Found 24 papers in cond-mat
Date of feed: Mon, 08 Jan 2024 01:30:00 GMT

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Stiffer alginate gels deposit more efficiently in microchannel flows. (arXiv:2401.02530v1 [cond-mat.soft])
Barrett T Smith, Sara M Hashmi

The behavior of crosslinking polymer solutions as they transition from liquid-like to solid-like material in flow determines success or failure in several applications, from 3D printing to oil recovery in the earth's subsurface to a wide variety of biological flows. Dilute polymer solutions flow easily, while concentrated polymers or crosslinked polymer gels can clog pores, nozzles, or channels. We have recently uncovered and described a third regime of flow dynamics in polymers that occurs when crosslinking happens during flow: intermittent flows. In a model system of alginate and calcium meeting at a Y-shaped junction in a microfluidic channel, a persistent and regular pattern of intermittent flow occurs when driven at a constant volume flow rate. At the junction, calcium crosslinks alginate to form an alginate gel, which subsequently deposits on the channel wall. As gel continues to deposit, it obstructs the channel, causing the driving pressure to increase to maintain a constant flow rate. At a critical pressure, corresponding to a critical shear stress, the fluid pulls the gel from the wall, removing the gel from the device and resulting in a clear channel. The gel deposit begins again, and the process then repeats as long as flow continues. Chemical concentrations and flow rate control both the frequency of ablation and the critical shear stress. In this work, we provide an analytical framework to quantitatively describe the intermittent behavior as a result of diffusively driven deposition in a high Peclet number flow where convection dominates. Fitting the experimental data to the model allows estimation of the deposition efficiency, or the fraction of flowing material that sticks to the channel walls. By correlating the results of the model with bulk rheology measurements, we find that deposition efficiency increases with the stiffness of the gel formed in flow.

On topological order in higher composites. (arXiv:2401.02551v1 [cond-mat.quant-gas])
Egor Babaev

The recent experiments reported observation of a state with symmetry-breaking appearing at the level of four-electron order parameter (electronic quadruplets condensation) in a multicomponent system. This is in contrast to the conventional case where order appears at the level of electron pairing fields. Related states were theoretically demonstrated in mixtures of ultracold atoms.

Here, we discuss the topological counterparts of this concept, i.e., topological order appearing only in higher-than-usual composites under somewhat related circumstances in multicomponent systems.

Distinguishing the Topological Charge of Vortex Beam via Fourier Back Plane Imaging with Chiral Gammadion Structure. (arXiv:2401.02587v1 [physics.optics])
Yangzhe Guo, Jing Li, Yurui Fang

In recent years, research on the interaction between Orbital Angular Momentum (OAM) and matter has seen a continuous influx of investigations. OAM possesses distinct properties, such as additional degrees of freedom, vortex characteristics, and topological properties, which expand its applications in optical communication, optical sensing, and optical force. Through experiments involving the interaction of a chiral metal swastika structure with a SAM-OAM beam generated by a q-plate, we have observed a phenomenon does not present in pure SAM beams. Fourier back focal plane (FBP) imaging under SAM beam excitation easily identifies the chirality and geometric properties of the structure. When the SAM-OAM beam excites the structure, FBP not only identifies its chirality and geometric properties but also distinguishes different OAM topological charges and signs, as well as the degree of elliptic polarization. The stokes parametric FBP imaging reveals asymmetric polarization distribution resulting from the interaction between a vortex beam and the chiral structure. Moreover, it clearly reflects the conversion process of SAM to OAM. The experimental results align well with simulation results. These findings hold valuable insights for the advancement of optical information storage and communication using OAM, opening up new possibilities for further exploration in this field.

