropy for advanced thermal management and energy conversion applications, and offer new design strategies for the development of thermally functional metamaterials operating in the near-field regime. △ Less

Submitted 4 December, 2025; originally announced December 2025.

Comments: Supplementary Information Available

Charge state equilibration of nitrogen-vacancy center ensembles in diamond: The role of electron tunneling

Authors: Audrius Alkauskas, Chris G. Van de Walle, Lukas Razinkovas, Ronald Ulbricht

Abstract: The charge state stability of nitrogen-vacancy (NV) centers critically affects their application as quantum sensors and qubits. Understanding charge state conversion and equilibration is critical not only for NV centers in diamond but also for defects and impurities in wide-bandgap materials in general. The mechanisms by which these centers change charge state upon optical or electronic excitation… ▽ More The charge state stability of nitrogen-vacancy (NV) centers critically affects their application as quantum sensors and qubits. Understanding charge state conversion and equilibration is critical not only for NV centers in diamond but also for defects and impurities in wide-bandgap materials in general. The mechanisms by which these centers change charge state upon optical or electronic excitation without the presence of mobile carriers remain unclear, potentially affecting the performance of applications ranging from phosphors to power electronics. Here, we elucidate this issue for the case of photoionization of NV center ensembles. Using pump-probe spectroscopy, we ionize negatively charged NV centers and monitor the recovery of $\NVm$ on timescales of up to several seconds. We find that the recovery rate depends strongly on the concentration of surrounding nitrogen donors. Remarkably, the equilibration dynamics exhibit no discernible dependence on temperature, ruling out thermally activated processes. The multiphonon-assisted electron tunneling model, supported by density-functional calculations, explains the measurements and identifies tunneling as the equilibration mechanism. △ Less

Submitted 30 November, 2025; originally announced December 2025.

Resonant states and nuclear dynamics in solid-state systems: the case of silicon-hydrogen bond dissociation

Authors: Woncheol Lee, Mark E. Turiansky, Dominic Waldhör, Byounghak Lee, Tibor Grasser, Chris G. Van de Walle

Abstract: Bond breaking in the presence of highly energetic carriers is central to many important phenomena in physics and chemistry, including radiation damage, hot-carrier degradation, activation of dopant-hydrogen complexes in semiconductors, and photocatalysis. Describing these processes from first principles has remained an elusive goal. Here we introduce a comprehensive theoretical framework for the d… ▽ More Bond breaking in the presence of highly energetic carriers is central to many important phenomena in physics and chemistry, including radiation damage, hot-carrier degradation, activation of dopant-hydrogen complexes in semiconductors, and photocatalysis. Describing these processes from first principles has remained an elusive goal. Here we introduce a comprehensive theoretical framework for the dissociation process, emphasizing the need for a non-adiabatic approach. We benchmark the results for the case of silicon-hydrogen bond dissocation, a primary process for hot-carrier degradation. Passivation of Si dangling bonds by hydrogen is vital in all Si devices because it eliminates electrically active mid-gap states; understanding the mechanism for dissociation of these bonds is therefore crucial for device technology. While the need for a non-adiabatic approach has been previously recognized, explicitly obtaining diabatic states for solid-state systems has been an outstanding challenge. We demonstrate how to obtain these states by applying a partitioning scheme to the Hamiltonian obtained from first-principles density functional theory. Our results demonstrate that bond dissociation can occur when electrons temporarily occupy the antibonding states, generating a highly repulsive excited-state potential that causes the hydrogen nuclear wavepacket to shift and propagate rapidly. Based on the Menzel-Gomer-Redhead (MGR) model, we show that after moving on this excited-state potential on femtosecond timescales, a portion of the nuclear wavepacket can continue to propagate even after the system relaxes back to the ground state, allowing us to determine the dissociation probability. Our results provide essential insights into the fundamental processes that drive carrier-induced bond breaking in general, and specifically elucidate hydrogen-related degradation in Si devices. △ Less

Submitted 29 November, 2025; originally announced December 2025.

Comments: 26 pages, 15 figures

Cr2O3/\b{eta}-Ga2O3 Heterojunction Diodes with Orientation-Dependent Breakdown Electric Field up to 12.9 MV/cm

Authors: Yizheng Liu, Haochen Wang, Carl Peterson, James S. Speck, Chris Van De Walle, Sriram Krishnamoorthy

Abstract: We report the fabrication of Cr2O3/\b{eta}-Ga2O3 heterojunction diodes using reactive magnetron sputtering of Cr2O3 on highly doped \b{eta}-Ga2O3 bulk substrates along (100), (010), (001), (110), and (011) orientation dependence of high electric field handling capability in \b{eta}-Ga2O3. Additional relative permittivity values in (110) and (011) orientations of \b{eta}-Ga2O3 were computed by usin… ▽ More We report the fabrication of Cr2O3/\b{eta}-Ga2O3 heterojunction diodes using reactive magnetron sputtering of Cr2O3 on highly doped \b{eta}-Ga2O3 bulk substrates along (100), (010), (001), (110), and (011) orientation dependence of high electric field handling capability in \b{eta}-Ga2O3. Additional relative permittivity values in (110) and (011) orientations of \b{eta}-Ga2O3 were computed by using first-principles calculation methods for accurate apparent charge density (ND-NA) extraction and breakdown electric field analysis from capacitance-voltage measurements. The HJDs fabricated on n+ (110) exhibited breakdown electric fields >10 MV/cm up to 12.9 MV/cm, showing the highest experimentally observed parallel-plane junction electric field among \b{eta}-Ga2O3-based junctions. Breakdown electric fields among (100), (010), (001), and (011) orientations showed distinct distribution in the range of 5.13-5.26 MV/cm, 5.10-7.05 MV/cm, 2.70-3.33 MV/cm, and 3.88-4.38 MV/cm, respectively, validating the orientational dependence of parallel-plane junction electric field at breakdown in low-symmetry monoclinic \b{eta}-Ga2O3. The parallel-plane breakdown electric fields (EBr,||) reported in this work were extracted when the device experienced catastrophic breakdown at 100 mA/cm^2 current density compliance, and should not be confused with critical electric field (Ec) as a function of drift layer doping concentration, which accounts for electric-field dependent impact ionization coefficients in Si, SiC and GaN. This study can guide the choice of crystal orientation for high performance gallium oxide-based devices that require high electric field handling capability. △ Less

Submitted 25 November, 2025; originally announced November 2025.

Dissimilarity measures for generalized Lotka-Volterra systems on networks

Authors: Nicolás A. Márquez, Maryam Chaib De Mares, Alejandro P. Riascos

Abstract: In this paper, we introduce a general framework to quantify dissimilarities between generalized Lotka-Volterra dynamical processes, ranging from classical predator-prey systems to multispecies communities interacting on networks. The proposed measures capture both transient and stationary dynamics, allowing systematic comparisons across systems with varying interaction parameters, network weights,… ▽ More In this paper, we introduce a general framework to quantify dissimilarities between generalized Lotka-Volterra dynamical processes, ranging from classical predator-prey systems to multispecies communities interacting on networks. The proposed measures capture both transient and stationary dynamics, allowing systematic comparisons across systems with varying interaction parameters, network weights, or topologies. Our analysis shows that even subtle structural changes can lead to markedly distinct outcomes: in two-species systems, interaction strength and initial conditions strongly affect divergence, while in small directed networks, differences that are invisible at the adjacency-matrix level produce divergent dynamics. In modular networks, the fraction and distribution of negative interactions control the transition from stable to unstable dynamics, with localized perturbations within cliques yielding different global outcomes than distributed ones. Beyond structural variations, the framework also applies when modified processes follow distinct nonlinear equations, demonstrating its versatility. Taken together, these results highlight that dynamical dissimilarity measures provide a powerful tool to analyze robustness, detect structural sensitivity, and predict instabilities in nonlinear systems. More broadly, this approach supports the comparative analysis of biological systems, where complex interaction networks and nonlinear dynamics are central to stability and resilience. △ Less

Submitted 16 November, 2025; originally announced November 2025.

Comments: 15 pages, 7 figures

Optimizing magnetic coupling in lumped element superconducting resonators for molecular spin qubits

Authors: Marcos Rubín-Osanz, Marina C. de Ory, Ignacio Gimeno, Wenzel Kersten, Marta Mas-Torrent, María C. Pallarés, Sebastián Roca-Jerat, David Rodriguez, Nerea González-Prato, J. Alejandro de Sousa, Lorenzo Tesi, Daniel Granados, Jaume Veciana, David Zueco, Anabel Lostao, Joerg Schmiedmayer, Inma Ratera, Joris van Slageren, Núria Crivillers, Alicia Gomez, Fernando Luis

Abstract: We engineer lumped-element superconducting resonators that maximize magnetic coupling to molecular spin qubits, achieving record single-spin couplings up to 100 kHz and collective couplings exceeding 10 MHz. The resonators were made interact with PTMr organic free radicals, model spin systems with $S=1/2$ and a quasi-isotropic $g \simeq 2$, dispersed in polymer matrices. The highest collective spi… ▽ More We engineer lumped-element superconducting resonators that maximize magnetic coupling to molecular spin qubits, achieving record single-spin couplings up to 100 kHz and collective couplings exceeding 10 MHz. The resonators were made interact with PTMr organic free radicals, model spin systems with $S=1/2$ and a quasi-isotropic $g \simeq 2$, dispersed in polymer matrices. The highest collective spin-photon coupling strengths are attained with resonators having large inductors, which therefore interact with most spins in the molecular ensemble. By contrast, the coupling of each individual spin $G_{1}$ is maximized in resonators having a minimum size inductor, made of a single microwire. The same platform has been used to study spin relaxation and spin coherent dynamics in the dispersive regime, when spins are energetically detuned from the resonator. We find evidences for the Purcell effect, i.e. the photon induced relaxation of those spins that are most strongly coupled to the circuit. The rate of this process has been used to infer the distribution of single spin photon couplings in a given device. For resonators with a 50 nm wide constriction fabricated at the center of its single maximum $G_{1}$ values reach $\sim 100$ kHz. Pumping the spins with strong pulses fed through an independent transmission line induces coherent Rabi oscillations. The spin excitation then proceeds via either direct resonant processes induced by the main pulse frequency or, in the case of square-shaped pulses, via the excitation of the cavity by side frequency components. The latter process measures the cavity mode hybridization with the spins and can be eliminated by using Gaussian shaped pulses. These results establish a scalable route toward integrated molecular-spin quantum processors. △ Less

Submitted 2 November, 2025; originally announced November 2025.

