Unusual Electronic States Research Articles

Observation of edge states derived from topological helix chains.

Introducing the concept of topology has revolutionized materials classification, leading to the discovery of topological insulators and Dirac-Weyl semimetals1-3. One of the most fundamental theories underpinning topological materials is the Su-Schrieffer-Heeger (SSH) model4,5, which was developed in 1979-decades before the recognition of topological insulators-to describe conducting polymers. Distinct from the vast majority of known topological insulators with two and three dimensions1-3, the SSH model predicts a one-dimensional analogue of topological insulators, which hosts topological bound states at the endpoints of a chain4-8. To establish this unique and pivotal state, it is crucial to identify the low-energy excitations stemming from bound states, but this has remained unknown in solids because of the absence of suitable platforms. Here we report unusual electronic states that support the emergent bound states in elemental tellurium, the single helix of which was recently proposed to realize an extended version of the SSH chain9,10. Using spin- and angle-resolved photoemission spectroscopy with a micro-focused beam, we have shown spin-polarized in-gap states confined to the edges of the (0001) surface. Our densityfunctional theory calculations indicate that these states are attributed to the interacting bound states originating from the one-dimensional array of SSH tellurium chains. Helices in solids offer a promising experimental platform for investigating exotic properties associated with the SSH chain and exploring topological phases through dimensionality control.

In-plane uniaxial-strain tuning of superconductivity and charge-density wave in CsV3Sb5

The kagome superconductor CsV3Sb5 with exotic electronic properties has attracted substantial research interest, and the interplay between the superconductivity and the charge-density wave is crucial for understanding its unusual electronic ground state. In this work, we performed resistivity and AC magnetic susceptibility measurements on CsV3Sb5 single crystals uniaxially-strained along [100] and [110] directions. We find that the uniaxial-strain tuning effect of T c (dT c/dε) and T CDW (dT CDW/dε) are almost identical along these distinct high-symmetry directions. These findings suggest the in-plane uniaxial-strain-tuning of T c and T CDW in CsV3Sb5 are dominated by associated c-axis strain, whereas the response to purely in-plane strains is likely small.

Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO3 and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a “mixed” HS/LS state with a stable abundance ratio realized in the Roverline{3 }c crystal structure with a single Fe site within a wide pressure range of ~ 50–106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments.

Cu-doping effects on the ferromagnetic semimetal CeAuGe

We present a study of Cu-substitution effects in 4f-Ce intermetallic compound CeAu1-xCuxGe, with potentially unusual electronic states, in the whole concentration range (x = 0.0 – 1.0). The parent CeAuGe compound, crystallizing in a non-centrosymmetric hexagonal structure, is a ferromagnetic semimetal with Curie temperature 10 K. Cu-doping on Au-site of CeAuGe, CeAu1-xCuxGe, changes the crystal structure from the non-centrosymmetric (P63mc) to centrosymmetric (P63/mmc) space group at the concentration x ∼ 0.5, where the c-lattice constant has a maximum value. Magnetic susceptibility and electrical resistivity measurements reveal that all Cu-doped compounds undergo magnetic phase transition near 10 K, with the maximum transition temperature of 12 K for x = 0.5. The neutron powder diffraction experiments show the ferromagnetic ordering of Ce3+ magnetic moments with a value of about 1.2 μB at 1.8 K, oriented perpendicular to the hexagonal c-axis. By using symmetry analysis, we have found the solutions for the magnetic structure in the ferromagnetic Shubnikov space groups Cmc'21′ and P21′/m' for x < 0.5 and x ≥ 0.5, respectively. Electrical resistivity ρ(T) exhibits a metallic temperature behaviour in all compositions. The resistivity ρ(T) has a local minimum in the paramagnetic state due to Kondo effects at high doping x = 0.8 and 1.0. At the small Cu-doping level, x = 0.2, the resistivity shows a broad feature at the ferromagnetic transition temperature and an additional transition-like peculiarity at 2.5 K in the ferromagnetic state.

Pauli paramagnetism of cubic V3Al , CrVTiAl, and related 18-electron Heusler compounds with a group-13 element

Calculations suggest that ordered Heusler alloys with 18 valance electrons could exhibit a variety of unusual electronic and magnetic states that are absent in the constituent elements. They include magnetic semiconductors, spin gapless semiconductors, compensated ferrimagnetic half-metals, and metallic antiferromagnets. Magnetic order has been predicted at exceptionally high temperature. Any of this would be of interest for spin electronics. Here, we investigate the magnetic properties of bulk, single-phase ${\mathrm{V}}_{3}\mathrm{Al}$, CrVTiAl and the corresponding Ga compounds, with and without $^{57}\mathrm{Fe}$ doping. Results are compared with data on the constituent elements. We conclude that all the as-cast alloys show some degree of B2-type ordering, but all of them are Pauli paramagnets with dimensionless susceptibilities close to the average of the atomic constituents. Prolonged annealing of the single-phase as-cast alloys leads to phase segregation. Density functional theory calculations on ${\mathrm{V}}_{3}X$ and $\text{CrVTi}X$ with $X=B$, Al, Ga, and In confirm that different atomic arrangements on the four interpenetrating face-centered cubic sublattices of the Heusler structure could indeed lead to unusual magnetic properties, but both magnetism and semiconductivity are destroyed by disorder. The energy and entropy differences between different ordered magnetic phases preclude the stabilization of any single one of them. All are metastable and inaccessible in alloys prepared from the melt.

Criteria for Realizing Room-Temperature Electrical Transport Applications of Topological Materials.