Stability of Hydrides in Sub-Neptune Exoplanets with Thick Hydrogen-Rich Atmospheres. (arXiv:2401.02637v1 [astro-ph.EP])
Taehyun Kim, Xuehui Wei, Stella Chariton, Vitali B. Prakapenka, Young-Jay Ryu, Shize Yang, Sang-Heon Shim

Many sub-Neptune exoplanets have been believed to be composed of a thick hydrogen-dominated atmosphere and a high-temperature heavier-element-dominant core. From an assumption that there is no chemical reaction between hydrogen and silicates/metals at the atmosphere-interior boundary, the cores of sub-Neptunes have been modeled with molten silicates and metals (magma) in previous studies. In large sub-Neptunes, pressure at the atmosphere-magma boundary can reach tens of gigapascals where hydrogen is a dense liquid. A recent experiment showed that hydrogen can induce the reduction of Fe$^{2+}$ in (Mg,Fe)O to Fe$^0$ metal at the pressure-temperature conditions relevant to the atmosphere-interior boundary. However, it is unclear if Mg, one of the abundant heavy elements in the planetary interiors, remains oxidized or can be reduced by H. Our experiments in the laser-heated diamond-anvil cell found that heating of MgO + Fe to 3500-4900 K (close to or above their melting temperatures) in a H medium leads to the formation of Mg$_2$FeH$_6$ and H$_2$O at 8-13 GPa. At 26-29 GPa, the behavior of the system changes, and Mg-H in an H fluid and H$_2$O were detected with separate FeH$_x$. The observations indicate the dissociation of the Mg-O bond by H and subsequent production of hydride and water. Therefore, the atmosphere-magma interaction can lead to a fundamentally different mineralogy for sub-Neptune exoplanets compared with rocky planets. The change in the chemical reaction at the higher pressures can also affect the size demographics (i.e., "radius cliff") and the atmosphere chemistry of sub-Neptune exoplanets.

Scaling Laws Governing the Elastic Properties of 3D-Graphenes. (arXiv:2401.02689v1 [cond-mat.mtrl-sci])
Ming Li, Guo Lu, Haodong Yu, Menglei Li, Fawei Zheng

In this study, we have comprehensively investigated the scaling law for elastic properties of three-dimensional honeycomb-like graphenes (3D-graphenes) using hybrid neural network potential based molecular dynamics simulations and theoretical analyses. The elastic constants as functions of honeycomb hole size, denoted by the graphene wall length $L$, were provided. All five independent elastic constants in the large $L$ limit are proportional to $L^{-1}$. The associated coefficients are combinations of two-dimensional graphene's elastic constants. High-order terms including $L^{-2}$ and $L^{-3}$ emerge for finite $L$ values. They have three origins, the distorted areas close to the joint lines of 3D-graphenes, the variation of solid angles between graphene plates, and the bending distortion of graphene plates. Significantly, the chirality becomes essential with the decreasing of $L$, because the joint line structures are different between the armchair and zigzag type 3D-graphenes. Our findings provide insights into the elastic properties of graphene-based superstructures and can be used for further studies on graphene-based materials.

Single Crystal Growth and Transport Properties of van der Waals Materials $AB$Te$_\mathbf{4}$ ($A/B$ = Ti, Zr, Hf). (arXiv:2401.02704v1 [cond-mat.mtrl-sci])
Yuto Hasuo, Takahiro Urata, Masaaki Araidai, Yuji Tsuchiya, Satoshi Awaji, Hiroshi Ikuta

Monolayers of $AB$Te$_4$ ($A$/$B$ = Ti, Zr, Hf) were theoretically predicted to be two-dimensional topological insulators, but little has been known about the physical properties of these compounds. Here, we report on the single crystal growth, bulk transport properties, and band structure calculations of these compounds. The magnetotransport properties indicate that all three compounds are multi-carrier systems. The experimental results of ZrTiTe$_4$ and HfTiTe$_4$ can be well fitted by the multi-carrier formula assuming two types of carriers, while three carrier components were necessary for HfZrTe$_4$. Interestingly, one of the carrier mobilities of HfZrTe$_4$ exceeded 1000 cm$^2$V$^{-1}$s$^{-1}$, which was nearly one order in magnitude larger than the carrier mobilities of ZrTiTe$_4$ and HfTiTe$_4$. Our band structure calculations showed that all three compounds are semimetals consistent with the magnetotransport properties. The band structure around the $\Gamma$-point of HfZrTe$_4$ exhibits features that are distinct from the other two compounds, which is likely the reason of the different carrier properties.