A CN complex as an alternative to the T center in Si

Authors: J. K. Nangoi, M. E. Turiansky, C. G. Van de Walle

Abstract: We present a first-principles study of a carbon-nitrogen (CN) impurity complex in silicon as an isoelectronic alternative to the T center [(CCH)$_\mathrm{Si}$]. The latter has been pursued for applications in quantum information science, yet its sensitivity to the presence of hydrogen is still problematic. Our proposed complex has no hydrogen, thereby eliminating this issue. First, we show that th… ▽ More We present a first-principles study of a carbon-nitrogen (CN) impurity complex in silicon as an isoelectronic alternative to the T center [(CCH)$_\mathrm{Si}$]. The latter has been pursued for applications in quantum information science, yet its sensitivity to the presence of hydrogen is still problematic. Our proposed complex has no hydrogen, thereby eliminating this issue. First, we show that the CN complex is stable against decomposition into substitutional and interstitial defects. Next, we show that due to being isoelectronic to the T center, the CN complex has a similar electronic structure, and therefore could be used in similar applications. We assess several low-energy configurations of the CN complex, finding (CN)$_\mathrm{Si}$ to be stable and have the largest Debye-Waller factor. We predict a zero-phonon line (ZPL) of 828 meV (in the telecom S-band) and a radiative lifetime of 4.2 $μ$s, comparable to the T center. Due to the presence of a bound exciton, choice of the exchange-correlation functional and also supercell-size scaling of the ZPL and transition dipole moment require special scrutiny; we rigorously justify our extrapolation schemes that allow computing values in the dilute limit. △ Less

Submitted 1 November, 2025; originally announced November 2025.

Comments: Submitted to Physical Review B

Ambient-Induced Selenium Segregation and Nanoparticle Formation in 2H-HfSe2: An Experimental and Theoretical Study

Authors: Stefany P. Carvalho, Guilherme S. L. Fabris, Ana Carolina F. de Brito, Raphael B. de Oliveira, Wesley Kardex C. de Oliveira, Catalina Ruano-Merchan, Carlos A. R. Costa, Luiz F. Zagonel, Douglas Galvao, Ingrid D. Barcelos

Abstract: We investigate the air-induced degradation of few-layer hafnium diselenide (HfSe$_2$) through combined experimental and theoretical approaches. AFM and SEM reveal the formation of selenium-rich spherical features upon ambient exposure, while EDS confirms Se segregation. \textit{Ab initio} molecular dynamics simulations show that Se atoms migrate to flake edges and that O/O$_2$ exposure leads to se… ▽ More We investigate the air-induced degradation of few-layer hafnium diselenide (HfSe$_2$) through combined experimental and theoretical approaches. AFM and SEM reveal the formation of selenium-rich spherical features upon ambient exposure, while EDS confirms Se segregation. \textit{Ab initio} molecular dynamics simulations show that Se atoms migrate to flake edges and that O/O$_2$ exposure leads to selective Hf oxidation, breaking Se--Hf bonds and expelling Se atoms. No stable Se--O bonds are observed, indicating structural reorganization rather than oxidation. These findings emphasize the material's instability in air and the importance of encapsulation for preserving HfSe$_2$ in practical applications. Scanning tunneling spectroscopy confirms the semiconducting character of the nanoparticles, with an electronic bandgap compatible with that of elemental Se. These results highlight the critical role of lattice defects and oxidation dynamics in the degradation process and underscore the need for encapsulation strategies to preserve the integrity of HfSe$_2$-based devices. △ Less

Submitted 31 October, 2025; originally announced November 2025.

Correlative Analysis of Iron-Driven Structural, Optical, and Magnetic Properties in Natural Biotite Crystals

Authors: Raphaela de Oliveira, Yara Galvão Gobato, Ronei C. de Oliveira, José R. de Toledo, Verônica C. Teixeira, Angelo Malachias, Cesar R. Rabahi, Chunwei Hsu, Adilson J. A. de Oliveira, Herre. S. J. van der Zant, Ingrid D. Barcelos, Alisson R. Cadore

Abstract: Biotite crystals are phyllosilicate trioctahedral micas with the general chemical formula K(Mg,Fe)3AlSi3O10(OH)2 that form a solid-solution series with iron-poor phlogopite and iron-rich annite endmembers. With a wide band gap energy and a layered structure with free surface charges, biotite nanosheets can be readily obtained by cleavage methods and used as dielectrics in nanodevice fabrication fo… ▽ More Biotite crystals are phyllosilicate trioctahedral micas with the general chemical formula K(Mg,Fe)3AlSi3O10(OH)2 that form a solid-solution series with iron-poor phlogopite and iron-rich annite endmembers. With a wide band gap energy and a layered structure with free surface charges, biotite nanosheets can be readily obtained by cleavage methods and used as dielectrics in nanodevice fabrication for the next generation of electronics and energy harvesting. Here, a comprehensive study of biotite samples with different iron concentrations and oxidation states is presented. Structural, optical, magneto-optical, and magnetic characterizations were performed using several experimental techniques, including state-of-the-art synchrotron-based techniques, to correlate the iron chemistry (content and oxidation state) with the macroscopic properties of both minerals. The study reveals a nanoscale-homogeneous Fe distribution via synchrotron X-ray fluorescence mapping, defect-mediated optical transitions modulated by Fe3+/Fe2+ ratios, and temperature-dependent magnetic transitions from paramagnetism to competing ferro-/antiferromagnetic interactions. Furthermore, the use of these biotite crystals as substrates for ultrathin heterostructures incorporating monolayer (ML) MoSe2 is explored by magneto photoluminescence at cryogenic temperatures. The results show that the presence of iron impurities in different oxidation states significantly impacts the valley properties for ML-MoSe2. Overall, these findings offer a comprehensive interpretation of the physical properties of bulk biotites in a correlative approach, serving as a robust reference for future studies aiming to explore biotites in their ultrathin form. △ Less

Submitted 6 October, 2025; originally announced October 2025.

Comments: 17 pages, 5 figs

How to Identify Suitable Gate Dielectrics for Transistors based on Two-Dimensional Semiconductors

Authors: Theresia Knobloch, Quentin Smets, Anton E. O. Persson, Pedram Khakbaz, Christoph Wilhelmer, Dennis Lin, Zherui Han, Yunyan Chung, Kevin P. OBrien, Chelsey Dorow, Cormac OCoileain, Mario Lanza, Dominic Waldhoer, Alexander Karl, Kailang Liu, Tianyou Zhai, Hailin Peng, Congwei Tan, Xiao Renshaw Wang, Georg S. Duesberg, John Robertson, Uygar Avci, Iuliana Radu, Eric Pop, Cesar J. Lockhart de la Rosa , et al. (1 additional authors not shown)

Abstract: The recent progress in nanosheet transistors has established two-dimensional (2D) semiconductors as viable candidates for future ultra-scaled electronic devices. Next to reducing contact resistance, identifying good gate dielectrics is a fundamental challenge, as the dielectric/channel interface dramatically impacts virtually all performance parameters. While several promising gate dielectrics hav… ▽ More The recent progress in nanosheet transistors has established two-dimensional (2D) semiconductors as viable candidates for future ultra-scaled electronic devices. Next to reducing contact resistance, identifying good gate dielectrics is a fundamental challenge, as the dielectric/channel interface dramatically impacts virtually all performance parameters. While several promising gate dielectrics have recently been reported, the evaluation of their quality and suitability is often fragmentary and focused on selected important performance metrics of the gate stack, such as the capacitive gate control, leakage currents, reliability, and ease of fabrication and integration. However, identifying a suitable gate stack is a complex problem that has not yet been approached systematically. In this perspective, we aim to formulate general criteria for good gate dielectrics. △ Less

Submitted 24 September, 2025; originally announced September 2025.

Comments: 21 pages, 4 figures, perspective article

Interplay of energy and charge transfer in WSe2/CrSBr heterostructures

Authors: José Roberto de Toledo, Caique Serati de Brito, Barbara L. T. Rosa, Alisson R. Cadore, César Ricardo Rabahi, Paulo E. Faria Junior, Ana Carolina Ferreira de Brito, Talieh S. Ghiasi, Josep Ingla-Aynés, Christian Schüller, Herre S. J. van der Zant, Stephan Reitzenstein, Ingrid D. Barcelos, Florian Dirnberger, Yara Galvão Gobato

Abstract: Van der Waals heterostructures (vdWHs) composed of transition-metal dichalcogenides (TMDs) and layered magnetic semiconductors offer great opportunities to manipulate exciton and valley properties of TMDs. Here, we present magneto-photoluminescence (PL) studies in a WSe2 monolayer (ML) on a CrSBr crystal, an anisotropic layered antiferromagnetic semiconductor. Our results reveal unique behavior of… ▽ More Van der Waals heterostructures (vdWHs) composed of transition-metal dichalcogenides (TMDs) and layered magnetic semiconductors offer great opportunities to manipulate exciton and valley properties of TMDs. Here, we present magneto-photoluminescence (PL) studies in a WSe2 monolayer (ML) on a CrSBr crystal, an anisotropic layered antiferromagnetic semiconductor. Our results reveal unique behavior of each of the ML-WSe2 PL peaks under magnetic field that is distinct from the pristine case. An intriguing feature is the clear enhancement of the PL intensity that we observe each time the external magnetic field tunes the energy of an exciton in CrSBr into resonance with one of the optical states of WSe2. This result suggests a magnetic field-controlled resonant energy transfer (RET) beyond other effects reported in similar structures. Our work provides deep insight on the importance of different mechanisms into magnetic vdWHs and underscores its great potential for light harvesting and emission enhancement of two-dimensional materials. △ Less

Submitted 31 August, 2025; originally announced September 2025.

