Unusual Quantum Research Articles

Altermagnetic topological insulator with $\mathcal{C}$-paired spin-valley locking

AbstractAltermagnetism emerges as the third fundamental collinear magnetism recently. Associating nontrivial topology and quantum orders to this newly established magnetism is an important physical topic and may lead to interesting physics such as unusual quantum spin Hall effect with time-reversal-symmetry breaking, topological superconductivity and spin-valley locking. Here, we first generalize the spinless Haldane model from the usual honeycomb lattice to the otherwise simpler square lattice. We then restore the doublet spin degree of freedom, the resulted spinful model resembles the Kane-Mele model and is able to describe the unprecedented physics which intertwine $\mathcal{C}$ C -paired spin-valley locking, Dirac physics, nontrivial band topology and altermagnetism. Our works shed lights on future studies and applications in spintronics and valleytronics for altermagnetic materials, the lattice theories developed in this article may also find their broad applications to optical lattice with ultra cold atoms, photonic lattice, etc. other than the usual electronic systems.

Superconductivity in breathing kagome-structured C14 Laves phase XOs2(X = Zr, Hf)

Recently, the emergence of superconductivity in kagome metals has generated significant interest due to its interaction with flat bands and topological electronic states, which exhibit a range of unusual quantum characteristics. This study thoroughly investigates largely unexplored breathing kagome structure C14 Laves phase compounds XOs2 (X = Zr, Hf) by XRD, electrical transport, magnetization, and specific heat measurements. Our analyses confirm the presence of the MgZn2-type structure in ZrOs2 and HfOs2 compounds, exhibiting type-II superconductivity with critical temperature ( TC ) values of 2.90(3) K and 2.69(6) K, respectively. Furthermore, specific heat measurements and electron–phonon coupling constants for both compounds indicate weakly coupled fully gapped superconductivity.

High spin axion insulator

Axion insulators possess a quantized axion field θ = π protected by combined lattice and time-reversal symmetry, holding great potential for device applications in layertronics and quantum computing. Here, we propose a high-spin axion insulator (HSAI) defined in large spin-s representation, which maintains the same inherent symmetry but possesses a notable axion field θ = (s + 1/2)2π. Such distinct axion field is confirmed independently by the direct calculation of the axion term using hybrid Wannier functions, layer-resolved Chern numbers, as well as the topological magneto-electric effect. We show that the guaranteed gapless quasi-particle excitation is absent at the boundary of the HSAI despite its integer surface Chern number, hinting an unusual quantum anomaly violating the conventional bulk-boundary correspondence. Furthermore, we ascertain that the axion field θ can be precisely tuned through an external magnetic field, enabling the manipulation of bonded transport properties. The HSAI proposed here can be experimentally verified in ultra-cold atoms by the quantized non-reciprocal conductance or topological magnetoelectric response. Our work enriches the understanding of axion insulators in condensed matter physics, paving the way for future device applications.

Asymmetric Tilt-Induced Quantum Beating of Conductance Oscillation in Magnetically Modulated Dirac Matter Systems.

Herein, we investigate the effect of tilt mismatch on the quantum oscillations of spin transport properties in two-dimensional asymmetrically tilted Dirac cone systems. This study involves the examination of conductance oscillation in two distinct junction types: transverse- and longitudinal-tilted Dirac cones (TTDCs and LTDCs). Our findings reveal an unusual quantum oscillation of spin-polarized conductance within the TTDC system, characterized by two distinct anomaly patterns within a single period, labeled as the linear conductance phase and the oscillatory conductance phase. Interestingly, these phases emerge in association with tilt-induced orbital pseudo-magnetization and exchange interaction. Our study also demonstrates that the structure of the LTDC can modify the frequency of spin conductance oscillation, and the asymmetric effect within this structure results in a quantum beating pattern in oscillatory spin conductance. We note that an enhancement in the asymmetric longitudinal tilt velocity ratio within the structure correspondingly amplifies the beating frequency. Our research potentially contributes valuable insights for detecting the asymmetry of tilted Dirac fermions in type-I Dirac semimetal-based spintronics and quantum devices.

Colloidal Multi-Dot Nanorods.

