arXiv subjects

P. Zhou

Publications and source records attributed to P. Zhou.

At least 19 recordsLinked to original sources

arXiv2024

Transition from antiferromagnets to altermagnets: Symmetry-Breaking Theory

Considering the similarity of the real-space configurations for the opposite spin sublattices in both antiferromagnets (AFM) and altermagnets (AM), the relationship between them should be profound. In this work, we demonstrate that AFM and AM can be connected with spin groups and their subgroups. Consequently, the breaking of the combined inversion or translation operation with time-reversal symmetry (PT or tT) in AFM will induce transition from AFM to AM. We systematically list all collinear spin point groups and space groups that can realize the transition for the three types of AFMs: PT-type, tT-type and PT-tT-type. Moreover, we propose that Floquet engineering using circularly polarized light and surface cutting engineering are effective approaches to break PT and tT symmetries of AFM, respectively, achieving the transition. Interestingly, the features and magnitude of altermagnetic spin splitting can be tuned by adjusting various parameters of Floquet engineering. Our work not only establishes a theoretical framework for the transition from AFM to AM, but also provides practical approaches utilizing the achievements in AFM for a hundred years to obtain AM, significantly expanding the scope of altermagnetic materials for both theoretical studies and future practical applications.

cond-mat.mtrl-sci↗

arXiv2024

Resonant molecular transitions in second harmonic generation spectroscopy of Fe-octaethylporphyrin adsorbed on Cu(001)

Metal-organic molecular adsorbates on metallic surfaces offer the potential to both generate materials for future (spin-)electronics applications as well as a better fundamental understanding of molecule-substrate interaction, provided that the electronic properties of such interfaces can be analyzed and/or manipulated in a targeted manner. To investigate electronic interactions at such interfaces, we measure optical second harmonic generation (SHG) from iron-octaethylporphyrin (FeOEP) adsorbed on Cu(001), and perform electronic structure calculations using coupled cluster methods including optical excitations. We find that the SHG response of FeOEP/Cu(001) is modified at 2.15-2.35 eV fundamental photon energy compared to the bare Cu(001) surface. Our polarization-dependent analysis shows that the $\chi_{zzz}^{(2)}$ non-linear susceptibility tensor element dominates this modification. The first-principles calculations confirm this effect and conclude a resonantly enhanced SHG by molecular transitions at $\hbar\omega \geq 2$ eV. We show that the enhancement of $\chi^{(2)}_{zzz}$ results from a strong charge-transfer character of the molecule-substrate interaction. Our findings demonstrate the suitability of surface SHG for the characterization of such interfaces and the potential to employ it for time-resolved SHG experiments on optically induced electronic dynamics.

cond-mat.mtrl-sci↗

arXiv2023

Detection of optical emission from the supernova remnant G7.7-3.7

We present the first optical study of the supernova remnant (SNR) G7.7-3.7, with the aim of determining its evolutionary phase since it has been suggested to be the remnant of SN 386 AD. We obtained narrow-band images in the filters H$\alpha$ + [NII], H$\beta$, [OIII], [SII] that revealed faint optical emission in the southern region of the SNR consisting of two filaments elongated in the east-west direction aligned with the X-ray emitting region of the remnant. The filaments were seen in H$\alpha$ + [NII], [OIII] images and marginally in the [SII] images, with a non-detection in H$\beta$. Long-slit spectroscopy of three regions along one filament revealed large ratios of [SII] / H$\alpha$ = (1.6-2.5), consistent with that expected for a shock-heated SNR. The [SII] doublet ratio observed in two of the regions implies an upper limit for the electron density of the gas, with estimates falling below 400 cm$^{-3}$ and 600 cm$^{-3}$ in the respective areas. We discuss potential physical mechanisms that formed the observed optical filaments and we suggest that most likely they resulted by a collision of the SNR with a dense circumstellar shell lying at the southern region of the remnant.

astro-ph.GA↗

arXiv2023

A Vacuum-Compatible Cylindrical Inertial Rotation Sensor with Picoradian Sensitivity