Journal ref: Nano Lett. 2025, XXXX, XXX, XXX-XXX

Weighted Hartree-Fock-Bogoliubov method for interacting fermions: An application to ultracold Fermi superfluids

Authors: Nikolai Kaschewski, Axel Pelster, Carlos A. R. Sá de Melo

Abstract: For several decades it has been known that divergences arise in the ground-state energy and chemical potential of unitary superfluids, where the scattering length diverges, due to particle-hole scattering. Leading textbooks and research articles recognize that there are serious issues but ignore them due to the lack of an approach that can regularize these divergences. We find a solution to this d… ▽ More For several decades it has been known that divergences arise in the ground-state energy and chemical potential of unitary superfluids, where the scattering length diverges, due to particle-hole scattering. Leading textbooks and research articles recognize that there are serious issues but ignore them due to the lack of an approach that can regularize these divergences. We find a solution to this difficulty by proposing a general method, called the weighted Hartree-Fock-Bogoliubov theory, to handle multiple decomposition channels originating from the same interaction. We distribute the interaction in weighted channels determined by minimization of the action, and we apply this idea to unpolarized Fermi superfluids. Using our method, we solve a long-standing difficulty in the partitioning of the interaction into Hartree, Fock, and Bogoliubov channels for Fermi superfluids, and we obtain a phase diagram at the saddle-point level, which contains multichannel nonperturbative corrections. In particular, we find a previously overlooked superfluid phase for weak interactions, which is dominated by particle-hole processes, in addition to the usual superfluid phase only containing particle-particle physics. △ Less

Submitted 26 August, 2025; originally announced August 2025.

Comments: 26 pages, 12 figures

Journal ref: Phys. Rev. Research 7, 033186 (2025)

Emergent Self-propulsion of Skyrmionic Matter in Synthetic Antiferromagnets

Authors: Clecio C. de Souza Silva, Matheus V. Correia, Juan C. Pina Velasquez

Abstract: Self-propulsion plays a crucial role in biological processes and nanorobotics, enabling small systems to move autonomously in noisy environments. Here, we theoretically demonstrate that a bound skyrmion-skyrmion pair in a synthetic antiferromagnetic bilayer can function as a self-propelled topological object, reaching speeds of up to a hundred million body lengths per second--far exceeding those o… ▽ More Self-propulsion plays a crucial role in biological processes and nanorobotics, enabling small systems to move autonomously in noisy environments. Here, we theoretically demonstrate that a bound skyrmion-skyrmion pair in a synthetic antiferromagnetic bilayer can function as a self-propelled topological object, reaching speeds of up to a hundred million body lengths per second--far exceeding those of any known synthetic or biological self-propelled particles. The propulsion mechanism is triggered by the excitation of back-and-forth relative motion of the skyrmions, which generates nonreciprocal gyrotropic forces, driving the skyrmion pair in a direction perpendicular to their bond. Remarkably, thermal noise induces spontaneous reorientations of the pair and momentary reversals of the propulsion, mimicking behaviors observed in motile bacteria and microalgae. △ Less

Submitted 20 August, 2025; originally announced August 2025.

Comments: Includes End Matter and Supplemental Material (total: 15 pages)

Journal ref: Phys. Rev. Lett. 135, 086701 (2025)

Role of electron-electron interactions in $M$-valley twisted transition metal dichalcogenides

Authors: Christophe De Beule, Liangtao Peng, E. J. Mele, Shaffique Adam

Abstract: We investigate the role of long-range Coulomb interactions in $M$-valley moirés using the self-consistent Hartree-Fock approximation. This platform was recently proposed [Nature 643, 376 (2025) and arXiv:2411.18828 (2024)] as a new class of experimentally realizable moiré materials using twisted transition metal dichalcogenides homobilayers with the 1T structure. While these seminal studies consid… ▽ More We investigate the role of long-range Coulomb interactions in $M$-valley moirés using the self-consistent Hartree-Fock approximation. This platform was recently proposed [Nature 643, 376 (2025) and arXiv:2411.18828 (2024)] as a new class of experimentally realizable moiré materials using twisted transition metal dichalcogenides homobilayers with the 1T structure. While these seminal studies considered the noninteracting theory without an electric displacement field, this work shows that both electron-electron interactions at finite doping and an interlayer bias strongly modify the moiré bands. For small twists ($\lesssim 5^\circ$) the density of states versus filling and interlayer bias displays qualitatively different behavior for twisting near aligned ($0^\circ$) and antialigned ($60^\circ$) stacking with tunable Van Hove singularities (VHSs). Moreover, interactions pin the VHS to the Fermi energy over a finite range of doping both at zero and finite bias depending on the stacking type, an effect known to enhance both superconductivity and strongly correlated states. At half filling, we obtain the phase diagram as a function of interaction strength, interlayer bias, and twist angle. We find a competition driven by band mixing between an isotropic ferromagnet and an antiferromagnet that are nearly degenerate over a wide range of experimentally accessible parameters. Our work demonstrates that correlated states in $M$-valley 1T tTMDs can be strongly tuned in situ both by applying an electric displacement field and by electron doping. △ Less

Submitted 22 September, 2025; v1 submitted 19 August, 2025; originally announced August 2025.

Fast hydrogen atom diffraction through monocrystalline graphene

Authors: Pierre Guichard, Arnaud Dochain, Raphaël Marion, Pauline de Crombrugghe de Picquendaele, Nicolas Lejeune, Benoît Hackens, Paul-Antoine Hervieux, Xavier Urbain

Abstract: We report fast atom diffraction through single-layer graphene using hydrogen atoms at kinetic energies from 150 to 1200 eV. High-resolution images reveal overlapping hexagonal patterns from coexisting monocrystalline domains. Time-of-flight tagging confirms negligible energy loss, making the method suitable for matter-wave interferometry. The diffraction is well described by the eikonal approximat… ▽ More We report fast atom diffraction through single-layer graphene using hydrogen atoms at kinetic energies from 150 to 1200 eV. High-resolution images reveal overlapping hexagonal patterns from coexisting monocrystalline domains. Time-of-flight tagging confirms negligible energy loss, making the method suitable for matter-wave interferometry. The diffraction is well described by the eikonal approximation, with accurate modeling requiring the full 3D interaction potential from DFT. Simpler models fail to reproduce the data, highlighting the exceptional sensitivity of diffraction patterns to atom-surface interactions and their potential for spectroscopic applications. △ Less

Submitted 11 August, 2025; originally announced August 2025.

Comments: 6 pages and 5 figures (main text), 6 pages and 5 figures (supplemental material)

Journal ref: Phys. Rev. Lett. 135, 263403 (2025)

Design of high-mobility p-type GaN via the piezomobility tensor

Authors: Jie-Cheng Chen, Joshua Leveillee, Chris G. Van de Walle, Feliciano Giustino

Abstract: Gallium nitride (GaN) is a wide-bandgap semiconductor of significant interest for applications in solid-state lighting, power electronics, and radio-frequency amplifiers. An important limitation of this semiconductor is its low intrinsic hole mobility, which hinders the development of \textit{p}-channel devices and the large-scale integration of GaN CMOS in next-generation electronics. Prior resea… ▽ More Gallium nitride (GaN) is a wide-bandgap semiconductor of significant interest for applications in solid-state lighting, power electronics, and radio-frequency amplifiers. An important limitation of this semiconductor is its low intrinsic hole mobility, which hinders the development of \textit{p}-channel devices and the large-scale integration of GaN CMOS in next-generation electronics. Prior research has explored the use of strain to improve the hole mobility of GaN, but a systematic analysis of all possible strain conditions and their impact on the mobility is lacking. In this study, we introduce a piezomobility tensor notation to characterize the relationship between applied strain and hole mobility in GaN. To map the strain-dependence of the hole mobility, we solve the \textit{ab initio} Boltzmann transport equation, accounting for electron-phonon scattering and GW quasiparticle energy corrections. We show that there exist three optimal strain configurations, two uniaxial strains and one shear strain, that can lead to significant mobility enhancement. In particular, we predict room-temperature hole mobility of up to 164~\mob\ for 2\% uniaxial compression and 148~\mob\ for 2\% shear strain. Our methodology provides a general framework for investigating strain effects on the transport properties of semiconductors from first principles. △ Less

Submitted 8 August, 2025; originally announced August 2025.

All Photonic Isolator using Atomically Thin (2D) Bismuth Telluride (Bi2Te3)

Authors: Saswata Goswami, Bruno Ipaves, Juan Gomez Quispe, Caique Campos de Oliveira, Surbhi Slathia, Abhijith M. B, Varinder Pal, Christiano J. S. de Matos, Samit K. Ray, Douglas S. Galvao, Pedro A. S. Autreto, Chandra Sekhar Tiwary

Abstract: This study demonstrates that two-dimensional (2D) Bi2Te3 exhibits strong light-matter interaction, enabling a broadband Kerr nonlinear optical response. This characteristic is advantageous for nonreciprocal light propagation in passive photonic isolators. Using Spatial Self-Phase Modulation (SSPM) spectroscopy, self-induced diffraction patterns in the far field were observed at excitation waveleng… ▽ More This study demonstrates that two-dimensional (2D) Bi2Te3 exhibits strong light-matter interaction, enabling a broadband Kerr nonlinear optical response. This characteristic is advantageous for nonreciprocal light propagation in passive photonic isolators. Using Spatial Self-Phase Modulation (SSPM) spectroscopy, self-induced diffraction patterns in the far field were observed at excitation wavelengths of 650 nm, 532 nm, and 405 nm to calculate the nonlinear refractive index (n2) and the third-order nonlinear optical susceptibility (chi^(3)) of the synthesized 2D Bi2Te3. The results show that 2D Bi2Te3 possesses a significantly higher nonlinear refractive index than graphene. The laser-induced hole coherence effect is responsible for the large magnitude of the third-order nonlinear susceptibility. Surface engineering techniques were also employed to enhance the response speed of the photonic system. Complementary ab initio simulations were performed to gain further insight into the observed nonlinear behavior. Leveraging the strong Kerr nonlinearity of 2D Bi2Te3, a nonlinear photonic isolator that breaks time-reversal symmetry and enables unidirectional light propagation was demonstrated. This work establishes Bi2Te3 as a novel 2D material for nonlinear photonics, expanding its potential applications in detectors, modulators, and optical switches. △ Less

Submitted 5 August, 2025; originally announced August 2025.