Beyond symmetry-protected BICs: transmission through asymmetric crossbar junctions in one-dimensional waveguides. (arXiv:2401.02707v1 [physics.optics])
Sofía Pinto, Rafael A. Molina, Pedro A. Orellana

Over the last few decades, the study of Bound States in the Continuum, their formation, and properties has attracted lots of attention, especially in optics and photonics. It is particularly noticeable that most of these investigations base their studies on symmetric systems. In this article, we study the formation of bound states in the continuum in electronic and photonic transport systems consisting of crossbar junctions formed by one-dimensional waveguides, considering asymmetric junctions with commensurable lengths for the upper and lower arms. We also study how BICs form in linear junction arrays as a function of the distance between consecutive junctions and their commensurability with the upper and lower arms. We solve the Helmholtz equation for the crossbar junctions and calculate the transmission probability, probability density in the intersections, and quality factor. The presence of quasi-BICs is reflected in the transmission probability as a sharp resonance in the middle of a symmetric Fano resonance along with Dirac's delta functions in the probability density and divergence in the quality factors.

Distance dependence of the energy transfer mechanism in WS$_2$-graphene heterostructures. (arXiv:2401.02716v1 [cond-mat.mes-hall])
David Tebbe, Marc Schütte, K. Watanabe, T. Taniguchi, Christoph Stampfer, Bernd Beschoten, Lutz Waldecker

We report on the mechanism of energy transfer in van der Waals heterostructures of the two-dimensional semiconductor WS$_2$ and graphene with varying interlayer distances, achieved through spacer layers of hexagonal boron nitride (hBN). We record photoluminescence and reflection spectra at interlayer distances between 0.5 nm and 5.8 nm (0-16 hBN layers). We find that the energy transfer is dominated by states outside the light cone, indicative of a F\"orster transfer process, with an additional contribution from a Dexter process at 0.5 nm interlayer distance. We find that the measured dependence of the luminescence intensity on interlayer distances above 1 nm can be quantitatively described using recently reported values of the F\"orster transfer rates of thermalized charge carriers. At smaller interlayer distances, the experimentally observed transfer rates exceed the predictions and furthermore depend on excess energy as well as on excitation density. Since the transfer probability of the F\"orster mechanism depends on the momentum of electron-hole pairs, we conclude that at these distances, the transfer is driven by non-thermalized charge carrier distributions.

Compression-induced crossovers for the ground-state of classical dipole lattices on a M\"obius strip. (arXiv:2401.02748v1 [cond-mat.mes-hall])
Ansgar Siemens (1), Felipe Augusto Oliveira Silveira (2), Peter Schmelcher (1 and 3) ((1) Zentrum für Optische Quantentechnologien, Fachbereich Physik, Universität Hamburg, (2) Departamento de Física, UNESP - Universidade Estadual Paulista, (3) Hamburg Center for Ultrafast Imaging, Universität Hamburg)

We explore the ground state properties of a lattice of classical dipoles spanned on the surface of a M\"{o}bius strip. The dipole equilibrium configurations depend significantly on the geometrical parameters of the M\"{o}bius strip, as well as on the lattice dimensions. As a result of the variable dipole spacing on the curved surface of the M\"{o}bius strip, the ground state can consist of multiple domains with different dipole orientations which are separated by domain walls. We analyze in particular the dependence of the ground state dipole configuration on the width of the M\"{o}bius strip and highlight two crossovers in the ground state that can be correspondingly tuned. A first crossover changes the dipole lattice from a phase which resists compression to a phase that favors it. The second crossover leads to an exchange of the topological properties of the two involved domains. We conclude with a brief summary and an outlook on more complex topologically intricate surfaces.

Dirac particle under dynamical confinement: Fermi acceleration, trembling motion and quantum force. (arXiv:2401.02837v1 [quant-ph])
J. Dittrich, S. Rakhmanov, D. Matrasulov

Quantum dynamics of a Dirac particle in a 1D box with moving wall is studied. Dirac equation with time-dependent boundary condition is mapped onto that with static one, but with time-dependent mass. Exact analytical solution of such modified Dirac equation is obtained for massless particle. For massive particle the problem is solved numerically. Time-dependences of the main characteristics of the dynamical confinement, such as average kinetic energy and quantum force are analyzed. It is found that the average kinetic energy remains bounded for the interval length bounded from below, in particular for the periodically oscillating wall.