We describe an inertial rotation sensor with a 30-cm cylindrical proof-mass suspended from a pair of 14-${\mu}$m thick BeCu flexures. The angle between the proof-mass and support structure is measured with a pair of homodyne interferometers which achieve a noise level of $\sim 5\ \text{prad}/\sqrt{\text{Hz}}$. The sensor is entirely made of vacuum compatible materials and the center of mass can be adjusted remotely.

physics.ins-det↗

arXiv2023

Hybrid single-pair charge-2 Weyl semimetals

Intuitively, the dispersion characteristics of Weyl nodes with opposite charges in single-pair charge-2 Weyl semimetals are the same, quadratic or linear. We theoretically predicted that single-pair hybrid charge-2 Weyl semimetals (the nodes with opposite charges show quadratic Weyl and linear charge-2 Dirac characteristics, respectively) can be protected by specific nonsymmorphic symmetries in spinless systems. Moreover, the symmetries force the pair of Weyl points locate at the center and corners of the first Brillouin zone (FBZ), respectively. Consequently, nontrivial surface states run through the entire FBZ of the system fascinating for future experimental detection and device applications. The hybrid phase is further verified with the help of first-principles calculations for the phonon states in realistic material of Na$_2$Zn$_2$O$_3$. The new phase will not only broaden the understanding of the Weyl semimetals, but also provide an interesting platform to investigate the interaction between the two types of Weyl fermions with different dispersions.

cond-mat.mtrl-sci↗

arXiv2022

A light-induced Weyl semiconductor-to-metal transition mediated by Peierls instability

Elemental tellurium is a strongly spin-orbit coupled Peierls-distorted semiconductor whose band structure features topologically protected Weyl nodes. Using time-dependent density functional theory calculations, we show that impulsive optical excitation can be used to transiently control the amplitude of the Peierls distortion, realizing a mechanism to switch tellurium between three states: Weyl semiconductor, Weyl metal and non-Weyl metal. Further, we present experimental evidence of this inverse-Peierls distortion using time-resolved optical second harmonic generation measurements. These results provide a pathway to multifunctional ultrafast Weyl devices and introduce Peierls systems as viable hosts of light-induced topological transitions.

cond-mat.str-el↗

arXiv2022

Second-Order Topological Insulator in Two-Dimensional C2N and Its Derivatives

Quadrupole phase, as a novel high-order topological phase, exhibits nontrivial gapless states at the boundaries whose dimension is lower than bulk by two. However, this phase has not been observed experimentally in two-dimensional (2D) materials up to now. In this work, using first-principles calculations and tight-binding (TB) model, we propose that the experimentally synthesized C2N is a 2D quadrupole topological insulator with one-dimensional gapped edge states and zero-dimensional gapless corner states. C2N is found to have a large bulk gap of 2.45 eV and an edge gap of 0.32 eV, making it an excellent candidate to evidently present the nontrivial corner states in experiments. The robustness of the corner states against the edge disorders has been explicitly identified. Moreover, another three C2N-like materials are also found to host the nontrivial quadrupole phase including an experimentally synthesized material aza-fused microporous polymers (CMP). The four 2D quadrupole topological phases proposed in our present work provide excellent candidates for studying the novel high-order topological properties in future experiments.

cond-mat.mtrl-sci↗

arXiv2022

Weyl-type nodal chains in X2MnO4 (X= Li, Na)

Recently, magnetic topological semimetals have received a lot of attention due to their potential applications in the field of spintronics. By using first-principles calculations, we propose that two ferromagnetic spinel materials of X2MnO4 (X=Li, Na) have Weyl-type nodal chains around the Fermi level. Their stabilities are validated by cohesive energies, phonon dispersions, and elastic constants. The nodal chains are composed of two types of nodal loops, which are protected by the glide operation Mz, the mirror operation M101 and their equivalent. The drumhead surface states are observed on the (001) surface and they exhibit nontrivial topological features. In addition, under different electron correlations and lattice strains, the semimetal states of these two materials are well kept. Our work provides two promising candidates for exploring the combination of magnetic materials and topological semimetal states.