Colloidal nanorod heterostructures consisting of multiple quantum dots within a nanorod (n-DNRs, where n is the number of quantum dots within a nanorod) are synthesized with alternating segments of CdSe "dot" and CdS "rod" via solution heteroepitaxy. The reaction temperature, time dependent ripening, and asymmetry of the wurtzite lattice and the resulting anisotropy of surface ligand steric hindrance are exploited to vary the morphology of the growing quantum dot segments. The alternating CdSe and CdS growth steps can be easily repeated to increment the dot number in unidirectional or bidirectional growth regimes. As an initial exploration of electron occupation effects on their optical properties, asymmetric 2-DNRs consisting of two dots of different lengths and diameters are synthesized and are shown to exhibit a change in color and an unusual photoluminescence quantum yield increase upon photochemical doping.

Altering Spin Distribution of Tb2Pc3 via Molecular Chirality Manipulation.

Manipulating the chirality of the spin-polarized electronic state is pivotal for understanding many unusual quantum spin phenomena, but it has not been achieved at the single-molecule level. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we successfully manipulate the chirality of spin distribution in a triple-decker single-molecule magnet tris(phthalocyaninato)bis(terbium(III)) (Tb2Pc3), which is evaporated on a Pb(111) substrate via molecular beam epitaxy. The otherwise achiral Tb2Pc3 becomes chiral after being embedded into the self-assembled monolayer films of bis(phthalocyaninato)terbium(III) (TbPc2). The chirality of the spin distribution in Tb2Pc3 is manifested via the spatial mapping of its Kondo resonance state from its ligand orbital. Our first-principles calculations revealed that the spin and molecular chirality are associated with a small rotation followed by a structural distortion of the top Pc, consistent with the experimental observation. By constructing tailored molecular clusters with the STM tip, a single Tb2Pc3 molecule can be manipulated among achiral and differently handed chiral configurations of spin distributions reversibly. This paves the way for designing chiral spin enantiomers for fundamental studies and developing functional spintronic devices.

Nonlinear and Nonreciprocal Transport Effects in Untwinned Thin Films of Ferromagnetic Weyl Metal SrRuO3

The identification of distinct charge transport features, deriving from nontrivial bulk band and surface states, has been a challenging subject in the field of topological systems. In topological Dirac and Weyl semimetals, nontrivial conical bands with Fermi-arc surface states give rise to negative longitudinal magnetoresistance due to chiral anomaly effect and unusual thickness dependent quantum oscillation from Weyl-orbit effect, which were demonstrated recently in experiments. In this work, we report the experimental observations of large nonlinear and nonreciprocal transport effects for both longitudinal and transverse channels in an untwinned Weyl metal of SrRuO3 thin film grown on a SrTiO3 substrate. From rigorous measurements with bias current applied along various directions with respect to the crystalline principal axes, the magnitude of nonlinear Hall signals from the transverse channel exhibits a simple sinα dependence at low temperatures, where α is the angle between bias current direction and orthorhombic [001]o, reaching a maximum when current is along orthorhombic [11¯0]o. On the contrary, the magnitude of nonlinear and nonreciprocal signals in the longitudinal channel attains a maximum for bias current along [001]o, and it vanishes for bias current along [11¯0]o. The observed α-dependent nonlinear and nonreciprocal signals in longitudinal and transverse channels reveal a magnetic Weyl phase with an effective Berry curvature dipole along [11¯0]o from surface states, accompanied by 1D chiral edge modes along [001]o. Published by the American Physical Society 2024

Charge-density wave mediated quasi-one-dimensional Kondo lattice in stripe-phase monolayer 1T-NbSe2

The heavy fermion physics is dictated by subtle competing exchange interactions, posing a challenge to their understanding. One-dimensional (1D) Kondo lattice model has attracted special attention in theory, because of its exact solvability and expected unusual quantum criticality. However, such experimental material systems are extremely rare. Here, we demonstrate the realization of quasi-1D Kondo lattice behavior in a monolayer van der Waals crystal NbSe2, that is driven into a stripe phase via Se-deficient line defects. Spectroscopic imaging scanning tunneling microscopy measurements and first-principles calculations indicate that the stripe-phase NbSe2 undergoes a novel charge-density wave transition, creating a matrix of local magnetic moments. The Kondo lattice behavior is manifested as a Fano resonance at the Fermi energy that prevails the entire film with a high Kondo temperature. Importantly, coherent Kondo screening occurs only in the direction of the stripes. Upon approaching defects, the Fano resonance exhibits prominent spatial 1D oscillations along the stripe direction, reminiscent of Kondo holes in a quasi-1D Kondo lattice. Our findings provide a platform for exploring anisotropic Kondo lattice behavior in the monolayer limit.

