arXiv subjects

Zhuoliang Ni

Publications and source records attributed to Zhuoliang Ni.

15 recordsLinked to original sources

arXiv2026

Observation of magnetically switchable quantum geometric photocurrents

In non-centrosymmetric materials, light can be rectified into two types of DC photocurrents, known as injection and shift currents, through the bulk photovoltaic effect. Recent theory has uncovered their deep relation with the two-state quantum geometry of resonant transitions: In non-magnetic crystals, where these currents have been routinely observed, the injection current responds to circular light and probes the Berry curvature, while the shift current responds to linear light and probes the geometric connection. Magnetic crystals have been predicted to show a new set of hitherto unobserved magnetically switchable photocurrents, with the roles of linear and circular light interchanged: A linear injection current, which probes the quantum metric, and a circular shift current, which probes the geometric torsion. In this work, we demonstrate the existence of such currents for the first time, demonstrating the switching of the current by flipping the Néel vector in a van der Waals antiferromagnet. Furthermore, their specific frequency and temperature dependence confirm the assignment of circular shift and linear injection currents. Our work demonstrates a new way to control photocurrents in magnets that are directly tied to geometry and have promising applications in antiferromagnetic spintronics and light harvesting.

cond-mat.mtrl-sci↗

arXiv2025

Broad-band THz emission by Spin-to-Charge Conversion in Topological Material -- Ferromagnet Heterostructures

Terahertz spintronic devices combine ultrafast operation with low power consumption, making them strong candidates for next-generation memory technologies. In this study, we use time-domain terahertz emission spectroscopy to investigate spin-to-charge conversion (SCC) in bilayer heterostructures comprising topological insulators (TIs) or Weyl semimetals (WSMs) with ferromagnetic metals (FMs). SCC is studied in TI materials \ce{Bi2Se3}, Pb-doped \ce{Bi2Se3}, and (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$, and the WSM NbP. Our results reveal that the dependence of SCC on TI thickness varies with interface quality, indicating that thickness dependence alone is not a reliable criterion for distinguishing between inverse spin Hall effect and the inverse Rashba--Edelstein effect mechanisms. We find efficient SCC in TIs depends on both \textit{in-situ} growth to prevent surface oxidation and proper composition. In NbP$\vert$FM bilayers, we observe THz emission with efficiency and bandwidth comparable to that of TIs, highlighting the broader potential of topological materials. Finally, broadband spectral measurements demonstrate that both TIs and WSMs can generate THz pulses with frequencies extending up to 8\,THz. These findings underscore the promise of topological materials as efficient platforms for ultrafast, broadband spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Imprinting spin patterns by local strain control in a van der Waals antiferromagnet

Van der Waals magnets provide opportunities for exploring low-dimensional magnetism and spintronic phenomena. The Mermin-Wagner theorem states that long-range correlations in reduced dimensions are stabilized and controlled by magnetic anisotropy. In this study, we meticulously create and control the in-plane easy-axis magnetic anisotropy within two-dimensional (2D) van der Waals antiferromagnet MnPSe3 via a novel method involving topography and therefore strain control by using a micro-patterned substrate. By transposing the MnPSe3 thin flakes onto a substrate patterned with micro-scale grooves, we introduce local uniaxial strain pattern, which not only locks the spin direction to the strain direction but also replicates the groove pattern in the spin orientation distribution. Our approach generates spin orientations that correspond to the substrate patterns, therefore having the potential to significantly advance spintronic devices by offering a unique method for manipulating and designing spin textures in easy-plane magnets.

cond-mat.mtrl-sci↗

arXiv2024

Observation of terahertz second harmonic generation from Dirac surface states in the topological insulator Bi$_2$Se$_3$

We report the observation of second harmonic generation with high conversion efficiency $\sim 0.005\%$ in the terahertz regime from thin films of the topological insulator Bi$_2$Se$_3$ that exhibit the linear photogalvanic effect, measured via time-domain terahertz spectroscopy and terahertz emission, respectively. As neither phenomena is observable from topologically trivial In-doped Bi$_2$Se$_3$, and since no enhancement is observed when subject to band bending, the efficient thickness-independent nonliear responses are attributable to the Dirac fermions of topological surface states of Bi$_2$Se$_3$. This observation of intrinsic terahertz second harmonic generation in an equilibrium system unlocks the full suite of both even and odd harmonic orders in the terahertz regime and opens new pathways to probing quantum geometry via intraband nonlinear processes. We hope our work will motivate the theoretical development of a full treatment of second harmonic generation for probing the quantum geometry in various inversion-breaking topological and twisted materials.