Towards Environmentally Responsive Hypersound Materials

Authors: Edson Rafael Cardozo de Oliveira, Gastón Grosman, Chushuang Xiang, Michael Zuarez-Chamba, Priscila Vensaus, Abdelmounaim Harouri, Cédric Boissiere, Galo J. A. A. Soler-Illia, Norberto Daniel Lanzillotti-Kimura

Abstract: The engineering of acoustic phonons in the gigahertz (GHz) range holds significant potential for technological breakthroughs in areas such as data processing, sensing and quantum communication. Novel approaches for nanophononic resonators responsive to external stimuli provide additional control and functionality for these devices. Mesoporous thin films (MTFs) for example, featuring nanoscale orde… ▽ More The engineering of acoustic phonons in the gigahertz (GHz) range holds significant potential for technological breakthroughs in areas such as data processing, sensing and quantum communication. Novel approaches for nanophononic resonators responsive to external stimuli provide additional control and functionality for these devices. Mesoporous thin films (MTFs) for example, featuring nanoscale ordered pores, support GHz-range acoustic resonances. These materials are sensitive to environmental changes, such as liquid and vapor infiltration, modifying their effective optical and elastic properties. Here, a SiO$_{2}$ MTF-based open-cavity nanoacoustic resonator is presented, in which the MTF forms the topmost layer and is exposed to the environment. Using a transient reflectivity setup, acoustic responses under varying humidity conditions are investigated. A pronounced shift in acoustic resonance frequency with changes in relative humidity is observed for the first time, demonstrating a simple way to tune hypersound confinement. In addition, resonators with varying pore sizes and thicknesses are compared, revealing that resonance frequencies are primarily influenced by material properties and film thickness, rather than pore size. The proposed open-cavity resonator design provides a versatile platform for future studies on the mechanical response of MTFs to liquid and vapor infiltration, opening the gate to environment-responsive hypersound devices. △ Less

Submitted 25 July, 2025; originally announced July 2025.

Comments: 10 pages, 4 figures. supplemental material: 4 pages, 3 figures

Revealing emergent many-body phenomena by analyzing large-scale space-time records of monitored quantum systems

Authors: Marcel Cech, Cecilia De Fazio, María Cea, Mari Carmen Bañuls, Igor Lesanovsky, Federico Carollo

Abstract: Recent advances in quantum simulators permit unitary evolution interspersed with locally resolved mid-circuit measurements. This paves the way for the observation of large-scale space-time structures in quantum trajectories and opens a window for the \emph{in situ} analysis of complex dynamical processes. We demonstrate this idea using a paradigmatic dissipative spin model, which can be implemente… ▽ More Recent advances in quantum simulators permit unitary evolution interspersed with locally resolved mid-circuit measurements. This paves the way for the observation of large-scale space-time structures in quantum trajectories and opens a window for the \emph{in situ} analysis of complex dynamical processes. We demonstrate this idea using a paradigmatic dissipative spin model, which can be implemented, e.g., on Rydberg quantum simulators. Here, already the trajectories of individual experimental runs reveal surprisingly complex statistical phenomena. In particular, we exploit free-energy functionals for trajectory ensembles to identify dynamical features reminiscent of hydrophobic behavior observed near the liquid-vapor transition in the presence of solutes in water. We show that these phenomena are observable in experiments and discuss the impact of common imperfections, such as readout errors and disordered interactions. △ Less

Submitted 1 July, 2025; originally announced July 2025.

Comments: 9+7 pages, 4+6 figures, comments welcome

Versatile multi-q antiferromagnetic charge order in correlated vdW metals

Authors: Y. Fujisawa, P. Wu, R. Okuma, B. R. M. Smith, D. Ueta, R. Kobayashi, N. Maekawa, T. Nakamura, C-H. Hsu, Chandan De, N. Tomoda, T. Higashihara, K. Morishita, T. Kato, Z. Y. Wang, Y. Okada

Abstract: Following the discovery of graphene, interest in van der Waals (vdW) materials has surged; yet, advancing "beyond graphene" physics requires the development of quantum material platforms that host versatile many-body states. Using scanning tunneling microscopy and spectroscopy at 300 mK, we uncover two competing states in vdW metal CeTe3: charge-ordered in-plane antiferromagnetic phases forming st… ▽ More Following the discovery of graphene, interest in van der Waals (vdW) materials has surged; yet, advancing "beyond graphene" physics requires the development of quantum material platforms that host versatile many-body states. Using scanning tunneling microscopy and spectroscopy at 300 mK, we uncover two competing states in vdW metal CeTe3: charge-ordered in-plane antiferromagnetic phases forming stripe and checkerboard patterns. Remarkably, the competition between them is tuned through a modest in-plane magnetic field (approximately 1.5 T), revealing significant cooperative phenomena between frustrated antiferromagnetism, charge order, and competing Fermi surface nesting. Underlying strongly intertwined many-body states are consistently signaled by density of states deformations exceeding plus/minus 30 meV scale across the Fermi level. Our findings provide a promising correlated vdW platform hosting versatile two-dimensional many-body physics, offering a fertile ground to explore topologically nontrivial multi-q charge-ordered antiferromagnetism, quantum criticality, unconventional superconductivity, and their potential interconnections. △ Less

Submitted 1 July, 2025; originally announced July 2025.

Unveiling the Electronic Origin of Anomalous Contact Conductance in Twisted Bilayer Graphene

Authors: Kevin J. U. Vidarte, Caio Lewenkopf, F. Crasto de Lima, R. Hiroki Miwa, Felipe Pérez Riffo, Eric Suárez Morell

Abstract: This study theoretically investigates the contact conductance in twisted bilayer graphene (TBG), providing a theoretical explanation for recent experimental observations from scanning tunneling microscopy (STM) and conductive atomic force microscopy (c-AFM). These experiments revealed a surprising non-monotonic current pattern as a function of the TBG rotation angle $θ$, with a peak at… ▽ More This study theoretically investigates the contact conductance in twisted bilayer graphene (TBG), providing a theoretical explanation for recent experimental observations from scanning tunneling microscopy (STM) and conductive atomic force microscopy (c-AFM). These experiments revealed a surprising non-monotonic current pattern as a function of the TBG rotation angle $θ$, with a peak at $θ\approx 5^\circ$, a finding that markedly departs from the well-known magic angle TBG behavior. To elucidate this phenomenon, we develop a comprehensive theoretical and computational framework. Our calculations, performed on both relaxed and rigid TBG structures, simulate contact conductance by analyzing the local density of states across a range of biases and rotational angles. Contrary to the current interpretation, our results demonstrate that the maximum conductance at $θ\approx 5^{\rm o}$ is not caused by structural relaxation or AA stacking zone changes. Instead, we attribute this peak to the evolution of the electronic band structure, specifically the shifting of van Hove singularities (vHs) to the Fermi level as the twist angle decreases. We further show that the precise location of this conductance maximum is dependent on the applied bias voltage. This interplay between twist angle, bias, and vHs energy provides a robust explanation for the experimental findings. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 9 pages, 6 figures

Dynamical quantum phase transition with divergent multipartite entanglement

Authors: Jie Chen, Ricardo Costa de Almeida, Hendrik Weimer

Abstract: We investigate the nonequilibrium quench dynamics of the one-dimensional transverse-field Ising model in both integrable and nonintegrable regimes. In particular, we report on a novel type of dynamical quantum phase transition (DQPT) that is characterized by a divergent multipartite entanglement at critical times in the post-quench dynamics. We quantify the multipartite entanglement of the state b… ▽ More We investigate the nonequilibrium quench dynamics of the one-dimensional transverse-field Ising model in both integrable and nonintegrable regimes. In particular, we report on a novel type of dynamical quantum phase transition (DQPT) that is characterized by a divergent multipartite entanglement at critical times in the post-quench dynamics. We quantify the multipartite entanglement of the state by the quantum Fisher information and demonstrate that the DQPT belongs to a different universality class than the ground-state phase transition. Furthermore, we perform a spectral analysis of the DQPT and demonstrate that it is a genuine nonequilibrium transition arising from the constructive interference of excited states of the system during the many-body dynamics. Finally, we discuss potential experimental realizations in Rydberg platforms as well as applications in the context of quantum metrology. △ Less

Submitted 16 June, 2025; originally announced June 2025.

Comments: 7 pages, 6 figures

How do Probabilistic Graphical Models and Graph Neural Networks Look at Network Data?

Authors: Michela Lapenna, Caterina De Bacco

Abstract: Graphs are a powerful data structure for representing relational data and are widely used to describe complex real-world systems. Probabilistic Graphical Models (PGMs) and Graph Neural Networks (GNNs) can both leverage graph-structured data, but their inherent functioning is different. The question is how do they compare in capturing the information contained in networked datasets? We address this… ▽ More Graphs are a powerful data structure for representing relational data and are widely used to describe complex real-world systems. Probabilistic Graphical Models (PGMs) and Graph Neural Networks (GNNs) can both leverage graph-structured data, but their inherent functioning is different. The question is how do they compare in capturing the information contained in networked datasets? We address this objective by solving a link prediction task and we conduct three main experiments, on both synthetic and real networks: one focuses on how PGMs and GNNs handle input features, while the other two investigate their robustness to noisy features and increasing heterophily of the graph. PGMs do not necessarily require features on nodes, while GNNs cannot exploit the network edges alone, and the choice of input features matters. We find that GNNs are outperformed by PGMs when input features are low-dimensional or noisy, mimicking many real scenarios where node attributes might be scalar or noisy. Then, we find that PGMs are more robust than GNNs when the heterophily of the graph is increased. Finally, to assess performance beyond prediction tasks, we also compare the two frameworks in terms of their computational complexity and interpretability. △ Less

Submitted 22 August, 2025; v1 submitted 13 June, 2025; originally announced June 2025.