cond-mat.mtrl-sci↗

arXiv2021

Completing the X-ray view of the recently discovered supernova remnant G53.41+0.03

Aims: We present a detailed X-ray study of the recently discovered supernova remnant (SNR) G53.41+0.03 that follows up and further expands on the previous, limited analysis of archival data covering a small portion of the SNR. Methods: With the new dedicated 70ks XMM-Newton observation we investigate the morphological structure of the SNR in X-rays, search for a presence of a young neutron star and characterise the plasma conditions in the selected regions by means of spectral fitting. Results: The first full view of SNR G53.41+0.03 shows an X-ray emission region well aligned with the reported half-shell radio morphology. We find three distinct regions of the remnant that vary in brightness and hardness of the spectra, and are all best characterised by a hot plasma model in a non-equilibrium ionisation state. Of the three regions, the brightest one contains the most mature plasma, with ionisation age $\tau \approx 4\times10^{10}$s cm$^{-3}$ (where $\tau = n_e t$), a lower electron temperature of kT$_\mathrm{e} \approx 1$ keV and the highest estimated gas density of n$_\mathrm{H}\approx 0.87$ cm$^{-3}$. The second, fainter but spectrally harder, region reveals a younger plasma ($\tau \approx 1.7\times10^{10}$s cm$^{-3}$) with higher temperature (kT$_\mathrm{e} \approx 2$ keV) and two to three times lower density (n$_\mathrm{H}\approx 0.34$ cm$^{-3}$). The third region is very faint, but we identify spectroscopically the presence of a hot plasma.Employing several methods for age estimation, we find the remnant to be $t \approx 1000-5000$ yrs old, confirming the earlier reports of a relatively young age. The environment of the remnant also contains a number of point sources, of which most are expected to be positioned in the foreground. Of the two point sources in the geometrical centre of the remnant one is consistent with the characteristics of a young neutron star.

astro-ph.HE↗

arXiv2021

The ambivalent competition of Coulomb and van-der-Waals interactions in Xe-Cs+ aggregates on Cu(111) surfaces

Microscopic insight into interactions is a key for understanding the properties of heterogenous interfaces. We analyze local attraction in non-covalently bonded Xe{Cs+ aggregates and monolayers on Cu(111) as well as repulsion upon electron transfer. Using two-photon photoemission spectroscopy, scanning tunneling microscopy, and coupled cluster calculations combined with an image-charge model we explain the intricate impact Xe has on Cs+/Cu(111). We find that attraction between Cs+ and Xe counterbalances the screened Coulomb repulsion between Cs+ ions on Cu(111). Furthermore, we observe that the Cs 6s electron is repelled from Cu(111) due to xenon's electron density. Together, this yields a dual, i.e., attractive or repulsive, response of Xe depending on the positive or negative charge of the respective counterparticle, which emphasizes the importance of the Coulomb interaction in these systems.

cond-mat.mes-hall↗

arXiv2020

Mapping the spectral index of Cas A: evidence for flattening from radio to infrared

Synchrotron radiation from supernova remnants are caused by electrons accelerated through diffusive shock acceleration (DSA). The standard DSA theory predicts an electron spectral index of $p=2$, corresponding to a radio spectral index of $\alpha=-0.5$. An extension of DSA predicts that the accelerated particles changes the shock structure, resulting in a spectrum that is steeper than $p>2$ ($\alpha<-0.5$) at low energies and flattens with energy. For Cas A, a synchrotron spectral flattening was previously reported for a small part of the remnant in the mid-infrared regime. Here, we present new measurements for spectral flattening using archival radio (4.72~GHz) and mid-infrared (3.6~$\mu$m) data, and produce a complete spectral index map to investigate the spatial variations within the remnant. We compare this to measurements of the radio spectral index from L-(1.285~GHz) and C-(4.64~GHz) band maps. Our result shows overall spectral flattening across the remnant ($\alpha _\mathrm{R-IR} \sim -0.5$ to $-0.7$), to be compared to the radio spectral index of $\alpha_{\rm R}=-0.77$. The flattest values coincide with the locations of most recent particle acceleration. In addition to overall flattening, we detect a relatively steeper region in the southeast of the remnant ($\alpha _\mathrm{R-IR} \sim -0.67$). We explore whether these locally steeper spectra could be the result of synchrotron cooling, which provides constraints on the local magnetic-field strengths/age of the plasma, suggesting $B\lesssim 2$~mG for an age of 100~yr, and even $B\lesssim 1$~mG using the age of Cas A, in agreement with other estimates.