cond-mat.str-el↗

arXiv2024

Magnetic field control of continuous Néel vector rotation and Néel temperature in a van der Waals antiferromagnet

In a collinear antiferromagnet, spins tend to cant towards the direction of an applied magnetic field, thereby decreasing the energy of the system. The canting angle becomes negligible when the magnetic field is small so that the induced anisotropic energy is substantially lower than the exchange energy. However, this tiny anisotropy can play a significant role when the intrinsic anisotropy of the antiferromagnet is small. In our work, we conduct direct imaging of the Néel vector in a two-dimensional easy-plane antiferromagnet, MnPSe$_3$, with negligible spin canting under an external in-plane magnetic field. The small inherent in-plane anisotropy allows for the continuous rotation of the Néel vector by ramping up the magnetic field in samples from the bulk to the monolayer. In monolayer samples, the applied magnetic field elevates the Néel temperature 10$\%$ at 5 tesla, as the combination of intrinsic and field-induced anisotropies set a critical temperature scale for fluctuations of the otherwise disordered Néel vector field. Our study illuminates the contribution of field-induced anisotropy in two dimensional magnets with in-plane anisotropy. We also demonstrate that the strain can tune the spin flop transition field strength by one order of magnitude.

cond-mat.mtrl-sci↗

arXiv2023

Three-state nematicity and magneto-optical Kerr effect in the charge density waves in kagome superconductors

The kagome lattice provides a fascinating playground to study geometrical frustration, topology and strong correlations. The newly-discovered kagome metals AV$_3$Sb$_5$ (where A can refer to K, Rb, or Cs) exhibit phenomena including topological band structure, symmetry-breaking charge-density waves and superconductivity. Nevertheless, the nature of the symmetry breaking in the charge-density wave phase is not yet clear, despite the fact that it is crucial in order to understand whether the superconductivity is unconventional. In this work, we perform scanning birefringence microscopy on all three members of this family and find that six-fold rotation symmetry is broken at the onset of the charge-density wave transition in all these compounds. We show that the three nematic domains are oriented at 120$^\circ$ to each other and propose that staggered charge-density wave orders with a relative $π$ phase shift between layers is a possibility that can explain these observations. We also perform magneto-optical Kerr effect and circular dichroism measurements. The onset of both signals is at the transition temperature, indicating broken time-reversal symmetry and the existence of the long-sought loop currents in that phase.

cond-mat.str-el↗

arXiv2023

Signatures of Z$_3$ Vestigial Potts-nematic order in van der Waals antiferromagnets

Layered van der Waals magnets have attracted much recent attention as a promising and versatile platform for exploring intrinsic two-dimensional magnetism. Within this broader class, the transition metal phosphorous trichalcogenides $M$P$X_3$ stand out as particularly interesting, as they provide a realization of honeycomb lattice magnetism and are known to display a variety of magnetic ordering phenomena as well as superconductivity under pressure. One example, found in a number of different materials, is commensurate single-$Q$ zigzag antiferromagnetic order, which spontaneously breaks the spatial threefold $(C_3)$ rotation symmetry of the honeycomb lattice. The breaking of multiple distinct symmetries in the magnetic phase suggests the possibility of a sequence of distinct transitions as a function of temperature, and a resulting intermediate $\mathbb{Z}_3$-nematic phase which exists as a paramagnetic vestige of zigzag magnetic order -- a scenario known as vestigial ordering. Here, we report the observation of key signatures of vestigial Potts-nematic order in rhombohedral FePSe$_3$. By performing linear dichroism imaging measurements -- an ideal probe of rotational symmetry breaking -- we find that the $C_3$ symmetry is already broken above the Néel temperature. We show that these observations are explained by a general Ginzburg-Landau model of vestigial nematic order driven by magnetic fluctuations and coupled to residual strain. An analysis of the domain structure as temperature is lowered and a comparison with zigzag-ordered monoclinic FePS$_3$ reveals a broader applicability of the Ginzburg-Landau model in the presence of external strain, and firmly establishes the $M$P$X_3$ magnets as a new experimental venue for studying the interplay between Potts-nematicity, magnetism and superconductivity.