Spin-1/2 kagome Heisenberg antiferromagnet: Machine learning discovery of the spinon pair density wave ground state. (arXiv:2401.02866v1 [cond-mat.str-el])
Tanja Duric, Jia Hui Chung, Bo Yang, Pinaki Sengupta

Spin-1/2 kagome antiferromagnet (AFM) is one of the most studied models in frustrated magnetism since it is a promising candidate to host exotic spin liquid states. However, despite numerous studies using both analytical and numerical approaches, the nature of the ground state and low-energy excitations in this system remain elusive. This is related to the difficulty in determining the spin gap in various calculations. We present the results of our investigation of the Kagome AFM using the recently developed group equivariant convolutional neural networks - an advanced machine learning technique for studying strongly frustrated models. The approach, combined with variational Monte Carlo, introduces significant improvement of the achievable results accuracy in comparison with approaches based on other neural network architectures that lack generalization quality for frustrated spin systems. Contrary to the results obtained previously with various methods, that predicted Z_2 or U(1) Dirac spin liquid states, our results strongly indicate that the ground state of the kagome lattice antiferromagnet is a spinon pair density wave that does not break time-reversal symmetry or any of the lattice symmetries. The found state appears due to the spinon Cooper pairing instability close to two Dirac points in the spinon energy spectrum and resembles the pair density wave state studied previously in the context of underdoped cuprate superconductors in connection with the pseudogap phase. The state has significantly lower energy than the lowest energy states found by the SU(2) symmetric density matrix renormalization group calculations and other methods.

GW Theory of Magic-Angle Twisted Bilayer Graphene. (arXiv:2401.02872v1 [cond-mat.str-el])
Jihang Zhu, Iacopo Torre, Marco Polini, Allan H. MacDonald

Strong correlations occur in magic-angle twisted bilayer graphene (MATBG) when the octet of flat moir\'e minibands centered on charge neutrality (CN) is partially occupied. The octet consists of a single valence band and a single conduction band for each of four degenerate spin-valley flavors. Motivated by the importance of Hartree electrostatic interactions in determining the filling-factor dependent band structure, we use a time-dependent Hartree (GW) approximation to gain insight into electronic correlations. We find that the electronic compressibility is dominated by Hartree interactions, that paramagnetic states are stable over a range of density near CN, and that the dependence of energy on flavor polarization is strongly overestimated by mean-field theory.

Solutions to the Landau-Lifshitz-Gilbert equation in the frequency space: Discretization schemes for the dynamic-matrix approach. (arXiv:2401.02933v1 [physics.comp-ph])
D. E. Gonzalez-Chavez, G. P. Zamudio, R. L. Sommer

The dynamic-matrix method addresses the Landau-Lifshitz-Gilbert (LLG) equation in the frequency domain by transforming it into an eigenproblem. Subsequent numerical solutions are derived from the eigenvalues and eigenvectors of the dynamic-matrix. In this work we explore discretization methods needed to obtain a numerical representation of the dynamic-operator, a foundational counterpart of the dynamic-matrix. Our approach opens a new set of linear algebra tools for the dynamic-matrix method and expose the approximations and limitations intrinsic to it. We present some application examples, including a technique to obtain the dynamical matrix directly from the magnetic free energy function of an ensemble of macrospins, and an algorithmic method to calculate numerical micromagnetic kernels, including plane wave kernels. Additionally, we also show how to exploit symmetries and reduce the numerical size of micromagnetic dynamic-matrix problems by a change of basis. This work contributes to the understanding of the current magnetization dynamics methods, and could help the development and formulations of novel analytical and numerical methods for solving the LLG equation within the frequency domain.

Temperature dependence of microwave losses in lumped-element resonators made from superconducting nanowires with high kinetic inductance. (arXiv:2401.02943v1 [cond-mat.mes-hall])
Ermes Scarano, Elisabet K. Arvidsson, August K. Roos, Erik Holmgren, David B. Haviland

We study the response of several microwave resonators made from superconducting NbTiN thin-film meandering nanowires with large kinetic inductance, having different circuit topology and coupling to the transmission line. Reflection measurements reveal the parameters of the circuit and analysis of their temperature dependence in the range 1.7-6 K extract the superconducting energy gap and critical temperature. The lumped-element LC resonator, valid in our frequency range of interest, allows us to predict the quasiparticle contribution to internal loss, independent of circuit topology and characteristic impedance. Our analysis shows that the internal quality factor is limited not by thermal-equilibrium quasiparticles, but an additional temperature-dependent source of internal microwave loss.