astro-ph.HE↗

arXiv2019

Effect of lattice excitations on transient near edge X-ray absorption spectroscopy

Time-dependent and constituent-specific spectral changes in soft near edge X-ray spectroscopy (XAS) of an [Fe/MgO]$_8$ metal/insulator heterostructure upon laser excitation are analyzed at the O K-edge with picosecond time resolution. The oxygen absorption edge of the insulator features a uniform intensity decrease of the fine structure at elevated phononic temperatures, which can be quantified by a simple simulation and fitting procedure presented here. Combining X-ray absorption spectroscopy with ultrafast electron diffraction measurements and ab initio calculations demonstrate that the transient intensity changes in XAS can be assigned to a transient lattice temperature. Thus, the sensitivity of transient near edge XAS to phonons is demonstrated.

cond-mat.mtrl-sci↗

arXiv2019

Microscopic non-equilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure

The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]$_n$ as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or non-local relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L$_3$ absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, non-thermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L$_3$ edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with non-equilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these non-local, non-equilibrium dynamics.

cond-mat.mtrl-sci↗

arXiv2018

A low-frequency view of mixed-morphology supernova remnant VRO 42.05.01, and its neighbourhood

Mixed-morphology supernova remnants (MM SNRs) are a mysterious class of objects that display thermal X-ray emission within their radio shell. They are an older class of SNRs, and as such are profoundly affected by the environment into which they evolve. VRO 42.05.01 is a MM SNR of puzzling morphology in the direction of the Galactic anticentre. Low-frequency radio observations of supernova remnants are sensitive to synchrotron electrons accelerated in the shock front. We aim to compare the low-frequency emission to higher frequency observations to understand the environmental and shock acceleration conditions that have given rise to the observed properties of this source. We present a LOFAR High Band Antenna map centred at 143 MHz of the region of the Galactic plane centred at $l = 166^\rm{o}, b = 3.5^\rm{o}$ at 143 MHz, with a resolution of 148'' and an rms noise of 4.4 mJy bm$^{-1}$ . Our map is sensitive to scales as large as $6^\rm{o}$. We compared the LOw Frequency ARay (LOFAR) observations to archival higher frequency radio, infrared, and optical data to study the emission properties of the source in different spectral regimes. We did this both for the SNR and for OA 184, an H II region within our field of view. We find that the radio spectral index of VRO 42.05.01 increases at low radio frequencies; i.e. the LOFAR flux is higher than expected from the measured spectral index value at higher radio frequencies. This observed curvature in the low-frequency end of the radio spectrum occurs primarily in the brightest regions of the source, while the fainter regions present a roughly constant power-law behaviour between 143 MHz and 2695 MHz. We favour an explanation for this steepening whereby radiative shocks have high compression ratios and electrons of different energies probe different length scales across the shocks, therefore sampling regions of different compression ratios.