Squeezed Protons and Infrared Plasmonic Resonance Energy Transfer.

Unusual nuclear quantum effects may emerge near noble metal nanostructures such as squeezed vibrational states in molecular junctions and plasmonic resonance energy transfer in the infrared domain. Herein, nuclear quantum effects near heavy metals are studied by nuclear-electronic orbital density functional theory (NEO-DFT) with an effective core potential. For a quantum proton sandwiched between a pair of gold tips modeled by two Au6 clusters, NEO-DFT calculations suggest that the quantum proton density can be squeezed as the tip distance decreases. For an HF molecule placed near a one-dimensional Au nanowire composed of up to 34 Au atoms, real-time NEO time-dependent density functional theory (RT-NEO-TDDFT) shows that the infrared plasmonic motion within the Au nanowire may resonantly transfer electronic energy to the HF proton vibrational stretch mode. Overall, these calculations illustrate the advantages of the NEO approach for probing nuclear quantum effects, such as squeezed proton vibrational states and infrared plasmonic resonance energy transfer.

Signature of spin-phonon coupling driven charge density wave in a kagome magnet

The intertwining between spin, charge, and lattice degrees of freedom can give rise to unusual macroscopic quantum states, including high-temperature superconductivity and quantum anomalous Hall effects. Recently, a charge density wave (CDW) has been observed in the kagome antiferromagnet FeGe, indicative of possible intertwining physics. An outstanding question is that whether magnetic correlation is fundamental for the spontaneous spatial symmetry breaking orders. Here, utilizing elastic and high-resolution inelastic x-ray scattering, we observe a c-axis superlattice vector that coexists with the 2times2times1 CDW vectors in the kagome plane. Most interestingly, between the magnetic and CDW transition temperatures, the phonon dynamical structure factor shows a giant phonon-energy hardening and a substantial phonon linewidth broadening near the c-axis wavevectors, both signaling the spin-phonon coupling. By first principles and model calculations, we show that both the static spin polarization and dynamic spin excitations intertwine with the phonon to drive the spatial symmetry breaking in FeGe.

ELECTRONIC AND MAGNETIC PROPERTIES OF ALLOYS BASED ON THE DIRAC SEMIMETAL Cd<sub>3</sub>As<sub>2</sub> DOPED BY Mn ATOMS WITH THE VARIABLE CONCENTRATIONS

Theoretical studies predict that the low magnetic doping of the Dirac semimetals (DS) leads to the appearance in them of unusual quantum states and properties: the states of Weil semimetals, axionic insulator, topological superconductor and so on. However the specific materials in which these phenomena can be observed, as well as the characteristic concentrations of magnetic atoms are still unknown. In the present work, an ab initio study of the electronic and magnetic properties of the DS Cd3As2 doped isoelectronically with Mn atoms at concentrations of 4, 6, and 8% was performed. When analyzing the results, the main attention is paid to breaking spatial and time reversal symmetry in alloys, the behavior of the electronic structure near the top of the Dirac cone, and the processes of spin ordering in Mn atoms. The results obtained are compared with earlier theoretical and experimental studies, and on their basis a detailed picture of the effect of isoelectronic magnetic doping on the properties of the DS Cd3As2 is given.

Intercell moiré exciton complexes in electron lattices.

Excitons, Coulomb-bound electron-hole pairs, play a crucial role in both optical excitation and correlated phenomena in solids. When excitons interact with other quasiparticles, few- and many-body excited states can appear. Here we report an interaction between exciton and charges enabled by unusual quantum confinement in two-dimensional moiré superlattices, which results in many-body ground states composed of moiré excitons and correlated electron lattices. In an H-stacked (60o-twisted) WS2/WSe2 heterobilayer, we found an interlayer moiré exciton whose hole is surrounded by its partner electron's wavefunction distributed among three adjacent moiré traps. This three-dimensional excitonic structure enables large in-plane electrical quadrupole moments in addition to the vertical dipole. Upon doping, the quadrupole facilitates the binding of interlayer moiré excitons to the charges in neighbouring moiré cells, forming intercell charged exciton complexes. Our work provides a framework for understanding and engineering emergent exciton many-body states in correlated moiré charge orders.