Electrical Seebeck-Contrast Observation of Magnon Hall Effect in Topological Ferromagnet Lu$_2$V$_2$O$_7$/Heavy Metal Heterostructures. (arXiv:2210.06606v2 [cond-mat.mtrl-sci] UPDATED)
Jinsong Xu, Jiaming He, J.-S. Zhou, Danru Qu, Ssu-Yen Huang, C. L. Chien

The observation of the magnon Hall effect (MHE) has relied solely on the challenging measurement of the thermal Hall conductivity. Here, we report a highly sensitive electrical Seebeck-contrast method for the observation of MHE in Lu$_2$V$_2$O$_7$/heavy metal heterostructures, that is highly desirable for the exploration of new MHE materials and their applications. Using measuring wires with very different Seebeck coefficients, we established a general method that can separate contributions (e.g., MHE) that generates a lateral temperature drop, from those [e.g., anomalous Nernst effect (ANE) and spin Seebeck effect (SSE)] that generate a lateral electric field. We show that a suitable heavy metal overlayer can eliminate the inherent ANE and SSE signals from the semiconducting Lu$_2$V$_2$O$_7$. The MHE in Lu$_2$V$_2$O$_7$ is quasi-isotropic among crystals with different orientations. In addition to the previously reported transverse MHE under an in-plane temperature gradient, we have uncovered longitudinal MHE under an out-of-plane temperature gradient.

New type of helical topological superconducting pairing at finite excitation energies. (arXiv:2210.11955v4 [cond-mat.mes-hall] UPDATED)
Masoud Bahari, Song-Bo Zhang, Chang-An Li, Sang-Jun Choi, Carsten Timm, Björn Trauzettel

We propose a new type of helical topological superconductivity away from the Fermi surface in three-dimensional time-reversal-symmetric odd-parity multiband superconductors. In these systems, pairing between electrons originating from different bands is responsible for the corresponding topological phase transition. Consequently, a pair of helical topological Dirac surface states emerges at finite excitation energies. These helical Dirac surface states are tunable in energy by chemical potential and strength of band-splitting. They are protected by time-reversal symmetry combined with crystalline two-fold rotation symmetry. We suggest concrete materials in which this phenomenon could be observed.

Realization of all-band-flat photonic lattices. (arXiv:2305.05906v2 [physics.optics] UPDATED)
Jing Yang, Yuanzhen Li, Yumeng Yang, Xinrong Xie, Zijian Zhang, Jiale Yuan, Han Cai, Da-Wei Wang, Fei Gao

Flatbands play an important role in correlated quantum matter and have novel applications in photonic lattices. Synthetic magnetic fields and destructive interference in lattices are traditionally used to obtain flatbands. However, such methods can only obtain a few flatbands with most bands remaining dispersive. Here we realize all-band-flat photonic lattices of an arbitrary size by precisely controlling the coupling strengths between lattice sites to mimic those in Fock-state lattices. This allows us to go beyond the perturbative regime of strain engineering and group all eigenmodes in flatbands, which simultaneously achieves high band flatness and large usable bandwidth. We map out the distribution of each flatband in the lattices and selectively excite the eigenmodes with different chiralities. Our method paves a new way in controlling band structure and topology of photonic lattices.

The BHL-BCL crossover: from nonlinear to linear quantum amplification. (arXiv:2306.05458v3 [cond-mat.quant-gas] UPDATED)
Juan Ramón Muñoz de Nova, Fernando Sols

