Topological Band Structure Research Articles

Superconductivity and nematic order in a new titanium-based kagome metal CsTi3Bi5 without charge density wave order

The cascade of correlated topological quantum states in the newly discovered vanadium-based kagome superconductors, AV3Sb5 (A = K, Rb, and Cs), with a Z2 topological band structure has sparked immense interest. Here, we report the discovery of superconductivity and electronic nematic order in high-quality single-crystals of a new titanium-based kagome metal, CsTi3Bi5, that preserves the translation symmetry, in stark contrast to the charge density wave superconductor AV3Sb5. Transport and magnetic susceptibility measurements show superconductivity with an onset superconducting transition temperature Tc of approximately 4.8 K. Using the scanning tunneling microscopy/spectroscopy and Josephson scanning tunneling spectroscopy, we demonstrate that the single crystals of CsTi3Bi5 exhibit two distinct superconducting gaps. Furthermore, the superconducting gaps break the six-fold crystal rotational symmetry down to two-fold. At low energies, we find that the quasiparticle interference patterns exhibit rotational-symmetry-breaking C2 patterns, revealing a nematic ordered normal state with the same nematic direction as in the superconducting state. Our findings uncover a novel superconducting state in CsTi3Bi5 and provide new insights for the intrinsic electron liquid crystal phases in kagome superconductors.

Growth Conditions and Interfacial Misfit Array in SnTe (111) Films Grown on InP (111)A Substrates by Molecular Beam Epitaxy.

Tin telluride (SnTe) is an IV-VI semiconductor with a topological crystalline insulator band structure, high thermoelectric performance, and in-plane ferroelectricity. Despite its many applications, there has been little work focused on understanding the growth mechanisms of SnTe thin films. In this manuscript, we investigate the molecular beam epitaxy synthesis of SnTe (111) thin films on InP (111)A substrates. We explore the effect of substrate temperature, Te/Sn flux ratio, and growth rate on the film quality. Using a substrate temperature of 340 °C, a Te/Sn flux ratio of 3, and a growth rate of 0.48 Å/s, fully coalesced and single crystalline SnTe (111) epitaxial layers with X-ray rocking curve full-width-at-half-maxima of 0.09° and root-mean-square surface roughness as low as 0.2 nm have been obtained. Despite the 7.5% lattice mismatch between the SnTe (111) film and the InP (111)A substrate, reciprocal space mapping indicates that the 15 nm SnTe layer is fully relaxed. We show that a periodic interfacial misfit (IMF) dislocation array forms at the SnTe/InP heterointerface, where each IMF dislocation is separated by 14 InP lattice sites/13 SnTe lattice sites, providing rapid strain relaxation and yielding the high quality SnTe layer. This is the first report of an IMF array forming in a rock-salt on zinc-blende material system and at an IV-VI on III-V heterointerface, and highlights the potential for SnTe as a buffer layer for epitaxial telluride film growth. This work represents an important milestone in enabling the heterointegration between IV-VI and III-V semiconductors to create multifunctional devices.

Generalized Bloch Theorem and Band-Structure Topology

Dispersion relations in a metal with a commensurate helical magnetic order are considered in the framework of one- and two-dimensional tight-binding models. The generalized Bloch theorem for translations combined with spin rotations, together with the Born–Karman periodic boundary conditions, leads to the appearance of multisheet dispersion curves (surfaces). It is demonstrated that the resulting band structure is topologically nontrivial, which can lead to a spin textured Fermi surface and cause transport anomalies.

Magnetism of pseudospin-1/2 pyrochlore antiferromagnet Na3Co(CO3)2Cl.

Pyrochlore magnets have attracted interest as systems for realizing critical phenomena, rich magnetic structures, associated topological band structures, and nontrivial quantum phases. Na3Co(CO3)2Cl is a pseudospin-1/2 antiferromagnet in which the Co2+ions form a pyrochlore network. Its structural and magnetic properties were investigated using magnetization, heat capacity, ESR, single-crystal X-ray diffraction, powder neutron diffraction and powder inelastic neutron scattering. Magnetization and heat capacity measurements indicated a ground-state doublet, which is regarded as pseudospin 1/2, dominated the magnetic properties at low temperatures, with a magnetic exchange of 9.6 K. As the temperature decreases, a magnetic transition is observed at 1.6 K, which is confirmed to be an all-in-all-out magnetic order. The crystal field excitations observed by inelastic neutron scattering experiments indicated the Ising nature of the ground-state doublet. This thorough study revealed that Na3Co(CO3)2Cl can be regarded as a pseudospin-1/2 pyrochlore lattice antiferromagnet with dominant Ising-type interactions.