Topological tailoring-induced Dirac cone in ultrathin niobium diboride nanosheets for electrocatalytic sulfur reduction reaction

Two-dimensional topological Dirac materials with novel electronic structures have attracted increasing attention in materials science, where the Weyl fermions can emerge in topological insulating multilayers with unusual quantum transport properties. However, their practical catalytic performance is seldom reported. Here, we design ultrathin niobium diboride (NbB2) nanosheets by utilizing topological electronic states to power catalytic activity. In the lithium-sulfur batteries, the stable topological surface states of NbB2 with high carrier mobility are a recipe for high-activity catalysts to spur the sluggish electrocatalytic sulfur reduction reaction, which originates from the Dirac cone electronic structure. Most remarkably, it delivers stable capacities with 1500 cycles under the 3.6 mA/cm2 (3 C), which is far superior to most electrode catalysts. The proposed functional Weyl and/or Dirac materials would have broad applications in other related fields such as the hydrogen evolution reaction, nitrogen reduction reaction, and energy conversion.

Quantum simulation of an exotic quantum critical point in a two-site charge Kondo circuit

Tuning a material to the cusp between two distinct ground states can produce physical properties that are unlike those in either of the neighbouring phases. Advances in the fabrication and control of quantum systems have raised the tantalizing prospect of artificial quantum simulators that can capture such behaviour. A tunable array of coupled qubits should have an appropriately rich phase diagram, but realizing such a system with either tunnel-coupled semiconductor quantum dots or metal nanostructures has proven difficult. The challenge for scaling up to clusters or lattices is to ensure that each element behaves essentially identically and that the coupling between elements is uniform, while also maintaining tunability of the interactions. Here we study a nanoelectronic circuit comprising two coupled hybrid metal–semiconductor islands, combining the strengths of both materials to form a potentially scalable platform. The semiconductor component allows for controllable inter-site couplings at quantum point contacts, while the metal component’s effective continuum of states means that different sites can be made equivalent by tuning local potentials. The couplings afforded by this architecture can realize an unusual quantum critical point resulting from frustrated Kondo interactions. The observed critical behaviour matches theoretical predictions, verifying the success of our experimental quantum simulation. The quantum critical behaviour of a two-impurity Kondo model variant is observed in a system of hybrid-semiconductor islands that could provide a scalable platform for solid-state quantum simulation

Emulating quantum photon-photon interactions in waveguides by double-wire media

Arrays of atoms coupled to photons propagating in a waveguide are now actively studied due to their prospects for generation and detection of quantum light. Quantum simulators based on waveguides with long-range couplings were also predicted to manifest unusual many-body quantum states. However, quantum tomography for large arrays with N ≳ 20 atoms remains elusive since it requires independent access to every atom. Here, we present a novel concept for analog emulation of such systems by unveiling an analogy between the setup of waveguide quantum electrodynamics and the classical problem of an electromagnetic wave propagating in a wire metamaterial. Experimentally measuring near electromagnetic fields, we emulate the two-particle localization arising from polariton-polariton interactions in the quantum problem. Our results demonstrate the potential of wire metamaterials to visualize quantum light-matter coupling in a table-top experiment and may be applied to emulate other exotic quantum effects, such as quantum chaos, and self-induced topological states.

Quantum Hall phase in graphene engineered by interfacial charge coupling

The quantum Hall effect can be substantially affected by interfacial coupling between the host two-dimensional electron gases and the substrate, and has been predicted to give rise to exotic topological states. Yet the understanding of the underlying physics and the controllable engineering of this interaction remains challenging. Here we demonstrate the observation of an unusual quantum Hall effect, which differs markedly from that of the known picture, in graphene samples in contact with an antiferromagnetic insulator CrOCl equipped with dual gates. Two distinct quantum Hall phases are developed, with the Landau levels in monolayer graphene remaining intact at the conventional phase, but largely distorted for the interfacial-coupling phase. The latter quantum Hall phase is even present close to the absence of a magnetic field, with the consequential Landau quantization following a parabolic relation between the displacement field and the magnetic field. This characteristic prevails up to 100 K in a wide effective doping range from 0 to 1013 cm−2.

