Exotic Quasiparticles Research Articles

Universal Ensemble-Embedding Graph Neural Network for Direct Prediction of Optical Spectra from Crystal Structures.

Optical properties in solids, such as refractive index and absorption, hold vast applications ranging from solar panels to sensors, photodetectors, and transparent displays. However, first-principles computation of optical properties from crystal structures is a complex task due to the high convergence criteria and computational cost. Recent progress in machine learning shows promise in predicting material properties, yet predicting optical properties from crystal structures remains challenging due to the lack of efficient atomic embeddings. Here, Graph Neural Network for Optical spectra prediction (GNNOpt) is introduced, an equivariant graph-neural-network architecture featuring universal embedding with automatic optimization. This enables high-quality optical predictions with a dataset of only 944 materials. GNNOpt predicts all optical properties based on the Kramers-Krönig relations, including absorption coefficient, complex dielectric function, complex refractive index, and reflectance. The trained model is applied to screen photovoltaic materials based on spectroscopic limited maximum efficiency and search for quantum materials based on quantum weight. First-principles calculations validate the efficacy of the GNNOpt model, demonstrating excellent agreement in predicting the optical spectra of unseen materials. The discovery of new quantum materials with high predicted quantum weight, such as SiOs, which host exotic quasiparticles with multifold nontrivial topology, demonstrates the potential of GNNOpt in predicting optical properties across a broad range of materials andapplications.

Signature of anyonic statistics in the integer quantum Hall regime

Anyons are exotic low-dimensional quasiparticles whose unconventional quantum statistics extend the binary particle division into fermions and bosons. The fractional quantum Hall regime provides a natural host, with the first convincing anyon signatures recently observed through interferometry and cross-correlations of colliding beams. However, the fractional regime is rife with experimental complications, such as an anomalous tunneling density of states, which impede the manipulation of anyons. Here we show experimentally that the canonical integer quantum Hall regime can provide a robust anyon platform. Exploiting the Coulomb interaction between two copropagating quantum Hall channels, an electron injected into one channel splits into two fractional charges behaving as abelian anyons. Their unconventional statistics is revealed by negative cross-correlations between dilute quasiparticle beams. Similarly to fractional quantum Hall observations, we show that the negative signal stems from a time-domain braiding process, here involving the incident fractional quasiparticles and spontaneously generated electron-hole pairs. Beyond the dilute limit, a theoretical understanding is achieved via the edge magnetoplasmon description of interacting integer quantum Hall channels. Our findings establish that, counter-intuitively, the integer quantum Hall regime provides a platform of choice for exploring and manipulating quasiparticles with fractional quantum statistics.

Observation of the scaling dimension of fractional quantum Hall anyons

Unconventional quasiparticles emerging in the fractional quantum Hall regime1,2 present the challenge of observing their exotic properties unambiguously. Although the fractional charge of quasiparticles has been demonstrated for nearly three decades3–5, the first convincing evidence of their anyonic quantum statistics has only recently been obtained6,7 and, so far, the so-called scaling dimension that determines the propagation dynamics of the quasiparticles remains elusive. In particular, although the nonlinearity of the tunnelling quasiparticle current should reveal their scaling dimension, the measurements fail to match theory, arguably because this observable is not robust to non-universal complications8–12. Here we expose the scaling dimension from the thermal noise to shot noise crossover and observe an agreement with expectations. Measurements are fitted to the predicted finite-temperature expression involving both the scaling dimension of the quasiparticles and their charge12,13, in contrast to previous charge investigations focusing on the high-bias shot-noise regime14. A systematic analysis, repeated on several constrictions and experimental conditions, consistently matches the theoretical scaling dimensions for the fractional quasiparticles emerging at filling factors ν = 1/3, 2/5 and 2/3. This establishes a central property of fractional quantum Hall anyons and demonstrates a powerful and complementary window into exotic quasiparticles.