The black-hole laser (BHL) effect is the self-amplification of Hawking radiation between a pair of horizons which act as a resonant cavity. In a flowing atomic condensate, the BHL effect arises in a finite supersonic region, where Bogoliubov-Cherenkov-Landau (BCL) radiation is resonantly excited by any static perturbation. Thus, experimental attempts to produce a BHL unavoidably deal with the presence of a strong BCL background, making the observation of the BHL effect still a major challenge in the analogue gravity field. Here, we perform a theoretical study of the BHL-BCL crossover using an idealized model where both phenomena can be unambiguously isolated. By drawing an analogy with an unstable pendulum, we distinguish three main regimes according to the interplay between quantum fluctuations and classical stimulation: quantum BHL, classical BHL, and BCL. Based on quite general scaling arguments, the nonlinear amplification of quantum fluctuations up to saturation is identified as the most robust trait of a quantum BHL. A classical BHL behaves instead as a linear quantum amplifier, where the output is proportional to the input. The BCL regime also acts as a linear quantum amplifier, but its gain is exponentially smaller as compared to a classical BHL. Complementary signatures of black-hole lasing are a decrease in the amplification for increasing BCL amplitude or a nonmonotonic dependence of the growth rate with respect to the background parameters. We also identify interesting analogue phenomena such as Hawking-stimulated white-hole radiation or quantum BCL-stimulated Hawking radiation. The results of this work not only are of interest for analogue gravity, where they help to distinguish each phenomenon and to design experimental schemes for a clear observation of the BHL effect, but they also open the prospect of finding applications of analogue concepts in quantum technologies.

Pumping with Symmetry. (arXiv:2306.16401v2 [cond-mat.mtrl-sci] UPDATED)
Julio Andrés Iglesias Martínez, Muamer Kadic, Vincent Laude, Emil Prodan

Re-configurable materials and meta-materials can jump between space symmetry classes during their deformations. Here, we introduce the concept of singular symmetry enhancement, which refers to an abrupt jump to a higher symmetry class accompanied by an un-avoidable reduction in the number of dispersion bands of the excitations of the material. Such phenomenon prompts closings of some of the spectral resonant gaps along singular manifolds in a parameter space. In this work, we demonstrate that these singular manifolds carry topological charges. As a concrete example, we show that a deformation of an acoustic crystal that encircles a $p11g$-symmetric configuration of the cavity resonators results in an adiabatic cycle that carries a Chern number in the bulk and displays Thouless pumping at the edges. The outcome is a very general principle for recognizing or engineering topological adiabatic processes in complex materials and meta-materials.

How heat propagates in liquid $^3$He. (arXiv:2309.00502v3 [cond-mat.stat-mech] UPDATED)
Kamran Behnia, Kostya Trachenko

In Landau's Fermi liquid picture, transport is governed by scattering between quasi-particles. The normal liquid $^3$He conforms to this picture but only at very low temperature. Here, we observe that the deviation from the standard behavior is concomitant with the fermion-fermion scattering time falling below the Planckian time, $\frac{\hbar}{k_{\rm B}T}$. We also observe that thermal diffusivity of this quantum liquid is bounded by a minimum set by fundamental physical constants, similarly to what was observed in classical liquids earlier. This points to collective excitations (a sound mode) as carriers of heat. We propose that this mode has a wavevector of 2$k_F$ and a mean free path equal to the de Broglie thermal length. This would provide an additional conducting channel with a $T^{1/2}$ temperature dependence, matching what is observed by experiments. Within a margin of 10\%, the experimental data from 0.007 K to 3 K can be accounted for if thermal conductivity is the sum of contributions from quasiparticles and sound: $\kappa=\kappa_{qp}+\kappa_s$; $\kappa_{qp}\propto T^{-1}$; $\kappa_s\propto T^{1/2}$.

Skyrmion-deriven topological spin and charge Hall effects in diffusive antiferromagnetic thin films. (arXiv:2309.08763v2 [cond-mat.mes-hall] UPDATED)
Amir N. Zarezad, Józef Barnaś, Anna Dyrdał, Alireza Qaiumzadeh

We investigate topological Hall effects in a metallic antiferromagnetic (AFM) thin film and/or at the interface of an AFM insulator-normal metal bilayer with a single skyrmion in the diffusive regime. To determine the spin and charge Hall currents, we employed a Boltzmann kinetic equation with both spin-dependent and spin-flip scatterings. The interaction between conduction electrons and static skyrmions is included in the Boltzmann equation via the corresponding emergent magnetic field arising from the skyrmion texture. We compute intrinsic and extrinsic contributions to the topological spin Hall effect and spin accumulation, induced by an AFM skyrmion. We show that although the spin Hall current vanishes rapidly outside the skyrmion, the spin accumulation can be finite at the edges far from the skyrmion, provided the spin diffusion length is longer than the skyrmion radius. In addition, We show that in the presence of a spin-dependent relaxation time, the topological charge Hall effect is finite and we determine the corresponding Hall voltage. Our results may help to explore antiferromagnetic skyrmions by electrical means in real materials.