cond-mat.str-el↗

arXiv2023

Kramers nodal lines and Weyl fermions in SmAlSi

Kramers nodal lines (KNLs) have recently been proposed theoretically as a special type of Weyl line degeneracy connecting time-reversal invariant momenta. KNLs are robust to spin orbit coupling and are inherent to all non-centrosymmetric achiral crystal structures, leading to unusual spin, magneto-electric, and optical properties. However, their existence in in real quantum materials has not been experimentally established. Here we gather the experimental evidence pointing at the presence of KNLs in SmAlSi, a non-centrosymmetric metal that develops incommensurate spin density wave order at low temperature. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and magneto-transport methods, we provide evidence suggesting the presence of KNLs, together with observing Weyl fermions under the broken inversion symmetry in the paramagnetic phase of SmAlSi. We discuss the nesting possibilities regarding the emergent magnetic orders in SmAlSi. Our results provide a solid basis of experimental observations for exploring correlated topology in SmAlSi.

cond-mat.mtrl-sci↗

arXiv2022

Observation of giant surface second harmonic generation coupled to nematic orders in the van der Waals antiferromagnet FePS$_3$

Second harmonic generation has been applied to study lattice, electronic and magnetic proprieties in atomically thin materials. However, inversion symmetry breaking is usually required for the materials to generate a large signal. In this work, we report a giant second-harmonic generation that arises below the Néel temperature in few-layer centrosymmetric FePS$_3$. Layer-dependent study indicates the detected signal is from the second-order nonlinearity of the surface. The magnetism-induced surface second-harmonic response is two orders of magnitude larger than those reported in other magnetic systems, with the surface nonlinear susceptibility reaching 0.08--0.13 nm$^2$/V in 2 L--5 L samples. By combing linear dichroism and second harmonic generation experiments, we further confirm the giant second-harmonic generation is coupled to nematic orders formed by the three possible Zigzag antiferromagnetic domains. Our study shows that the surface second-harmonic generation is also a sensitive tool to study antiferromagnetic states in centrosymmetric atomically thin materials.

cond-mat.mtrl-sci↗

arXiv2022

Cavity-Enhanced Linear Dichroism in a van der Waals Antiferromagnet

Optical birefringence is a fundamental optical property of crystals widely used for filtering and beam splitting of photons. Birefringent crystals concurrently possess the property of linear dichroism (LD) that allows asymmetric propagation or attenuation of light with two different polarizations. This property of LD has been widely studied from small molecules to polymers and crystals but has rarely been engineered per will. Here, we use the newly discovered spin-charge coupling in van der Waals antiferromagnetic (AFM) insulator FePS3 to induce large in-plane optical anisotropy and consequently LD. We report that the LD in this AFM insulator is tunable both spectrally and magnitude-wise as a function of cavity coupling. We demonstrate near-unity LD in the visible-near infrared range in cavity-coupled FePS3 crystals and derive its dispersion as a function of cavity length and FePS3 thickness. Our results hold wide implications for use of cavity tuned LD as a diagnostic probe for strongly correlated quantum materials as well as opens new opportunities for miniaturized, on-chip beam-splitters and tunable filters.

physics.optics↗

arXiv2021

Direct imaging of antiferromagnetic domains and anomalous layer-dependent mirror symmetry breaking in atomically thin MnPS$_3$

We have developed a sensitive cryogenic second-harmonic generation microscopy to study a van der Waals antiferromagnet MnPS$_3$. We find that long-range Néel antiferromagnetic order develops from the bulk crystal down to the bilayer, while it is absent in the monolayer. Before entering the long-range antiferromagnetic ordered phase in all samples, an upturn of the second harmonic generation below 200 K indicates the formation of the short-range order and magneto-elastic coupling. We also directly image the two antiphase (180$^{\circ}$) antiferromagnetic domains and thermally-induced domain switching down to bilayer. An anomalous mirror symmetry breaking shows up in samples thinner than ten layers for the temperature both above and below the Néel temperature, which indicates a structural change in few-layer samples. Minimal change of the second harmonic generation polar patterns in strain tuning experiments indicate that the symmetry crossover at ten layers is most likely an intrinsic property of MnPS$_3$ instead of an extrinsic origin of substrate-induced strain. Our results show that second harmonic generation microscopy is a direct tool for studying antiferromagnetic domains in atomically thin materials, and opens a new way to study two-dimensional antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2021

Imaging the Néel vector switching in the monolayer antiferromagnet MnPSe$_3$ with strain-controlled Ising order