Pseudomagnetotransport in Strained Graphene

Authors: Alina Mreńca-Kolasińska, Christophe De Beule, Jia-Tong Shi, Aitor Garcia-Ruiz, Denis Kochan, Klaus Richter, Ming-Hao Liu

Abstract: In graphene, long-wavelength deformations that result in elastic shear strain couple to the low-energy Dirac electrons as pseudogauge fields. Using a scalable tight-binding model, we consider analogs to magnetotransport in mesoscopic strained graphene devices with nearly uniform pseudomagnetic fields. In particular, we consider transverse pseudomagnetic focusing in a bent graphene ribbon and show… ▽ More In graphene, long-wavelength deformations that result in elastic shear strain couple to the low-energy Dirac electrons as pseudogauge fields. Using a scalable tight-binding model, we consider analogs to magnetotransport in mesoscopic strained graphene devices with nearly uniform pseudomagnetic fields. In particular, we consider transverse pseudomagnetic focusing in a bent graphene ribbon and show that a focused valley-polarized current can be generated with characteristic conductance oscillations. Importantly, our scaling method allows for quantum transport calculations with realistic device geometries, and leaves the Dirac physics and pseudogauge fields invariant as long as the atomic displacements vary slowly with respect to the scaled lattice. Our results show that pseudomagnetotransport is a promising new route for graphene straintronics, and our scaling method provides a new framework for the modeling, design, and interpretation of straintronics experiments and applications. △ Less

Submitted 27 May, 2025; originally announced May 2025.

Comments: 7 pages + 9 pages supplementary file, 12 figures

Structural, magnetic, and nanoacoustic characterization of Co/Pt superlattices

Authors: E. R. Cardozo de Oliveira, C. Xiang, C. Borrazás, S. Sandeep, J. E. Gómez, M. Vásquez Mansilla, N. Findling, L. Largeau, N. D. Lanzillotti-Kimura, M. Granada

Abstract: Superlattices presenting a spatial modulation of the elastic properties appear as a main tool to reach the THz regime in nanoacoustic devices. The exploration of alternative materials with multifunctional properties remains a fertile domain of research. In this work, we study the structural, magnetic, and acoustic characteristics of nanometric superlattices made of Pt/Co. The samples present a w… ▽ More Superlattices presenting a spatial modulation of the elastic properties appear as a main tool to reach the THz regime in nanoacoustic devices. The exploration of alternative materials with multifunctional properties remains a fertile domain of research. In this work, we study the structural, magnetic, and acoustic characteristics of nanometric superlattices made of Pt/Co. The samples present a well defined periodicity, as determined by X-ray reflectometry, whereas scanning transmission electron microscopy with local compositional analysis reveals that the superlattices present a modulation in composition instead of sharp interfaces. The policrystalline nature of the superlattices is evidenced both by X ray diffraction and transmission electron microscopy. Magnetization measurements show a perpendicular magnetic anisotropy for the higher Co concentrations. Picosecond acoustic experiments evidence that the studied samples support short-lived acoustic modes up to 900 GHz, and up to 7 acoustic echoes at lower frequencies.These are promising results for the development of magnetoacoustic devices working at ultrahigh frequencies. △ Less

Submitted 25 May, 2025; originally announced May 2025.

Comments: 21 pages, 8 figures, 1 table

Relating thermodynamic quantities of convex-hard-body fluids to the body's shape

Authors: Thomas Franosch, Cristiano De Michele, Rolf Schilling

Abstract: For a fluid of convex hard particles, characterized by a length scale $σ_\text{min}$ and an anisotropy parameter $ε$, we develop a formalism allowing one to relate thermodynamic quantities to the body's shape. In a first step its thermodynamics is reduced to that of spherical particles. The latter have a hard core of diameter $σ_\text{min }$ and a soft shell with a thickness $εσ_\text{min}/2$. Bes… ▽ More For a fluid of convex hard particles, characterized by a length scale $σ_\text{min}$ and an anisotropy parameter $ε$, we develop a formalism allowing one to relate thermodynamic quantities to the body's shape. In a first step its thermodynamics is reduced to that of spherical particles. The latter have a hard core of diameter $σ_\text{min }$ and a soft shell with a thickness $εσ_\text{min}/2$. Besides their hard core repulsion at $σ_\text{min }$ they interact by effective entropic forces which will be calculated. Based on this mapping, a second step provides a perturbative method for the systematic calculation of thermodynamic quantities with the shape anisotropy $ε$ as smallness parameter. In leading order in $ε$, the equation of state is derived as a functional of the particle's shape. To illustrate these findings, they are applied to a one- and two-dimensional fluid of ellipses and compared with results from different analytical approaches, and our computer simulations. The mapping to spherical particles also implies that any phase transition of spherical particles, e.g., the liquid-hexatic transition, also exists for the nonspherical ones, and shifts linearly with $ε$ for weak shape anisotropy. This is supported by our Monte-Carlo simulation. △ Less

Submitted 14 June, 2025; v1 submitted 20 May, 2025; originally announced May 2025.

Comments: 26 pages, 11 figures

Journal ref: Physical Review Research 7, 023260 (2025)

Pyrite Bismuth Telluride Heterojunction for Hybrid Electromagnetic to Thermoelectric Energy Harvesting

Authors: Karthik R, Yiwen Zheng, Caique Campos de Oliveira, Punathil Raman Sreeram, Pedro Alves da Silva Autreto, Aniruddh Vashisth, Chandra Sekhar Tiwary

Abstract: The rapid proliferation of wireless networks and connected devices has led to pervasive electromagnetic (EM) energy dissipation into the environment, an underutilized resource for energy harvesting. Here, we demonstrate a pyrite (FeS$_2$)-bismuth telluride (Bi$_2$Te$_3$) heterojunction that enables hybrid electromagnetic-to-thermoelectric energy conversion. Fabricated via a simple cold-press compa… ▽ More The rapid proliferation of wireless networks and connected devices has led to pervasive electromagnetic (EM) energy dissipation into the environment, an underutilized resource for energy harvesting. Here, we demonstrate a pyrite (FeS$_2$)-bismuth telluride (Bi$_2$Te$_3$) heterojunction that enables hybrid electromagnetic-to-thermoelectric energy conversion. Fabricated via a simple cold-press compaction of powders, the heterojunction forms a Schottky interface at FeS$_2$, facilitating efficient RF absorption and localized heating. This heat is harvested by Bi$_2$Te$_3$ through thermoelectric conversion. Under 35~MHz RF irradiation at 1~W input power, the device achieved a local temperature rise of 46~$^\circ$C and a thermal gradient of 5.5~K across the Bi$_2$Te$_3$, resulting in a peak power density of approximately 13~mW/cm$^2$. Molecular dynamics (MD) simulations and density functional theory (DFT) calculations further elucidate the heat transport behavior and interfacial thermoelectric performance. This work introduces a new class of heterostructures for RF-responsive energy harvesting, offering a scalable route toward self-powered IoT and wireless sensing systems. △ Less

Submitted 12 May, 2025; originally announced May 2025.

Comments: 29 pages, 8 Figures

Supersonic Flow Past an Obstacle in a Quasi-Two-Dimensional Lee-Huang-Yang Quantum Fluid

Authors: G. H. dos Santos, L. F. Calazans de Brito, A. Gammal, A. M. Kamchatnov

Abstract: A supersonic flow past an obstacle can generate a rich variety of wave excitations. This paper investigates, both analytically and numerically, two types of excitations generated by the flow of a Lee-Huang-Yang quantum fluid past an obstacle: linear radiation and oblique dark solitons. We show that wave crests of linear radiation can be accurately described by the proper modification of the Kelvin… ▽ More A supersonic flow past an obstacle can generate a rich variety of wave excitations. This paper investigates, both analytically and numerically, two types of excitations generated by the flow of a Lee-Huang-Yang quantum fluid past an obstacle: linear radiation and oblique dark solitons. We show that wave crests of linear radiation can be accurately described by the proper modification of the Kelvin original theory, while the oblique dark soliton solution is obtained analytically by transformation of the 1D soliton solution to the obstacle's reference frame. A comparison between analytical predictions and numerical simulations demonstrates good agreement, validating our theoretical approach. △ Less

Submitted 25 August, 2025; v1 submitted 10 May, 2025; originally announced May 2025.

Comments: 9 pages, 6 figures

Planar fault-tolerant circuits for non-Clifford gates on the 2D color code

Authors: Andreas Bauer, Julio C. Magdalena de la Fuente

Abstract: We introduce a family of scalable planar fault-tolerant circuits that implement logical non-Clifford operations on a 2D color code, such as a logical $T$ gate or a logical non-Pauli measurement that prepares a magic $|T\rangle$ state. The circuits are relatively simple, consisting only of physical $T$ gates, $CX$ gates, and few-qubit measurements. They can be implemented with an array of qubits on… ▽ More We introduce a family of scalable planar fault-tolerant circuits that implement logical non-Clifford operations on a 2D color code, such as a logical $T$ gate or a logical non-Pauli measurement that prepares a magic $|T\rangle$ state. The circuits are relatively simple, consisting only of physical $T$ gates, $CX$ gates, and few-qubit measurements. They can be implemented with an array of qubits on a 2D chip with nearest-neighbor couplings, and no wire crossings. The construction is based on a spacetime path integral representation of a non-Abelian 2+1D topological phase, which is related to the 3D color code. We turn the path integral into a circuit by expressing it as a spacetime $ZX$ tensor network, and then traversing it in some chosen time direction. We describe in detail how fault tolerance is achieved using a "just-in-time" decoding strategy, for which we repurpose and extend state-of-the-art color-code matching decoders. △ Less

Submitted 8 May, 2025; originally announced May 2025.

Clarification of the Spontaneous Polarization Direction in Crystals with Wurtzite Structure

Authors: Simon Fichtner, Mohamed Yassine, Chris van de Walle, Oliver Ambacher

Abstract: The wurtzite structure is one of the most frequently found crystal structures in modern semiconductors and its inherent spontaneous polarization is a defining materials property. Despite this significance, confusion has been rampant in the literature with respect to the orientation of the spontaneous polarization inside the unit cell of the wurtzite structure, especially for the technologically ve… ▽ More The wurtzite structure is one of the most frequently found crystal structures in modern semiconductors and its inherent spontaneous polarization is a defining materials property. Despite this significance, confusion has been rampant in the literature with respect to the orientation of the spontaneous polarization inside the unit cell of the wurtzite structure, especially for the technologically very relevant III-N compounds (AlN, GaN, InN). In particular, the spontaneous polarization has been reported to either point up or down for the same unit cell orientation, depending on the literature source - with important implications for, e.g., the carrier type and density expected at interfaces of heterostructures involving materials with wurtzite-structure. This perspective aims to resolve this ambiguity by reviewing available reports on the direction of the energetically preferred polarization direction in the presence of external electric fields, as well as atomically resolved scanning transmission electron microscopy images. While we use ferroelectric wurtzite AlScN as a key example, our conclusions are generalizable to other compounds with the same crystal structure. We demonstrate that a metal-polar unit cell must be associated with an upward polarization vector - which is contrary to long-standing conventional wisdom. △ Less

Submitted 6 May, 2025; originally announced May 2025.