astro-ph.HE↗

arXiv2017

Open Type Nodal line Topological Semimetal in Two Dimensional B2C

Topological semimetals, including Dirac semimetals, Weyl semimetals, and nodal line semimetals, receive enormous research interest due to their intrinsic topological nature and fascinating properties. In present work, with the help of density functional theory (DFT), we find that nodal line semimetals and Dirac semimetals can coexist in the low energy electron of two dimensional (2D) monolayer B2C. Intriguingly, besides type-I and type-II Dirac fermions, a kind of open nodal line appears around the Fermi level. The low energy electronic nature of B2C sheet can be described by a tight-binding (TB) model relied on the basis of B-py, pz and C-py, pz states. One of the most merit of the system is that the energy windows of these four types topological semimetals are different that can be easily distinguished in experiments. Moreover, the system provides an excellent platform for studying the interplay between different type semimetals.

cond-mat.mtrl-sci↗

arXiv2017

Experimental Approach to the Thermodynamics of the Pure Two-Dimensional Spin-1/2 Triangular Lattice Antiferromagnet in Ba8CoNb6O24

Frustrated quantum magnets pose well-defined questions concerning quantum fluctuation effects and the nature of the many-body wavefunction, which challenge theory, numerics, experiment and materials synthesis. The S = 1/2 triangular-lattice antiferromagnet (TLAF) presents a case where classical order is strongly suppressed by quantum fluctuations, leading to extensive renormalization of physical properties at all energy scales. However, purely two-dimensional (2D) models are difficult to realise in the 3D world and their physics is controlled by the Mermin-Wagner theorem, which describes the dominant effects of additional thermal fluctuations. Here we report the magnetic properties Ba8CoNb6O24, whose Co2+ions have an effective spin 1/2 and construct a regular TLAF with very large interlayer spacing. We find no magnetic ordering down to 0.028 K, strong low-energy spin fluctuations in qualitative agreement with theoretical analysis and a diverging correlation length, all indicating a Mermin-Wagner trend towards zero-temperature ordering in this ideal 2D system.

cond-mat.str-el↗

arXiv2017

Mermin-Wagner physics, (H,T) phase diagram, and candidate quantum spin-liquid phase in the spin-1/2 triangular-lattice antiferromagnet Ba8CoNb6O24

Ba$_8$CoNb$_6$O$_{24}$ presents a system whose Co$^{2+}$ ions have an effective spin 1/2 and construct a regular triangular-lattice antiferromagnet (TLAFM) with a very large interlayer spacing, ensuring purely two-dimensional character. We exploit this ideal realization to perform a detailed experimental analysis of the $S = 1/2$ TLAFM, which is one of the keystone models in frustrated quantum magnetism. We find strong low-energy spin fluctuations and no magnetic ordering, but a diverging correlation length down to 0.1 K, indicating a Mermin-Wagner trend towards zero-temperature order. Below 0.1 K, however, our low-field measurements show an nexpected magnetically disordered state, which is a candidate quantum spin liquid. We establish the $(H,T)$ phase diagram, mapping in detail the quantum fluctuation corrections to the available theoretical analysis. These include a strong upshift in field of the maximum ordering temperature, qualitative changes to both low- and high-field phase boundaries, and an ordered regime apparently dominated by the collinear "up-up-down" state. Ba$_8$CoNb$_6$O$_{24}$ therefore offers fresh input for the development of theoretical approaches to the field-induced quantum phase transitions of the $S = 1/2$ Heisenberg TLAFM.

cond-mat.str-el↗

arXiv2017

Robust short-range-ordered nematicity in FeSe evidenced by high-pressure NMR

We report high-pressure $^{77}$Se NMR studies on FeSe single crystals that reveal a prominent inhomogeneous NMR linewidth broadening upon cooling, with the magnetic field applied along the tetragonal [110] direction. The data indicate the existence of short-range-ordered, inhomogeneous electronic nematicity, which has surprisingly long time scales over milliseconds. The short-range order survives temperatures up to $8$ times the structural transition temperature, and remains robust against pressure, in contrast to the strong pressure-dependence of the orbital ordering, structural transition, and the ground state magnetism. Such an extended region of static nematicity in the ($P$,$T$) space of FeSe indicates an enormously large fluctuating regime, and provide fresh insights and constraints to the understanding of electronic nematicity in iron-based superconductors.

cond-mat.supr-con↗