Large out-of-plane spin–orbit torque in topological Weyl semimetal TaIrTe4

The unique electronic properties of topological quantum materials, such as protected surface states and exotic quasiparticles, can provide an out-of-plane spin-polarized current needed for external field-free magnetization switching of magnets with perpendicular magnetic anisotropy. Conventional spin–orbit torque (SOT) materials provide only an in-plane spin-polarized current, and recently explored materials with lower crystal symmetries provide very low out-of-plane spin-polarized current components, which are not suitable for energy-efficient SOT applications. Here, we demonstrate a large out-of-plane damping-like SOT at room temperature using the topological Weyl semimetal candidate TaIrTe4 with a lower crystal symmetry. We performed spin–torque ferromagnetic resonance (STFMR) and second harmonic Hall measurements on devices based on TaIrTe4/Ni80Fe20 heterostructures and observed a large out-of-plane damping-like SOT efficiency. The out-of-plane spin Hall conductivity is estimated to be (4.05 ± 0.23)×104 (ℏ ⁄ 2e) (Ωm)−1, which is an order of magnitude higher than the reported values in other materials.

Topological Quantum Well States in Pb/Sb Thin-Film Heterostructures.

Composite topological heterostructures, wherein topologically protected states are electronically tuned due to their proximity to other matter, are key avenues for exploring emergent physical phenomena. Particularly, pairing a topological material with a superconductor such as Pb is a promising means for generating a topological superconducting phase with exotic Majorana quasiparticles, but oft-neglected is the emergence of bulklike spin-polarized states that are quite relevant to applications. Using high-resolution photoemission spectroscopy and first-principles calculations, we report the emergence of bulk-like spin-polarized topological quantum well states with long coherence lengths in Pb films grown on the topological semimetal Sb. The results establish Pb/Sb heterostructures as topological superconductor candidates and advance the current understanding of topological coupling effects required for realizing emergent physics and for designing advanced spintronic device architectures.

Chiral and flat-band magnetic quasiparticles in ferromagnetic and metallic kagome layers.

Magnetic kagome metals are a promising platform to develop unique quantum transport and optical phenomena caused by the interplay between topological electronic bands, strong correlations, and magnetic order. This interplay may result in exotic quasiparticles that describe the coupled electronic and spin excitations on the frustrated kagome lattice. Here, we observe novel elementary magnetic excitations within the ferromagnetic Mn kagome layers in TbMn6Sn6 using inelastic neutron scattering. We observe sharp, collective acoustic magnons and identify flat-band magnons that are localized to a hexagonal plaquette due to the special geometry of the kagome layer. Surprisingly, we observe another type of elementary magnetic excitation; a chiral magnetic quasiparticle that is also localized on a hexagonal plaquette. The short lifetime of localized flat-band and chiral quasiparticles suggest that they are hybrid excitations that decay into electronic states.

The Influence of Electrochemical Bias on the Exciton Dynamics of MoS2 thin Films

2D transition metal dichalcogenides are an attractive family of materials in the field of electronics and optoelectronics. They are excellent candidates for sensitive photodetectors, light harvesting devices (photovoltaics, photocatalysts or photoelectrodes), lasers or non-linear optical devices. All these applications rely on light-matter interactions, which processes will ultimately decide the efficiency of the derived devices. Understanding carrier dynamics and the interaction of various exotic quasi-particles (excitons, trions) in these materials is of paramount importance to design better performing devices.Pump-probe transient absorption/reflection spectroscopy (TAS/TRS) is a powerful technique to study photophysical processes involved in charge carrier generation and recombination on the ultrafast timescale. In MoS2 photoelectrodes after light excitation different types of excitons are generated, and their decay can be monitored with these techniques. Coupling electrochemical techniques with these ultrafast methods, allows to probe the decay of the excited state in these materials under working conditions. In this manner the effect of trap state filling [1] or the effect of charge extraction [2] on the charge carrier dynamics of photoelectrochemical systems can be revealed.In my presentation I will show ultrafast spectroelectrochemical measurements on ITO/MoS2 photoelectrodes and reveal how the decay of excitons are influenced by the applied electrochemical bias. By comparing results from TAS/TRS spectroscopy the separation of carrier dynamics on the surface and bulk of these systems can be performed. These measurements reveal that the dissociation of excitons occurs at the ITO/MoS2 interface, resulting in a long living exciton population on the surface of these samples. Charging/discharging studies carried out in these systems reveal that the trap states involved in the dissociation of excitons in these systems can be permanently filled. The electrochemical filling of these trap states allows the tuning of the excited state lifetime of these systems, which can aid the better design of photoelectrochemical devices based on MoS2.