Journal ref: Appl. Phys. Lett. 125, 040501 (2024)

Interface-Induced Stability of Nontrivial Topological Spin Textures: Unveiling Room-Temperature Hopfions and Skyrmions

Authors: F. Katmis, V. Lauter, R. Yagan, L. S. Brandt, A. M. Cheghabouri, H. Zhou, J. W. Freeland, C. I. L. de Araujo, M. E. Jamer, D. Heiman, M. C. Onbasli, J. S. Moodera

Abstract: Topological spin configurations, such as soliton-like spin texture and Dirac electron assemblies, have emerged in recent years in both fundamental science and technological applications. Achieving stable topological spin textures at room-temperature is crucial for enabling these structures as long-range information carriers. However, their creation and manipulation processes have encountered diffi… ▽ More Topological spin configurations, such as soliton-like spin texture and Dirac electron assemblies, have emerged in recent years in both fundamental science and technological applications. Achieving stable topological spin textures at room-temperature is crucial for enabling these structures as long-range information carriers. However, their creation and manipulation processes have encountered difficulties due to multi-step field training techniques and competitive interactions. Thus, a spontaneous ground state for multi-dimensional topological spin textures is desirable, as skyrmions form swirling, hedgehog-like spin structures in two dimensions, while hopfions emerge as their twisted three-dimensional counterparts. Here, we report the first observation of robust and reproducible topological spin textures of hopfions and skyrmions observed at room temperature and in zero magnetic field, which are stabilized by geometric confinement and protected by interfacial magnetism in a ferromagnet/topological insulator/ferromagnet trilayer heterostructure. These skyrmion-hopfion configurations are directly observed at room temperature with Lorenz transmission electron microscopy. Using micromagnetic modelling, the experimental observations of hopfion-skyrmion assemblies are reproduced. Our model reveals a complete picture of how spontaneously organized skyrmion lattices encircled by hopfion rings are controlled by surface electrons, uniaxial anisotropy and Dzyaloshinskii-Moriya interaction, all at ambient temperature. Our study provides evidence that topological chiral spin textures can facilitate the development of magnetically defined information carriers. These stable structures hold promise for ultralow-power and high-density information processing, paving the way for the next generation of topologically defined devices. △ Less

Submitted 21 June, 2025; v1 submitted 13 April, 2025; originally announced April 2025.

Impact of Absorption due to Zero-Field Splitting on Loss in Dielectrics

Authors: Mark E. Turiansky, Chris G. Van de Walle

Abstract: The coherence times of superconducting qubits are limited by loss mechanisms, whose microscopic origins have remained elusive. We propose a mechanism caused by transitions between zero-fieldsplit states of paramagnetic impurities or defects. We derive the absorption cross section for a magnetic dipole transition and apply it to calculate the loss tangent. For Cr, Fe, and V impurities in sapphire,… ▽ More The coherence times of superconducting qubits are limited by loss mechanisms, whose microscopic origins have remained elusive. We propose a mechanism caused by transitions between zero-fieldsplit states of paramagnetic impurities or defects. We derive the absorption cross section for a magnetic dipole transition and apply it to calculate the loss tangent. For Cr, Fe, and V impurities in sapphire, we find loss tangents at 4.5 GHz in the range of 10$^{-9}$-10$^{-8}$, comparable to the loss measured in experiments. This value suggests that magnetic loss may be a limiting factor in the coherence times of superconducting qubits. △ Less

Submitted 6 April, 2025; originally announced April 2025.

Is Herbertsmithite far from an ideal antiferromagnet? Ab-initio answer including in-plane Dzyaloshinskii-Moriya interactions and coupling with extra-plane impurities

Authors: Flaurent Heully-Alary, Nadia Ben Amor, Nicolas Suaud, Laura Messio, Coen de Graaf, Nathalie Guihéry

Abstract: Herbertsmithite is known as the archetype of a S=1/2 nearest-neighbor Heisenberg antiferromagnet on the Kagomé lattice, theoretically presumed to be a quantum gapless spin liquid. However, more and more experiments reveal that the model suffers from deviations from the ideal one, evidenced at very low temperatures. This detailed ab initio study focuses on two such deviations that have never been q… ▽ More Herbertsmithite is known as the archetype of a S=1/2 nearest-neighbor Heisenberg antiferromagnet on the Kagomé lattice, theoretically presumed to be a quantum gapless spin liquid. However, more and more experiments reveal that the model suffers from deviations from the ideal one, evidenced at very low temperatures. This detailed ab initio study focuses on two such deviations that have never been quantitatively calculated: the anisotropic exchange interactions and the Heisenberg exchange with extra-plane magnetic impurities. The Dzyaloshinskii-Moriya interaction is found to have an in-plane component almost three times larger than the out-of-plane component, but typically obviated in theoretical studies. Moreover, it is shown that the extra-plane magnetic impurities have a strong ferromagnetic interaction (minus half the main exchange $J_1$ ) with the Kagomé magnetic sites. Combined with an estimated occurrence of these magnetic impurities of $\sim15\%$, the present results indicate that two-dimensional magnetic models only describe part of the physics. △ Less

Submitted 3 November, 2025; v1 submitted 25 March, 2025; originally announced March 2025.

Journal ref: SciPost Phys. Core 8, 092 (2025)

Theory for Lattice Relaxation in Marginal Twist Moirés

Authors: Christophe De Beule, Gayani N. Pallewela, Mohammed M. Al Ezzi, Liangtao Peng, E. J. Mele, Shaffique Adam

Abstract: Atomically thin moiré materials behave like elastic membranes where at very small twist angles, the van der Waals adhesion energy much exceeds the strain energy. In this ``marginal twist" regime, regions with low adhesion energy expand, covering most of the moiré unit cell, while all the unfavorable energy configurations shrink to form topological defects linked by a periodic network of domain wal… ▽ More Atomically thin moiré materials behave like elastic membranes where at very small twist angles, the van der Waals adhesion energy much exceeds the strain energy. In this ``marginal twist" regime, regions with low adhesion energy expand, covering most of the moiré unit cell, while all the unfavorable energy configurations shrink to form topological defects linked by a periodic network of domain walls. We find analytical expressions that successfully capture this strong-coupling regime for both the triangular soliton network and the honeycomb soliton network matching predictions from LAMMPS molecular dynamics simulations, and numerical solutions of continuum elasticity theory. There is an emergent universality where the theory is characterized by a single twist-angle dependent parameter. Our formalism is essential to understand experiments on a wide-range of materials of current interest including twisted bilayer graphene, both parallel and antiparallel stacked tWSe2 and tMoTe2, and any other twisted homobilayer with the same stacking symmetry. △ Less

Submitted 24 March, 2025; originally announced March 2025.

Comments: 6 + 10 pages, 4 + 11 figures

Universal fault tolerant quantum computation in 2D without getting tied in knots

Authors: Margarita Davydova, Andreas Bauer, Julio C. Magdalena de la Fuente, Mark Webster, Dominic J. Williamson, Benjamin J. Brown

Abstract: We show how to perform scalable fault-tolerant non-Clifford gates in two dimensions by introducing domain walls between the surface code and a non-Abelian topological code whose codespace is stabilized by Clifford operators. We formulate a path integral framework which provides both a macroscopic picture for different logical gates as well as a way to derive the associated microscopic circuits. We… ▽ More We show how to perform scalable fault-tolerant non-Clifford gates in two dimensions by introducing domain walls between the surface code and a non-Abelian topological code whose codespace is stabilized by Clifford operators. We formulate a path integral framework which provides both a macroscopic picture for different logical gates as well as a way to derive the associated microscopic circuits. We also show an equivalence between our approach and prior proposals where a 2D array of qubits reproduces the action of a transversal gate in a 3D stabilizer code over time, thus, establishing a new connection between 3D codes and 2D non-Abelian topological phases. We prove a threshold theorem for our protocols under local stochastic circuit noise using a just-in-time decoder to correct the non-Abelian code. △ Less

Submitted 15 May, 2025; v1 submitted 19 March, 2025; originally announced March 2025.

Comments: 62 pages, 35 figures; v2: minor edits

Itinerant Magnetism in Twisted Bilayer WSe$_2$ and MoTe$_2$

Authors: Liangtao Peng, Christophe De Beule, Yiyang Lai, Du Li, Li Yang, E. J. Mele, Shaffique Adam

Abstract: Using a self-consistent Hartree-Fock theory, we show that the recently observed ferromagnetism in twisted bilayer WSe$_2$ [Nat. Commun. 16, 1959 (2025)] can be understood as a Stoner-like instability of interaction-renormalized moiré bands. We quantitatively reproduce the observed Lifshitz transition as function of hole filling and applied electric field that marks the boundary between layer-hybri… ▽ More Using a self-consistent Hartree-Fock theory, we show that the recently observed ferromagnetism in twisted bilayer WSe$_2$ [Nat. Commun. 16, 1959 (2025)] can be understood as a Stoner-like instability of interaction-renormalized moiré bands. We quantitatively reproduce the observed Lifshitz transition as function of hole filling and applied electric field that marks the boundary between layer-hybridized and layer-polarized regimes. The former supports a ferromagnetic valley-polarized ground state below half-filling, developing a topological charge gap at half-filling for smaller twist angles. At larger twist angles, the system hosts a gapped triangular Néel antiferromagnet. On the other hand, the layer-polarized regime supports a stripe antiferromagnet below half-filling and a wing-shaped multiferroic ground state above half-filling. We map the evolution of these states as a function of filling factor, electric field, twist angle, and interaction strength. Our results demonstrate that long-range exchange in a symmetry-unbroken parent state with strongly renormalized moiré bands provides a broadly applicable framework to understand itinerant magnetism in moiré TMDs. △ Less

Submitted 16 December, 2025; v1 submitted 12 March, 2025; originally announced March 2025.