Electronic structure of noncentrosymmetric B20 compound HfSn and tuning of multifold band-crossing points

We present a detailed theoretical study of the electronic structure of hafnium tin HfSn crystallizing in a B20 structure, renowned for the diversity of physical and peculiar topological properties. The chiral crystal structure of these materials protects multifold band crossings located at high-symmetry points. We employ density functional methods to reveal basic features of the band structure and Fermi surface topology of HfSn, on top of which a tight-binding model is built. The compound exhibits a fourfold band crossing pinned at the Γ point. We investigate routes that can shift such crossings towards the Fermi level, offering a way to possibly tune the compound's properties. Specifically, we show that the energy position of the fourfold crossing can be easily manipulated via external perturbations such as strain and pressure. Considering that this point carries a topological charge larger than 1, such tuning is of great importance. We anticipate that the approach presented in the current study can be utilized to investigate symmetry-protected crossings in a wide class of materials. Published by the American Physical Society 2024

Berry curvature signatures in chiroptical excitonic transitions

The topology of the electronic band structure of solids can be described by its Berry curvature distribution across the Brillouin zone. We theoretically introduce and experimentally demonstrate a general methodology based on the measurement of energy- and momentum-resolved optical transition rates, allowing to reveal signatures of Berry curvature texture in reciprocal space. By performing time- and angle-resolved photoemission spectroscopy of atomically thin WSe 2 using polarization-modulated excitations, we demonstrate that excitons become an asset in extracting the quantum geometrical properties of solids. We also investigate the resilience of our measurement protocol against ultrafast scattering processes following direct chiroptical transitions.

Orbital Ingredients and Persistent Dirac Surface State for the Topological Band Structure in FeTe0.55Se0.45

FeTe0.55Se0.45 (FTS) occupies a special spot in modern condensed matter physics at the intersections of electron correlation, topology, and unconventional superconductivity. The bulk electronic structure of FTS is predicted to be topologically nontrivial due to the band inversion between the dxz and pz bands along Γ−Z. However, there remain debates in both the authenticity of the Dirac surface states (DSSs) and the experimental deviations of band structure from the theoretical band inversion picture. Here we resolve these debates through a comprehensive angle-resolved photoemission spectroscopy investigation. We first observe a persistent DSS independent of kz. Then, by comparing FTS with FeSe, which has no band inversion along Γ−Z, we identify the spectral weight fingerprint of both the presence of the pz band and the inversion between the dxz and pz bands. Furthermore, we propose a renormalization scheme for the band structure under the framework of a tight-binding model preserving crystal symmetry. Our results highlight the significant influence of correlation on modifying the band structure and make a strong case for the existence of topological band structure in this unconventional superconductor. Published by the American Physical Society 2024

Surface-dominated conductance scaling in Weyl semimetal NbAs

Protected surface states arising from non-trivial bandstructure topology in semimetals can potentially enable advanced device functionalities in compute, memory, interconnect, sensing, and communication. This necessitates a fundamental understanding of surface-state transport in nanoscale topological semimetals. Here, we investigate quantum transport in a prototypical topological semimetal NbAs to evaluate the potential of this class of materials for beyond-Cu interconnects in highly-scaled integrated circuits. Using density functional theory (DFT) coupled with non-equilibrium Green’s function (NEGF) calculations, we show that the resistance-area RA product in NbAs films decreases with decreasing thickness at the nanometer scale, in contrast to a nearly constant RA product in ideal Cu films. This anomalous scaling originates from the disproportionately large number of surface conduction states which dominate the ballistic conductance by up to 70% in NbAs thin films. We also show that this favorable RA scaling persists even in the presence of surface defects, in contrast to RA sharply increasing with reducing thickness for films of conventional metals, such as Cu, in the presence of surface defects. These results underscore the potential of topological semimetals as future back-end-of-line (BEOL) interconnect metals.