Identifying Chern numbers of superconductors from local measurements

Fascination in topological materials originates from their remarkable response properties and exotic quasiparticles which can be utilized in quantum technologies. In particular, large-scale efforts are currently focused on realizing topological superconductors and their Majorana excitations. However, determining the topological nature of superconductors with current experimental probes is an outstanding challenge. This shortcoming has become increasingly pressing due to rapidly developing designer platforms which are theorized to display very rich topology and are better accessed by local probes rather than transport experiments. We introduce a robust machine learning protocol for classifying the topological states of two-dimensional (2D) chiral superconductors and insulators from local density of states (LDOS) data. Since the LDOS can be measured with standard experimental techniques, our protocol contributes to overcoming the almost three decades standing problem of identifying the topological phase of 2D superconductors with broken time-reversal symmetry.

Edge mode percolation and equilibration in the topological insulator cadmium arsenide

Two-dimensional topological insulators can feature one-dimensional charge transport via edge modes, which offer a rich ground for studying exotic quasi-particles and for quantum materials applications. In this work, we use lateral junction devices, defined by nanoscale finger gates, to study edge mode transport in the two-dimensional topological insulator Cd3As2. The finger gate can be tuned to transmit an integer number of quantum Hall edge modes and exhibits full equilibration in the bipolar regime. When the Fermi level of the channel crosses a Landau level, reflected modes percolate through the channel, resulting in an anomalous conductance peak. The device does not fully pinch off when the channel is tuned into the topological gap, which is a sign of remnant modes in the channel. These modes are expected from band inversion, while residual bulk conduction associated with the disorder potential may also play a role.

Digital Simulation of Projective Non-Abelian Anyons with 68 Superconducting Qubits

Non-Abelian anyons are exotic quasiparticle excitations hosted by certain topological phases of matter. They break the fermion-boson dichotomy and obey non-Abelian braiding statistics: their interchanges yield unitary operations, rather than merely a phase factor, in a space spanned by topologically degenerate wavefunctions. They are the building blocks of topological quantum computing. However, experimental observation of non-Abelian anyons and their characterizing braiding statistics is notoriously challenging and has remained elusive hitherto, in spite of various theoretical proposals. Here, we report an experimental quantum digital simulation of projective non-Abelian anyons and their braiding statistics with up to 68 programmable superconducting qubits arranged on a two-dimensional lattice. By implementing the ground states of the toric-code model with twists through quantum circuits, we demonstrate that twists exchange electric and magnetic charges and behave as a particular type of non-Abelian anyons, i.e., the Ising anyons. In particular, we show experimentally that these twists follow the fusion rules and non-Abelian braiding statistics of the Ising type, and can be explored to encode topological logical qubits. Furthermore, we demonstrate how to implement both single- and two-qubit logic gates through applying a sequence of elementary Pauli gates on the underlying physical qubits. Our results demonstrate a versatile quantum digital approach for simulating non-Abelian anyons, offering a new lens into the study of such peculiar quasiparticles.

Cross-Correlation Investigation of Anyon Statistics in the ν=1/3 and 2/5 Fractional Quantum Hall States

To date, only two methods have provided convincing signs of exotic anyon quasiparticles. A reexamination of one method extends it to other types of anyons and establishes how much information the technique can provide.

Spin disorder in a stacking polytype of a layered magnet

Strongly correlated ground states and exotic quasiparticle excitations in low-dimensional systems are central research topics in the solid-state research community. The present work develops a layered material and explores the physical properties. Single crystals of $3R\text{\ensuremath{-}}{\mathrm{Na}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ were synthesized via a flux method. Single-crystal x-ray diffraction and transmission electron microscopy reveal a crystal structure with ABC-type stacking and an $R\text{\ensuremath{-}}3$ space group, which establishes this material as a stacking polytype to previously reported $2H\text{\ensuremath{-}}{\mathrm{Na}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$. Magnetic- and heat-capacity measurements demonstrate dominant antiferromagnetic interactions, the absence of long-range magnetic order down to 0.5 K, and field-dependent short-range magnetic correlations. A structural transition at $\ensuremath{\sim}23$ K observed in dielectric measurements may be related to displacements of the Na positions. Our results demonstrate that $3R\text{\ensuremath{-}}{\mathrm{Na}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ displays low-dimensional magnetism, disordered structure and spins, and the system displays a rich structure variety.