Carbon in GaN as a nonradiative recombination center

Authors: Fangzhou Zhao, Hongyi Guan, Mark E. Turiansky, Chris G. Van de Walle

Abstract: Trap-assisted nonradiative recombination has been shown to limit the efficiency of optoelectronic devices. While substitutional carbon ($\mathrm{C_N}$) has been suggested to be a nonradiative recombination center in GaN devices, a complete recombination cycle including the two charge-state transition levels has not been previously described. In this work, we investigate the trap-assisted recombina… ▽ More Trap-assisted nonradiative recombination has been shown to limit the efficiency of optoelectronic devices. While substitutional carbon ($\mathrm{C_N}$) has been suggested to be a nonradiative recombination center in GaN devices, a complete recombination cycle including the two charge-state transition levels has not been previously described. In this work, we investigate the trap-assisted recombination process due to $\mathrm{C_N}$ in GaN, including multiphonon emission (MPE), radiative recombination, trap-assisted Auger-Meitner (TAAM) recombination, as well as thermal emission of holes. Our study shows the key role of TAAM processes at the high carrier densities relevant for devices. We also reveal the carrier-density regimes where thermal emission and radiative recombination are expected to play an observable role. Our results highlight that carbon concentrations exceeding $\sim$10$^{17}$ cm$^{-3}$ can have a noticeable impact on device efficiency, not just in GaN active layers but also in InGaN and AlGaN. Our comprehensive formalism not only offers detailed results for carbon but provides a general framework for assessing the multiple processes that participate in trap-assisted recombination in semiconductors. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Exceptionally High Nonlinear Optical Response in Two-dimensional Type II Dirac Semimetal Nickel di-Telluride (NiTe2)

Authors: Saswata Goswami, Caique Campos de Oliveira, Bruno Ipaves, Preeti Lata Mahapatra, Varinder Pal, Suman Sarkar, Pedro A. S. Autreto, Samit K. Ray, Chandra Sekhar Tiwary

Abstract: Nickel ditelluride (NiTe2) is a newly identified Type-II Dirac semimetal, showing novel characteristics in electronic transport and optical experiments. In this study, we explored the nonlinear optical properties of two-dimensional NiTe2 using experimental and computational techniques (density functional theory-based approach). Few layered two-dimensional NiTe2 (2D-NiTe2) are synthesized using liq… ▽ More Nickel ditelluride (NiTe2) is a newly identified Type-II Dirac semimetal, showing novel characteristics in electronic transport and optical experiments. In this study, we explored the nonlinear optical properties of two-dimensional NiTe2 using experimental and computational techniques (density functional theory-based approach). Few layered two-dimensional NiTe2 (2D-NiTe2) are synthesized using liquid phase exfoliation (LPE), which is characterized using X-ray diffraction technique, transmission electron, and atomic force microscopy. The nonlinear refractive index and third-order nonlinear susceptibility of the prepared 2D-NiTe2 are determined from the self-induced diffraction pattern generated using different wavelengths ( 405, 532, and 650 nm) in the far field. In addition, the diffraction pattern generated by spatial self-phase modulation (SSPM) is further verified by varying concentration (2D-NiTe2 in the IPA solvent), wavelength (of incoming laser beams), and cuvette width (active path length). The high value of third-order nonlinear susceptibility (in order of 10-9 e.s.u.) determined using SSPM in the 2D-NiTe2 can be attributed to the laser-induced hole coherence effect. Lastly, utilizing the reverse saturable absorption property of 2D-hBN, asymmetric light propagation is also demonstrated in the 2D-NiTe2/2D-hBN heterostructure. △ Less

Submitted 15 February, 2025; originally announced February 2025.

Comments: 56 pages, 19 figures

NextGenPB: an analytically-enabled super resolution and local (de)refinement Poisson-Boltzmann Equation solver

Authors: Vincenzo Di Florio, Patrizio Ansalone, Sergii V. Siryk, Sergio Decherchi, Carlo de Falco, Walter Rocchia

Abstract: The Poisson-Boltzmann equation (PBE) is a relevant partial differential equation commonly used in biophysical applications to estimate the electrostatic energy of biomolecular systems immersed in electrolytic solutions. A conventional mean to improve the accuracy of its solution, when grid-based numerical techniques are used, consists in increasing the resolution, locally or globally. This, howeve… ▽ More The Poisson-Boltzmann equation (PBE) is a relevant partial differential equation commonly used in biophysical applications to estimate the electrostatic energy of biomolecular systems immersed in electrolytic solutions. A conventional mean to improve the accuracy of its solution, when grid-based numerical techniques are used, consists in increasing the resolution, locally or globally. This, however, usually entails higher complexity, memory demand and computational cost. Here, we introduce NextGenPB, a linear PBE, adaptive-grid, FEM solver that leverages analytical calculations to maximize the accuracy-to-computational-cost ratio. Indeed, in NextGenPB (aka NGPB), analytical corrections at the surface of the solute enhance the solution's accuracy without requiring grid adaptation. This leads to more precise estimates of the electrostatic potential, fields, and energy at no perceptible additional cost. Also, we apply computationally efficient yet accurate boundary conditions by taking advantage of local grid de-refinement. To assess the accuracy of our methods directly, we expand the traditionally available analytical case set to many non-overlapping dielectric spheres. Then, we use an existing benchmark set of real biomolecular systems to evaluate the energy convergence concerning grid resolution. Thanks to these advances, we have improved state-of-the-art results and shown that the approach is accurate and largely scalable for modern high-performance computing architectures. Lastly, we suggest that the presented core ideas could be instrumental in improving the solution of other partial differential equations with discontinuous coefficients. △ Less

Submitted 13 February, 2025; originally announced February 2025.

Comments: 54 pages, 14 figures

ACM Class: J.2

Journal ref: Computer Physics Communications, 2025, Vol. 317, 109816

Distinguishing thermal fluctuations from polaron formation in halide perovskites

Authors: Bai-Qing Zhao, Xuan-Yan Chen, Chuan-Nan Li, Jinshan Li, Chris G. Van de Walle, Xie Zhang

Abstract: Recent angle-resolved photoelectron spectroscopy (ARPES) measurements of the hole effective mass in CsPbBr$_3$ revealed an enhancement of $\sim$50 % compared to the bare mass computed from first principles for CsPbBr$_3$ at $T = 0 K$. This large enhancement was interpreted as evidence of polaron formation. Employing accurate finite-temperature first-principles calculations, we show that the calcul… ▽ More Recent angle-resolved photoelectron spectroscopy (ARPES) measurements of the hole effective mass in CsPbBr$_3$ revealed an enhancement of $\sim$50 % compared to the bare mass computed from first principles for CsPbBr$_3$ at $T = 0 K$. This large enhancement was interpreted as evidence of polaron formation. Employing accurate finite-temperature first-principles calculations, we show that the calculated hole effective mass of CsPbBr$_3$ at $T = 300 K$ can explain experimental results without invoking polarons. Thermal fluctuations are particularly strong in halide perovskites compared to conventional semiconductors such as Si and GaAs, and cannot be ignored when comparing with experiment. We not only resolve the debate on polaron formation in halide perovskites, but also demonstrate the general importance of including thermal fluctuations in first-principles calculations for strongly anharmonic materials. △ Less

Submitted 11 February, 2025; v1 submitted 9 February, 2025; originally announced February 2025.

Duality breaking, mobility edges, and the connection between topological Aubry-André and quantum Hall insulators in atomic wires with fermions

Authors: Bar Alluf, C. A. R. Sa de Melo

Abstract: It is well known that the Aubry-Andr{é} model lacks mobility edges due to its energy-independent self-duality but may exhibit edge states. When duality is broken, we show that mobility regions arise and non-trivial topological phases emerge. By varying the degree of duality breaking, we identify mobility regions and establish a connection between Aubry-Andr{é} atomic wires with fermions and quantu… ▽ More It is well known that the Aubry-Andr{é} model lacks mobility edges due to its energy-independent self-duality but may exhibit edge states. When duality is broken, we show that mobility regions arise and non-trivial topological phases emerge. By varying the degree of duality breaking, we identify mobility regions and establish a connection between Aubry-Andr{é} atomic wires with fermions and quantum Hall systems for a family of Hamiltonians that depends on the relative phase of laser fields, viewed as a synthetic dimension. Depending on the filling factor and the degree of duality breaking, we find three classes of non-trivial phases: conventional topological insulator, conventional topological Aubry-Andr{é} insulator, and unconventional (hybrid) topological Aubry-Andr{é} insulator. Finally, we discuss appropriate Chern numbers that illustrate the classification of topological phases of localized fermions in atomic wires. △ Less

Submitted 26 February, 2025; v1 submitted 22 January, 2025; originally announced January 2025.

Comments: 16 pages, 14 figures

Report number: 023322

Characterization of Chromium Impurities in $β$-Ga$_2$O$_3$

Authors: Mark E. Turiansky, Sai Mu, Lukas Razinkovas, Kamyar Parto, Sahil D. Patel, Sean Doan, Ganesh Pokharel, Steven J. Gomez Alvarado, Stephen D. Wilson, Galan Moody, Chris G. Van de Walle

Abstract: Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al$_2$O$_3$, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behav… ▽ More Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al$_2$O$_3$, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behavior of Cr in Ga$_2$O$_3$ through theoretical and experimental techniques. $β$-Ga$_2$O$_3$ samples are grown with the floating zone method and show evidence of a sharp photoluminescence signal, reminiscent of ruby. We calculate the energetics of formation of Cr from first principles, demonstrating that Cr preferentially incorporates as a neutral impurity on the octahedral site. Cr possesses a quartet ground-state spin and has an internal transition with a zero-phonon line near 1.8 eV. By comparing the calculated and experimentally measured luminescence lineshape function, we elucidate the role of coupling to phonons and uncover features beyond the Franck-Condon approximation. The combination of strong emission with a small Huang-Rhys factor of 0.05 and a technologically relevant host material render Cr in Ga$_2$O$_3$ attractive as a quantum defect. △ Less

Submitted 31 December, 2024; originally announced January 2025.