Magnetic field induces giant nonlinear optical response in Weyl semimetals. (arXiv:2310.02578v2 [cond-mat.mes-hall] UPDATED)
Grigory Bednik, Vladyslav Kozii

We study the second-order optical response of Weyl semimetals in the presence of a magnetic field. We consider an idealized model of a perfectly linear Weyl node and use the Kubo formula at zero temperature to calculate the intrinsic contribution to photocurrent and second harmonic generation conductivity components. We obtain exact analytical expressions applicable at arbitrary values of frequency, chemical potential, and magnetic field. Our results show that finite magnetic field significantly enhances the nonlinear optical response in semimetals, while magnetic resonances lead to divergences in nonlinear conductivity. In realistic systems, these singularities are regularized by a finite scattering rate, but result in pronounced peaks which can be detected experimentally, provided the system is clean and interactions are weak. We also perform a semiclassical calculation that complements and confirms our microscopic results at small magnetic fields and frequencies.

Phase Boundaries, Isotope Effect and Superconductivity of Lithium Under Hydrostatic Conditions. (arXiv:2312.17498v2 [cond-mat.supr-con] UPDATED)
Stefano Racioppi (1), Iren Saffarian-Deemyad (2), William Holle (2), Francesco Belli (1), Richard Ferry (3), Curtis Kenney-Benson (3), Jesse S. Smith (3), Eva Zurek (1), Shanti Deemyad (2) ((1) 1Department of Chemistry, State University of New York at Buffalo, Buffalo, New York, USA, (2) Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah, USA, (3) High Pressure Collaborative Access Team, X-Ray Sciences Division, Argonne National Laboratory, Argonne, IL, USA)

We present theoretical and experimental studies of superconductivity and low temperature structural phase boundaries in lithium. We mapped the structural phase diagram of 6Li and 7Li under hydrostatic conditions between 5 top 55GPa and within the temperature range of 15 to 75K, observing the FCC-hR1-cI16 phase transitions. 6Li and 7Li show some differences at the structural boundaries, with a potential shift of the phase boundaries of 6Li to lower pressures. Density functional theory calculations and topological analysis of the electron density elucidates the superconducting properties and interatomic interactions within these phases of lithium.

RKKY signals characterizing the topological phase transitions in Floquet Dirac semimetals. (arXiv:2401.01111v3 [cond-mat.mes-hall] UPDATED)
Hou-Jian Duan, Shi-Ming Cai, Xing Wei, Yong-Chi Chen, Yong-Jia Wu, Ming-Xun Deng, Ruiqiang Wang, Mou Yang

Recently, the Floquet ${\rm Na_3Bi}$-type material has been proposed as an ideal platform for realizing various phases, i.e., the spin-degenerate Dirac semimetal (DSM) can be turned into the Weyl semimetal (WSM), and even to the Weyl half-metal (WHM). Instead of the conventional electrical methods, we use the RKKY interaction to characterize the topological phase transitions in this paper. It is found that detecting the Ising term $J_I$ is feasible for distinguishing the phase transition of DSM/WSM, since the emergence of $J_I$ is induced by the broken spin degeneracy. For the case with impurities deposited on $z$ axis (the line connecting the Weyl points), the Heisenberg term $J_H$ coexists with $J_I$ in the WSM, while $J_H$ is filtered out and only $J_I$ survives in the WHM. This magnetic filtering effect is a reflection of the fully spin-polarized property (one spin band is in the WSM phase while the other is gapped) of the WHM, and it can act a signal to capture the phase transition of WSM/WHM. This signal can not be disturbed unless the direction of the impurities greatly deviates from $z$ axis. Interestingly, as the impurities are moved into the $x$-$y$ plane, there arises another signal (a dip structure for $J_H$ at the phase boundary), which can also identify the phase transition of WSM/WHM. Furthermore, we have verified that all magnetic signals are robust to the term that breaks the electron-hole symmetry. Besides characterizing the phase transitions, our results also suggest that the Floquet DSMs are power platforms for controlling the magnetic interaction.