Quasiparticle Andreev scattering in the ν = 1/3 fractional quantum Hall regime

The scattering of exotic quasiparticles may follow different rules than electrons. In the fractional quantum Hall regime, a quantum point contact (QPC) provides a source of quasiparticles with field effect selectable charges and statistics, which can be scattered on an ‘analyzer’ QPC to investigate these rules. Remarkably, for incident quasiparticles dissimilar to those naturally transmitted across the analyzer, electrical conduction conserves neither the nature nor the number of the quasiparticles. In contrast with standard elastic scattering, theory predicts the emergence of a mechanism akin to the Andreev reflection at a normal-superconductor interface. Here, we observe the predicted Andreev-like reflection of an e/3 quasiparticle into a − 2e/3 hole accompanied by the transmission of an e quasielectron. Combining shot noise and cross-correlation measurements, we independently determine the charge of the different particles and ascertain the coincidence of quasielectron and fractional hole. The present work advances our understanding on the unconventional behavior of fractional quasiparticles, with implications toward the generation of novel quasi-particles/holes and non-local entanglements.

A quantum spin liquid candidate isolated in a two-dimensional CoIIRhIII bimetallic oxalate network.

A quantum spin liquid (QSL) is an elusive state of matter characterized by the absence of long-range magnetic order, even at zero temperature, and by the presence of exotic quasiparticle excitations. In spite of their relevance for quantum communication, topological quantum computation and the understanding of strongly correlated systems, like high-temperature superconductors, the unequivocal experimental identification of materials behaving as QSLs remains challenging. Here, we present a novel 2D heterometallic oxalate complex formed by high-spin Co(ii) ions alternating with diamagnetic Rh(iii) in a honeycomb lattice. This complex meets the key requirements to become a QSL: a spin ½ ground state for Co(ii), determined by spin-orbit coupling and crystal field, a magnetically-frustrated triangular lattice due to the presence of antiferromagnetic correlations, strongly suppressed direct exchange interactions and the presence of equivalent interfering superexchange paths between Co centres. A combination of electronic paramagnetic resonance, specific heat and ac magnetic susceptibility measurements in a wide range of frequencies and temperatures shows the presence of strong antiferromagnetic correlations concomitant with no signs of magnetic ordering down to 15 mK. These results show that bimetallic oxalates are appealing QSL candidates as well as versatile systems to chemically fine tune key aspects of a QSL, like magnetic frustration and superexchange path geometries.

Non-volatile chirality switching by all-optical magnetization reversal in ferromagnetic Weyl semimetal Co3Sn2S2

Weyl semimetals show unique physical properties exemplified by the colossal anomalous Hall effect, arising from exotic quasiparticles called Weyl fermions emerging around the Weyl nodes. Manipulating these topologically protected Weyl nodes is anticipated to play a leading role towards the on-demand control of quantum properties in Weyl semimetals. We demonstrate non-volatile chirality switching in a ferromagnetic Weyl semimetal Co3Sn2S2 via all-optical magnetization reversal. When excited by circularly polarized mid-infrared light pulses, the sign reversal of the anomalous Hall conductivity stemming from the Berry curvature is observed, manifesting the switching of the chirality of the Weyl nodes accompanying with the magnetization reversal. Magneto-optical imaging measurements reveal that the mechanism of the magnetization/chirality switching is attributed to the helicity-dependent deterministic magnetization associated with the magnetic circular dichroism.

Many-body localization of Z3 Fock parafermions

We study the effects of a random magnetic field on a one-dimensional (1D) spin-1 chain with correlated nearest-neighbor XY interaction. We show that this spin model can be exactly mapped onto the 1D disordered tight-binding model of ${\mathbb{Z}}_{3}$ Fock parafermions (FPFs), exotic anyonic quasiparticles that generalize usual spinless fermions. Thus, we have a peculiar case of a disordered Hamiltonian that despite being bilinear in the creation and annihilation operators, exhibits a many-body localization (MBL) transition owing to the nontrivial statistics of FPFs. This is in sharp contrast to conventional bosonic and fermionic quadratic disordered Hamiltonians that show single-particle (Anderson) localization. We perform finite-size exact diagonalization calculations of level-spacing statistics, fractal dimensions, and entanglement entropy, and provide convincing evidence for the MBL transition at finite disorder strength.