The family of monolayer two-dimensional (2D) materials hosts a wide range of interesting phenomena, including superconductivity, charge density waves, topological states and ferromagnetism, but direct evidence for antiferromagnetism in the monolayer has been lacking. Nevertheless, antiferromagnets have attracted enormous interest recently in spintronics due to the absence of stray fields and their terahertz resonant frequency. Despite the great advantages of antiferromagnetic spintronics, controlling and detecting Néel vectors have been limited in bulk materials. In this work, we developed a sensitive second harmonic generation (SHG) microscope and detected long-range Néel antiferromagnetic (AFM) order and Néel vector switching down to the monolayer in MnPSe$_3$. Temperature-dependent SHG measurement in repetitive thermal cooling surprisingly collapses into two curves, which correspond to the switching of an Ising type Néel vector reversed by the time-reversal operation, instead of a six-state clock ground state expected from the threefold rotation symmetry in the structure. We imaged the spatial distribution of the Néel vectors across samples and rotated them by an arbitrary angle irrespective of the lattice in the sample plane by applying strain. By studying both a Landau theory and a microscopic model that couples strain to nearest-neighbor exchange, we conclude that the phase transition of the XY model in the presence of strain falls into the Ising universality class instead of the XY one, which could explain the extreme strain tunability. Finally, we found that the 180° AFM domain walls are highly mobile down to the monolayer after thermal cycles, paving the way for future control of the antiferromagnetic domains by strain or external fields on demand for ultra-compact 2D AFM terahertz spintronics.

cond-mat.mtrl-sci↗

arXiv2021

Giant topological longitudinal circular photo-galvanic effect in the chiral multifold semimetal CoSi

The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids. A remarkable example occurs when chiral semimetals with topologically protected band degeneracies are illuminated with circularly polarized light. Under the right conditions, the part of the generated photocurrent that switches sign upon reversal of the light's polarization, known as the circular photogalvanic effect, is predicted to depend only on fundamental constants. The conditions to observe quantization are non-universal, and depend on material parameters and the incident frequency. In this work, we perform terahertz emission spectroscopy with tunable photon energy from 0.2 eV - 1.1 eV in the chiral topological semimetal CoSi. We identify a large longitudinal photocurrent peaked at 0.4 eV reaching $\sim$ 550 $μA/V^{2}$, which is much larger than the photocurrent in any chiral crystal reported in the literature. Using first-principles calculations we establish that the peak originates from topological band crossings, reaching 3.3$\pm$0.3 in units of the quantization constant. Our calculations indicate that the quantized CPGE is within reach in CoSi upon doping and increase of the hot-carrier lifetime. The large photo-conductivity suggests that topological semimetals could potentially be used as novel mid-infrared detectors.

cond-mat.mtrl-sci↗

arXiv2020

Linear and nonlinear optical responses in the chiral multifold semimetal RhSi

Chiral topological semimetals are materials that break both inversion and mirror symmetries. They host interesting phenomena such as the quantized circular photogalvanic effect (CPGE) and the chiral magnetic effect. In this work, we report a comprehensive theoretical and experimental analysis of the linear and non-linear optical responses of the chiral topological semimetal RhSi, which is known to host multifold fermions. We show that the characteristic features of the optical conductivity, which display two distinct quasi-linear regimes above and below 0.4 eV, can be linked to excitations of different kinds of multifold fermions. The characteristic features of the CPGE, which displays a sign change at 0.4 eV and a large non-quantized response peak of around 160 $μ\textrm{A V}^{-2}$ at 0.7 eV, are explained by assuming that the chemical potential crosses a flat hole band at the Brillouin zone center. Our theory predicts that, in order to observe a quantized CPGE in RhSi, it is necessary to increase the chemical potential as well as the quasiparticle lifetime. More broadly our methodology, especially the development of the broadband terahertz emission spectroscopy, could be widely applied to study photo-galvanic effects in noncentrosymmetric materials and in topological insulators in a contact-less way and accelerate the technological development of efficient infrared detectors based on topological semimetals.

cond-mat.mtrl-sci↗

arXiv2017

Evolution of Weyl orbit and quantum Hall effect in Dirac semimetal Cd3As2

Owing to the coupling between open Fermi arcs on opposite surfaces, topological Dirac semimetals exhibit a new type of cyclotron orbit in the surface states known as Weyl orbit. Here, by lowering the carrier density in Cd3As2 nanoplates, we observe a crossover from multiple- to single-frequency Shubnikov-de Haas (SdH) oscillations when subjected to out-of-plane magnetic field, indicating the dominant role of surface transport. With the increase of magnetic field, the SdH oscillations further develop into quantum Hall state with non-vanishing longitudinal resistance. By tracking the oscillation frequency and Hall plateau, we observe a Zeeman-related splitting and extract the Landau level index as well as sub-band number. Different from conventional two-dimensional systems, this unique quantum Hall effect may be related to the quantized version of Weyl orbits. Our results call for further investigations into the exotic quantum Hall states in the low-dimensional structure of topological semimetals.

cond-mat.mtrl-sci↗