Towards comprehension of the surface state properties in the intrinsic magnetic topological insulators

Presently, the unusual quantum phenomena recently discovered in intrinsic antiferromagnetic topological insulators are far from being fully understood. Motivated by recent controversial ARPES results on the ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ family compounds, we theoretically study the underlying physics of spectral properties of such materials. By using a continual model, we address the issue of the topological surface fermions which simultaneously experience an exchange field of a regular antiferromagnetic alignment and a surface electrostatic potential. The emergent net exchange field acting on the surface state is shown to depend on the surface potential strength, of which variation could be associated with concentration fluctuations of antisite and other defects that are inevitably present in ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ material. In this scenario, we discuss the possible microscopic mechanism for change of both size and sign of the surface exchange gap from sample to sample. Furthermore, we provide a natural explanation for the appearance of a finite density of states inside the exchange gap in the case of strong fluctuations in the surface potential. Thus, our results allow for a unified interpretation of various spectroscopic measurements on intrinsic antiferromagnetic topological insulators.

Nontrivial Berry's phase and exotic quantum phenomena in the vicinity of the quantum limit in the three-dimensional Dirac system Bi0.97Sb0.03

Quantum oscillations are known as the manifestation of the Fermi surface of metals in the presence of a strong magnetic field and of associated geometrical phases such as Berry's phase, playing a critical role in topological quantum materials. This study reports quantum oscillations on the three-dimensional (3D) Dirac material Bi0.97Sb0.03 single crystals in pulsed magnetic fields up to 50 T with three possible orientations. Under a magnetic field, we observe very pronounced Shubnikov-de Haas (SdH) oscillations originating from the Landau quantization of T-point holes. The SdH oscillations were well indexed with a large Landé g factor. The anisotropic behaviors of the Landé g factor were also deduced based on a detailed analysis of SdH oscillations. The Landau fan diagram revealed a nontrivial Berry's phase. Beyond the quantum limit (QL) under an extremely strong magnetic field, unusual quantum oscillations (UQO) were found. The possible origin of this phenomenon may be electronic instability caused by interactions. These results offer insights into the unusual electronic properties under the extremely strong fields caused by the unique band topology in this case.

Interference of nuclear wavepackets in a pair of proton transfer reactions

Quantum mechanics revolutionized chemists' understanding of molecular structure. In contrast, the kinetics of molecular reactions in solution are well described by classical, statistical theories. To reveal how the dynamics of chemical systems transition from quantum to classical, we study femtosecond proton transfer in a symmetric molecule with two identical reactant sites that are spatially apart. With the reaction launched from a superposition of two local basis states, we hypothesize that the ensuing motions of the electrons and nuclei will proceed, conceptually, in lockstep as a superposition of probability amplitudes until decoherence collapses the system to a product. Using ultrafast spectroscopy, we observe that the initial superposition state affects the reaction kinetics by an interference mechanism. With the aid of a quantum dynamics model, we propose how the evolution of nuclear wavepackets manifests the unusual intersite quantum correlations during the reaction.

Unusual constructive quantum interference in isolated armchair graphene nanoribbons

A new and unusual type of Constructive Quantum Interference (CQI) is revealed in selected isolated armchair graphene nanoribbons (AGNRs). CQI, normally encountered in molecular junctions, occurs when two nearly isoenergetic states interfere constructively. Such almost isoenergetic states are identified here as the energy differences between two pairs of topological end states developed in “wider” AGNRs, through topological metal-insulator-like phase transition, which for 13-AGNRS occurs at a critical length L c ≈ 50 Å. At L c a sudden jump in conductivity by a factor ∼2.5 occurs, which persists up to L ≈ 80 Å and is interpreted as a signature of CQI. Such unusual CQI, in which the zigzag region effectively plays the role of “linkers”, is unique for 13-AGNRs. This is due to their aromaticity i.e., resonance between two different aromatic Clar-sextets, and the resulting symmetry and energy separation of the frontier orbitals. The experimental measurements for 13-AGNRs fully support our results. At a critical length L c a multidimensional phase transition occurs in 13-AGNRs associated with two pairs of nearly isoenergetic end-states and discontinuities in bandgaps and conductivity, which suddenly rises by a factor of ~2.5, signifying a region of constructive quantum interference (CQI). • In wider graphene nanoribbons more than one pair of topological end states occur • These end states appear at critical length L c as metal-insulator phase transitions • For 13-armchair nanoribbons only, this leads to constructive quantum interference • Which is ultimately due to resonance between two aromatic Clar sextets • The role of “linkers” is played by the zigzag ends where end-states are localized