Observation of unconventional proximity induced superconducting effects in Bi2Se3 flakes

Topological superconductors (TSCs) are the materials with superconducting gap in bulk and exotic quasiparticles form gapless edge state. These quasiparticles can be employed as topological quantum bits for execution of quantum computation. Intrinsic TSCs are very rare in nature but artificial TSCs can be engineered by combining topological insulators with s-wave conventional superconductors. Hence recently hybrid structure composed of topological insulator and superconductor has been subject of research in condensed matter physics. In this paper, we use Bi2Se3 topological insulator thin flakes and tungsten (W) superconducting electrodes are deposited by using FIB based gas injection system (GIS). Fabricated Bi2Se3 flake/W hybrid structures were used to investigate induced superconductivity in Bi2Se3 flake via electric transport measurements. The resistance of the flake shows a drastic and significant decrease (8.4%) when the electrodes become superconducting. The superconducting transition (8.4%) is observed even though the length of Bi2Se3 flake between the superconducting contacts was 1.38 μm (≫ superconducting coherence length). Surprisingly, we find that resistance of Bi2Se3 flake shows an abrupt increase just before the transition temperature of superconducting electrodes (T < Tc) due to presence of electron-electron interaction. We also observe that the magnetoresistance of the flake is negative below transition temperature (T < Tc), but above transition temperature (T > Tc) magnetoresistance becomes positive due to the effects of superconducting electrodes which we discussed here in detail and it may help in constructing the topological superconductor.

Coexistence of charge-2 Dirac and Weyl phonons in chiral space groups

Unconventional topological phonons with unusual properties in crystalline solids have attracted increasing attention. By performing symmetry arguments and low-energy effective Hamiltonian, we confirm the coexistence of charge-2 Dirac and Weyl phonons at high-symmetry points and high-symmetry lines by analyzing all possible space groups. We reveal that such a particular system only emerges in chiral space groups in the presence of time-reversal symmetry. Unlike conventional Weyl pairs with opposite chirality that host one surface arc, there are two surface arcs by connecting one charge-2 Dirac point with topological charge = $\ifmmode\pm\else\textpm\fi{}2$ and two Weyl points with topological charge = $\ensuremath{\mp}1$. Due to the constraints of symmetry arguments and projection direction in chiral crystals, the surface arcs are long enough to span over the whole surface Brillouin zone. We take ${\mathrm{Na}}_{2}{\mathrm{Zn}}_{2}{\mathrm{O}}_{3}$ as an example to confirm our predication, and two branches of surface states across the whole surface Brillouin zone are clearly visible. Our work not only presents exotic quasiparticles but also provides an idea for realizing long nontrivial surface arcs.

Composite fermion mass: Experimental measurements in ultrahigh quality two-dimensional electron systems

Composite fermions (CFs), exotic quasi-particles formed by pairing an electron and an even number of magnetic flux quanta emerge at high magnetic fields in an interacting electron system, and can explain phenomena such as the fractional quantum Hall state (FQHS) and other many-body phases. CFs possess an effective mass ($m_{CF}$) whose magnitude is inversely related to the most fundamental property of a FQHS, namely its energy gap. We present here experimental measurements of $m_{CF}$ in ultra-high quality two-dimensional electron systems confined to GaAs quantum wells of varying thickness. An advantage of measuring $m_{CF}$ over gap measurements is that mass values are insensitive to disorder and are therefore ideal for comparison with theoretical calculations, especially for high-order FQHS. Our data reveal that $m_{CF}$ increases with increasing well width, reflecting a decrease in the energy gap as the electron layer becomes thicker and the in-plane Coulomb energy softens. Comparing our measured masses with available theoretical results, we find significant quantitative discrepancies, highlighting that more rigorous and accurate calculations are needed to explain the experimental data.

Majorana fermion arcs and the local density of states of UTe2

$\text{UTe}_2$ is a leading candidate for chiral p-wave superconductivity, and for hosting exotic Majorana fermion quasiparticles. Motivated by recent STM experiments in this system, we study particle-hole symmetry breaking in chiral p-wave superconductors. We compute the local density of states from Majorana fermion surface states in the presence of Rashba surface spin-orbit coupling, which is expected to be sizeable in heavy-fermion materials like UTe$_2$. We show that time-reversal and surface reflection symmetry breaking lead to a natural pairing tendency towards a triplet pair density wave state, which naturally can account for broken particle-hole symmetry.