Ab-initio investigation of the structural, electronic and optical properties of CsPbBr3 perovskite under pressure using the DFT-1/2 method

Pressure-induced phase transitions in metal halide perovskites lead to significantly different material properties and offer great potential for diverse applications. The electronic, structural, and optical properties of Cesium lead bromide (CsPbBr3) crystals are investigated using density functional theory (DFT) to draw an integrated picture of orbital morphology, absorption spectra, and band gap values. Spin-orbit coupling and DFT-1/2 correction are used to gain a better insight into electronic properties and band splitting in the cubic phase. Calculations of pressure-induced phase transitions, electronic band gap engineering, and manipulation of optical absorption edges of CsPbBr3 are conducted. In the cubic and tetragonal crystal lattices, a band gap closure and opening occurs demonstrating a Dirac cone upon application and increase of pressure. Indeed, in the tetragonal lattice of CsPbBr3, a topological band occurs. Orthorhombic perovskite crystal lattice undergoes successive direct-indirect and indirect-direct band gap transitions, followed by a space group change at extreme pressures. However, under pressure, orthorhombic non-perovskite lattice experiences a sudden band gap breakdown. Phase and band gap transitions are explained by orbital hybridizations and structural symmetry evolution (e.g., bond length contraction/dilation and octahedral cage rotation/distortion). These results provide comprehensive guidelines for further experimental studies on pressure engineering, advanced photovoltaic and optoelectronic applications, discovering and understanding new topological band structures of perovskite materials.

DFT+U, TB-mBJ, and hybrid approaches: investigating electronic topology and magnetic properties in Sn-doped Co2TiGa

Cobalt-based Heusler compounds represent a new class of Heusler alloys which absorbed a lot of attention due to their performance in spintronics and magnetic devices. in this paper we have studied and analyzed the electronic structure and half-metallic ferromagnetic properties of Co2TiGa and Sn substituted at Ga site via the density functional theory (DFT) based first principle calculations with GGA-PBE, PBE+U, TB-MBJ exchange correlation potential and HSE06 approach. The band structure topology indicates that the parent ternary Heusler compound is ferromagnetic half-metallic with a half-metallic gap (band gap in the minority channel). The gap is increased according to the correction made on the exchange correlation potential. The half-metallic ferromagnetic behavior is confirmed by the total magnetizations which are very close to integrals Bohr magneton (1 μ B and 3μ B for Co2TiGa and Mn2TiGa0.5Sn0.5, respectively). The origin of half-metallic ferromagnetic is the p − d exchange and double-exchange interaction between the Co d-t2g states and neighboring Ti atom. The substitution effect on half-metallicity for Co2TiGa0.5 Sn0.5 was investigated in the tetragonal structure. The spontaneous magnetization obeyed the Slater-Pauling rule, indicating the Co2TiGa0.5 Sn0.5 is half-metal with 100% spin polarization.

Dimerized Hofstadter model in two-leg ladder quasi-crystals

We theoretically study topological features, band structure, and localization properties of a dimerized two-leg ladder with an oscillating on-site potential. The periodicity of the on-site potential can take either rational or irrational values. We consider two types of dimerized configurations; symmetric and asymmetric models. For rational values of the periodicity as long as inversion symmetry is preserved both symmetric and asymmetric ladders can host topological phases. Additionally, the energy spectrum of the models exhibits a fractal structure known as the Hofstadter butterfly spectrum, dependent on the dimerization of the hopping and the strength of the on-site potential. In the case of irrational values for the periodicity, a metal-insulator phase transition occurs with small values of the critical strength of the on-site potential in the dimerized cases. Our models incorporate the effects of lattice configuration and quasi-periodicity, paving the way for establishing platforms that host both topological and non-topological phase transitions.