Heterostructure Engineering for Wurtzite LaN

Authors: Andrew J. E. Rowberg, Sai Mu, Chris G. Van de Walle

Abstract: Wurtzite LaN (wz-LaN) is a semiconducting nitride with favorable piezoelectric and ferroelectric properties, making it promising for applications in electronics. We use first-principles density functional theory with a hybrid functional to investigate several features that are key for its use in heterostructures. First, for the purposes of growing wz-LaN on a substrate or designing a heterostructu… ▽ More Wurtzite LaN (wz-LaN) is a semiconducting nitride with favorable piezoelectric and ferroelectric properties, making it promising for applications in electronics. We use first-principles density functional theory with a hybrid functional to investigate several features that are key for its use in heterostructures. First, for the purposes of growing wz-LaN on a substrate or designing a heterostructure, we show that it can be lattice-matched with a number of cubic materials along their [111] axes. We also evaluate the bound charge at such interfaces, taking into account both the polarization discontinuity and the piezoelectric polarization due to pseudomorphic strain. Second, we investigate band alignments and assess the results for interfaces with zincblende-, rocksalt-, and perovskite-structure compounds, and with chemically similar wurtzite and rocksalt nitrides. Our results provide guidance for the development of electronic devices based on wz-LaN. △ Less

Submitted 30 December, 2024; originally announced December 2024.

Comments: 9 pages, 3 figures

Journal ref: J. Chem. Phys. 162, 194713 (2025)

First-principles theory of direct-gap optical emission in hexagonal Ge and its enhancement via strain engineering

Authors: Christopher A. Broderick, Xie Zhang, Mark E. Turiansky, Chris G. Van de Walle

Abstract: The emergence of hexagonal Ge (2H-Ge) as a candidate direct-gap group-IV semiconductor for Si photonics mandates rigorous understanding of its optoelectronic properties. Theoretical predictions of a "pseudo-direct" band gap, characterized by weak oscillator strength, contrast with a claimed high radiative recombination coefficient $B$ comparable to conventional (cubic) InAs. We compute $B$ in 2H-G… ▽ More The emergence of hexagonal Ge (2H-Ge) as a candidate direct-gap group-IV semiconductor for Si photonics mandates rigorous understanding of its optoelectronic properties. Theoretical predictions of a "pseudo-direct" band gap, characterized by weak oscillator strength, contrast with a claimed high radiative recombination coefficient $B$ comparable to conventional (cubic) InAs. We compute $B$ in 2H-Ge from first principles and quantify its dependence on temperature, carrier density and strain. For unstrained 2H-Ge, our calculated spontaneous emission spectra corroborate that measured photoluminescence corresponds to direct-gap emission, but with $B$ being approximately three orders of magnitude lower than in InAs. We confirm a pseudo-direct- to direct-gap transition under $\sim 2$\% [0001] uniaxial tension, which can enhance $B$ by up to three orders of magnitude, making it comparable to that of InAs. Beyond quantifying strong enhancement of $B$ via strain engineering, our analysis suggests the dominance of additional, as-yet unquantified recombination mechanisms in this nascent material. △ Less

Submitted 16 December, 2024; v1 submitted 11 December, 2024; originally announced December 2024.

Interacting Virtual Topological Phases in Defect-Rich 2D Materials

Authors: Felipe Crasto de Lima, Roberto H. Miwa, Caio Lewenkopf, Adalberto Fazzio

Abstract: We investigate the robustness of {\it virtual} topological states -- topological phases away from the Fermi energy -- against the electron-electron interaction and band filling. As a case study, we employ a realistic model to investigate the properties of vacancy-driven topological phases in transition metal dichalcogenides (TMDs) and establish a connection between the degree of localization of to… ▽ More We investigate the robustness of {\it virtual} topological states -- topological phases away from the Fermi energy -- against the electron-electron interaction and band filling. As a case study, we employ a realistic model to investigate the properties of vacancy-driven topological phases in transition metal dichalcogenides (TMDs) and establish a connection between the degree of localization of topological wave functions, the vacancy density, and the electron-electron interaction strength with the topological phase robustness. We demonstrate that electron-electron interactions play a crucial role in degrading topological phases thereby determining the validity of single-particle approximations for topological insulator phases. Our findings can be naturally extended to {\it virtual} topological phases of a wide range of materials. △ Less

Submitted 8 May, 2025; v1 submitted 11 December, 2024; originally announced December 2024.

Journal ref: Phys. Rev. B 111, 195135 (2025)

Magnetic properties and growth kinetics of Co/Gd bilayers with perpendicular magnetic anisotropy

Authors: T. J. Kools, J. Hintermayr, Y. L. W. van Hees, K. Poissonnier, M. C. H. de Jong, J. T. J. M. Janssen, B. Koopmans, R. Lavrijsen

Abstract: Ultrathin 3d-4f synthetic ferrimagnets with perpendicular magnetic anisotropy (PMA) exhibit a range of intriguing magnetic phenomena, including all-optical switching of magnetization (AOS), fast current-induced domain wall motion (CIDWM), and the potential to act as orbital-to-spin angular momentum converters. For spintronic applications involving these materials, the Curie temperature is a crucia… ▽ More Ultrathin 3d-4f synthetic ferrimagnets with perpendicular magnetic anisotropy (PMA) exhibit a range of intriguing magnetic phenomena, including all-optical switching of magnetization (AOS), fast current-induced domain wall motion (CIDWM), and the potential to act as orbital-to-spin angular momentum converters. For spintronic applications involving these materials, the Curie temperature is a crucial factor in determining not only the threshold energy for AOS, but also the material's resistance to temperature rise during CIDWM. However, the relationship between the Curie temperature, the thicknesses of the individual layers, and the specifics of the growth process remains an open question. In this work, we thoroughly investigate the Curie temperature of one of the archetype synthetic ferrimagnets with PMA, the Pt/Co/Gd trilayer, grown by DC magnetron sputtering and characterized with MOKE and SQUID. We provide an interpretation of the experiments we designed to address these outstanding questions through modeling of the deposition process and the induced magnetization at the Co/Gd interface. Our findings demonstrate that the Curie temperature and, by extension, the conditions for PMA and magnetic compensation, of these ultrathin 3d-4f synthetic ferrimagnets are not only impacted by the interface quality, which can be influenced by the sputtering process, but also to a significant extent by finite-size effects in the 4f-material. This work offers new methods and understanding to predict and manipulate the critical temperature and magnetostatic properties of 3d-4f synthetic ferrimagnets for spintronic applications and magneto-photonic integration. △ Less

Submitted 4 December, 2024; originally announced December 2024.

Effective dimensional reduction of complex systems based on tensor networks

Authors: Wout Merbis, Madelon Geurts, Clélia de Mulatier, Philippe Corboz

Abstract: The exact treatment of Markovian models of complex systems requires knowledge of probability distributions exponentially large in the number of components $n$. Mean-field approximations provide an effective reduction in complexity of the models, requiring only a number of phase space variables polynomial in system size. However, this comes at the cost of losing accuracy close to critical points in… ▽ More The exact treatment of Markovian models of complex systems requires knowledge of probability distributions exponentially large in the number of components $n$. Mean-field approximations provide an effective reduction in complexity of the models, requiring only a number of phase space variables polynomial in system size. However, this comes at the cost of losing accuracy close to critical points in the systems dynamics and an inability to capture correlations in the system. In this work, we introduce a tunable approximation scheme for Markovian spreading models on networks based on Matrix Product States (MPS). By controlling the bond dimensions of the MPS, we can investigate the effective dimensionality needed to accurately represent the exact $2^n$ dimensional steady-state distribution. We introduce the entanglement entropy as a measure of the compressibility of the system and find that it peaks just after the phase transition on the disordered side, in line with the intuition that more complex states are at the 'edge of chaos'. We compare the accuracy of the MPS with exact methods on different types of small random networks and with Markov Chain Monte Carlo methods for a simplified version of the railway network of the Netherlands with 55 nodes. The MPS provides a systematic way to tune the accuracy of the approximation by reducing the dimensionality of the systems state vector, leading to an improvement over second-order mean-field approximations for sufficiently large bond dimensions. △ Less

Submitted 11 March, 2025; v1 submitted 20 November, 2024; originally announced November 2024.

Comments: 28 pages, 12 figures, v2: version accepted in JPhys: Complexity

Interface second harmonic generation enhancement in hetero-bilayer van der Waals nanoantennas

Authors: Andrea Tognazzi, Paolo Franceschini, Jonas Biechteler, Enrico Baù, Alfonso Carmelo Cino, Andreas Tittl, Costantino De Angelis, Luca Sortino

Abstract: Layered van der Waals (vdW) materials have emerged as a promising platform for nanophotonics due to large refractive indexes and giant optical anisotropy. Unlike conventional dielectrics and semiconductors, the absence of covalent bonds between layers allows for novel degrees of freedom in designing optically resonant nanophotonic structures down to the atomic scale, from the precise stacking of v… ▽ More Layered van der Waals (vdW) materials have emerged as a promising platform for nanophotonics due to large refractive indexes and giant optical anisotropy. Unlike conventional dielectrics and semiconductors, the absence of covalent bonds between layers allows for novel degrees of freedom in designing optically resonant nanophotonic structures down to the atomic scale, from the precise stacking of vertical heterostructures to controlling the twist angle between crystallographic axes. Specifically, while transition metal dichalcogenides monolayers exhibit giant second order nonlinear responses, their bulk counterparts with 2H stacking have zero second order response. In this work, we show second harmonic generation (SHG) arising from the interface of WS$_2$/MoS$_2$ hetero-bilayer thin films with an additional SHG enhancement in nanostructured optical antennas mediated by both the excitonic resonances and the anapole condition. When both conditions are met, we observe up to $10^2$ SHG signal enhancement. Our results highlights vdW materials as a platform for designing unique multilayer optical nanostructures and metamaterial, paving the way for advanced applications in nanophotonics and nonlinear optics. △ Less

Submitted 9 November, 2024; originally announced November 2024.

Comments: Manuscript + Supplementary (21 pages, 3 Main figures 8 supplementary figures)

Journal ref: Light: Science & Applications, 14, 346 (2025)