Atomic-scale magnetic doping of monolayer stanene by revealing Kondo effect from self-assembled Fe spin entities

Atomic-scale spin entity in a two-dimensional topological insulator lays the foundation to manufacture magnetic topological materials with single atomic thickness. Here, we have successfully fabricated Fe monomer, dimer and trimer doped in the monolayer stanene/Cu(111) through a low-temperature growth and systematically investigated Kondo effect by combining scanning tunneling microscopy/spectroscopy (STM/STS) with density functional theory (DFT) and numerical renormalization group (NRG) method. Given high spatial and energy resolution, tunneling conductance (dI/dU) spectra have resolved zero-bias Kondo resonance and resultant magnetic-field-dependent Zeeman splitting, yielding an effective spin Seff = 3/2 with an easy-plane magnetic anisotropy on the self-assembled Fe atomic dopants. Reduced Kondo temperature along with attenuated Kondo intensity from Fe monomer to trimer have been further identified as a manifestation of Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between Sn-separated Fe atoms. Such magnetic Fe atom assembly in turn constitutes important cornerstones for tailoring topological band structures and developing magnetic phase transition in the single-atom-layer stanene.

Topological band structure due to modified Kramers degeneracy for electrons in a helical magnetic field

Topological band structure due to modified Kramers degeneracy for electrons in a helical magnetic field

Hidden non-collinear spin-order induced topological surface states

Rare-earth monopnictides are a family of materials simultaneously displaying complex magnetism, strong electronic correlation, and topological band structure. The recently discovered emergent arc-like surface states in these materials have been attributed to the multi-wave-vector antiferromagnetic order, yet the direct experimental evidence has been elusive. Here we report observation of non-collinear antiferromagnetic order with multiple modulations using spin-polarized scanning tunneling microscopy. Moreover, we discover a hidden spin-rotation transition of single-to-multiple modulations 2 K below the Néel temperature. The hidden transition coincides with the onset of the surface states splitting observed by our angle-resolved photoemission spectroscopy measurements. Single modulation gives rise to a band inversion with induced topological surface states in a local momentum region while the full Brillouin zone carries trivial topological indices, and multiple modulation further splits the surface bands via non-collinear spin tilting, as revealed by our calculations. The direct evidence of the non-collinear spin order in NdSb not only clarifies the mechanism of the emergent topological surface states, but also opens up a new paradigm of control and manipulation of band topology with magnetism.

Magnetization direction-controlled topological band structure in TlTiX (X = Si, Ge) monolayers

The quantum anomalous Hall (QAH) insulator is a vital material for the investigation of emerging topological quantum effects, but its extremely low working temperature limits experiments. Apart from the temperature challenge, effective regulation of the topological state of QAH insulators is another crucial concern. Here, by first-principles calculations, we find a family of stable two-dimensional materials TlTiX (X = Si, Ge) are large-gap QAH insulators. Their extremely robust ferromagnetic (FM) ground states are determined by both the direct- and super-exchange FM coupling. In the absence of spin–orbit coupling (SOC), there exist a spin-polarized crossing point located at each K and Kʹ points, respectively. The SOC effect results in the spontaneous breaking of C 2 symmetry and introduces a mass term, giving rise to a QAH state with sizable band gap. The tiny magnetocrystalline anisotropic energy (MAE) implies that an external magnetic field can be easily used to align magnetization deviating from z direction to the x–y plane, thereby leading to a transformation of the electronic state from the QAH state to the Weyl half semimetals state, which indicate monolayers TlTiX (X = Si, Ge) exhibit a giant magneto topological band effect. Finally, we examined the impact of stress on the band gap and MAE, which underlies the reasons for the giant magneto topological band effect attributed to the crystal field. These findings present novel prospects for the realization of large-gap QAH states with the characteristic of easily modifiable topological states.

Topological Structure Realized in Cove-Edged Graphene Nanoribbons via Incorporation of Periodic Pentagon Rings.

Cove-edged zigzag graphene nanoribbons are predicted to show metallic, topological, or trivial semiconducting band structures, which are precisely determined by their cove offset positions at both edges as well as the ribbon width. However, due to the challenge of introducing coves into zigzag-edged graphene nanoribbons, only a few cove-edged graphene nanoribbons with trivial semiconducting bandgaps have been realized experimentally. Here, we report that the topological band structure can be realized in cove-edged graphene nanoribbons by embedding periodic pentagon rings on the cove edges through on-surface synthesis. Upon noncontact atomic force microscopy and scanning tunneling spectroscopy measurements, the chemical and electronic structures of cove-edged graphene nanoribbons with periodic pentagon rings have been characterized for different lengths. Combined with theoretical calculations, we find that upon inducing periodic pentagon rings the cove-edged graphene nanoribbons exhibit nontrivial topological structures. Our results provide insights for the design and understanding of the topological character in cove-edged graphene nanoribbons.

Study of dielectric polarization and electrical transport in Bi1·2Sb0·8Te0·4Se2.6 nanofilms

Studying the dielectric response of topological insulators (TIs) can unveil their unique physical mechanisms such as charge transport and spin-orbit coupling effects. However, due to the manifestation of material's topological nature and band structure primarily in nanofilm, such thickness poses challenges for dielectric testing. To date, research on TI dielectric aspects remains relatively unexplored. Therefore, this paper successfully synthesizes nanofilm of quaternary topological insulator Bi1·2Sb0·8Te0·4Se2.6 (BSTS) using laser molecular beam epitaxy (LMBE) technique. Utilizing a wide-frequency dielectric spectrometer and a comprehensive physical properties measurement system (PPMS), we measured and thoroughly analyzed the dielectric polarization and charge transport characteristics of BSTS. We observed various polarization responses in the frequency range of 101–103 Hz, with the dipole orientation gradually failing to keep pace with the frequency increase in the range of 103–105 Hz, and the relaxation polarization unable to establish itself in the range of 105–107 Hz, with polarization primarily contributed by displacement polarization. Subsequently, we further analyzed the dependence of BSTS dielectric polarization response on temperature and film thickness, which will help reveal the influence of external factors on TI dielectric response, providing crucial insights for controlling TI materials' dielectric response. This not only deepens our understanding of the fundamental physical properties of this novel material but also offers important scientific basis and technological support for its applications in quantum computing, photonics, spintronics, and other fields.

Study on Extreme Quantum Matter and Functionality

Novel quantum-functional compounds that surpass the limits of traditional materials have been proposed. A prime example is the chiral superconductor that is applicable to quantum computing applications, realizing the ‘chiral superconducting state’ where Majorana quasi-particles are implemented. Another example is Kondo-Weyl semimetal, where correlated Kondo physics is intertwined with topological electronic band structure. The emergence of novel quantum-functional materials, which breaks the boundaries of traditional research fields and fosters convergence among diverse disciplines, stands as a key issue in condensed matter research. In this article, we introduce recent progress and future research direction in the study on extreme quantum matter and functionality.

Nontrivial Topological Phases in Ternary Borides M2XB2 (M=W, Mo; X=Co, Ni)

The nontrivial band topologies protected by certain symmetries have attracted significant interest in condensed matter physics. The discoveries of nontrivial topological phases in real materials provide a series of archetype materials to further explore the topological physics. Ternary borides M2XB2 (M = W, Mo; X = Co, Ni) have been widely investigated as the wear-resistant and high-hardness materials. Based on first-principles calculations, we find the nontrivial topological properties in these materials. Taking W2NiB2 as an example, this material shows the nodal line semimetal state in the absence of spin-orbit coupling. Two types of nodal lines appear near the Fermi level simultaneously. One is protected by the combined space-inversion and time-reversal symmetry, and the other is by the mirror symmetry. Part of these two-type nodal lines form nodal chains. When spin-orbit coupling is included, these nodal lines are fully gapped and the system becomes a strong topological insulator with nontrivial Z 2 index (1;000). Our calculations demonstrate that a nontrivial spin-momentum locked surface Dirac cone appears on the (1¯10) surface. We also find that other isostructural ternary borides Mo2NiB2, Mo2CoB2, and W2CoB2 possess similar topological band structures. Therefore, our work not only enriches the understanding of band topology for ternary borides, but also lays the foundation for the further study of topological phases manipulation and potential spintronic applications in realistic materials.

Quantum oscillations evidence for topological bands in kagome metal ScV6Sn6

Metals with kagome lattice provide bulk materials to host both the flat-band and Dirac electronic dispersions. A new family of kagome metals is recently discovered in AV6Sn6. The Dirac electronic structures of this material needs more experimental evidence to confirm. In the manuscript, we investigate this problem by resolving the quantum oscillations in both electrical transport and magnetization in ScV6Sn6. The revealed orbits are consistent with the electronic band structure models. Furthermore, the Berry phase of a dominating orbit is revealed to be around π, providing direct evidence for the topological band structure, which is consistent with calculations. Our results demonstrate a rich physics and shed light on the correlated topological ground state of this kagome metal.