The unusual electronic states found in topological materials can enable a new generation of devices and technologies, yet a long-standing challenge has been finding materials without deleterious parallel bulk conduction. This can arise either from defects or thermally activated carriers. Here, the criteria that materials need to meet to realize transport properties dominated by the topological states, a necessity for a topological device, are clarified. This is demonstrated for 3D topological insulators, 3D Dirac materials, and 1D quantum anomalous Hall insulators, though this can be applied to similar systems. The key parameters are electronic bandgap, dielectric constant, and carrier effective mass, which dictate under what circumstances (defect density, temperature, etc.) the unwanted bulk state will conduct in parallel to the topological states. As these are fundamentally determined by the basic atomic properties, simple chemical arguments can be used to navigate the phase space to ultimately find improved materials. This will enable rapid identification of new systems with improved properties, which is crucial to designing new material systems and push a new generation of topological technologies.

Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN

Atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. Here, we demonstrate a blue emitting atomic-resolved heterojunction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provides a future playground for optoelectronics.

Electronic and structural properties of the honeycomb iridates A2IrO3 ( A=Na , Li) at elevated pressures

The honeycomb lattice iridates ${A}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ ($A=\mathrm{Na}$, Li) are spin-orbit assisted Mott insulators proximate to Kitaev's quantum spin liquid. The insulating state as well as the magnetic properties are believed to arise due to a delicate balance of several energy scales. We report on high-pressure electrical transport and x-ray-diffraction measurements on ${A}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ ($A=\mathrm{Na}$, Li) in an attempt to study their structural and electronic evolution with pressure. We found that while $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ undergoes a structural phase transition into the dimerized state at a pressure of $P$ \ensuremath{\sim} 4 GPa, in $\mathrm{N}{\mathrm{a}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ the conservation of the original $C2/m$ structure up to at least 58 GPa is observed. In addition, $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ undergoes a sluggish structural rearrangement at the pressure range 20--40 GPa coinciding with a significant decrease in resistance. Despite dissimilar structural evolution and different mechanisms of the electrical conductivity, Arrhenius conductivity for $\mathrm{N}{\mathrm{a}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ and Mott variable-range hopping in $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$, both systems show a very similar $R$($P$) behavior. Namely, after a nonmonotonic decrease of the resistance $R$ and the charge gap \ensuremath{\Delta}, the \ensuremath{\Delta} stabilizes at about 45 GPa and even increases slightly with pressure; the $R$($T$) shows insulating behavior up to the highest pressure measured, 80 and 55 GPa, respectively. This resilient nonmetallic behavior of the studied iridates suggests a formation close to a localized-itinerant crossover of unusual electronic states, whose possible features are discussed. Unforeseeably, the $R$($P$) behavior is not dependent on the buffer element $A$, which seems essential for understanding the nature of the electrical conductivity in iridates.

Unconventional Hund metal in a weak itinerant ferromagnet

The physics of weak itinerant ferromagnets is challenging due to their small magnetic moments and the ambiguous role of local interactions governing their electronic properties, many of which violate Fermi-liquid theory. While magnetic fluctuations play an important role in the materials’ unusual electronic states, the nature of these fluctuations and the paradigms through which they arise remain debated. Here we use inelastic neutron scattering to study magnetic fluctuations in the canonical weak itinerant ferromagnet MnSi. Data reveal that short-wavelength magnons continue to propagate until a mode crossing predicted for strongly interacting quasiparticles is reached, and the local susceptibility peaks at a coherence energy predicted for a correlated Hund metal by first-principles many-body theory. Scattering between electrons and orbital and spin fluctuations in MnSi can be understood at the local level to generate its non-Fermi liquid character. These results provide crucial insight into the role of interorbital Hund’s exchange within the broader class of enigmatic multiband itinerant, weak ferromagnets.

Role of Structural Parameters on the Leaky Electronic States in the Continuum of Superlattice Structures

&#x0D; &#x0D; &#x0D; We present the concept of a leaky electronic state in the continuum in a semiconductor superlattice with a structural defect. The required conditions for such unusual electronic state were investigated through self- consistent simulations. The number of quantum wells and the position of the dopants in the superlattice are essential for the creation of the leaky electronic state. The influence of these parameters was analyzed through the oscillator strength of the optical transition between the ground state and the first and second leaky electronic states in the continuum. To produce two leak electronic states in the continuum, it is necessary one asymmetric superlattice with five quantum wells on the left side of the n-doped structural defect quantum well and one quantum well on the right one, and their oscillator strengths are 0.22 and 0.05, respectively.&#x0D; &#x0D; &#x0D;

Yu-Shiba-Rusinov bands in ferromagnetic superconducting diamond.

The combination of different exotic properties in materials paves the way for the emergence of their new potential applications. An example is the recently found coexistence of the mutually antagonistic ferromagnetism and superconductivity in hydrogenated boron-doped diamond, which promises to be an attractive system with which to explore unconventional physics. Here, we show the emergence of Yu-Shiba-Rusinov (YSR) bands with a spatial extent of tens of nanometers in ferromagnetic superconducting diamond using scanning tunneling spectroscopy. We demonstrate theoretically how a two-dimensional (2D) spin lattice at the surface of a three-dimensional (3D) superconductor gives rise to the YSR bands and how their density-of-states profile correlates with the spin lattice structure. The established strategy to realize new forms of the coexistence of ferromagnetism and superconductivity opens a way to engineer the unusual electronic states and also to design better-performing superconducting devices.

Helimagnetic Structure and Heavy-Fermion-Like Behavior in the Vicinity of the Quantum Critical Point in Mn_{3}P.

Antiferromagnet Mn_{3}P with Neel temperature T_{N}=30 K is composed of Mn tetrahedrons and zigzag chains formed by three inequivalent Mn sites. Due to the nearly frustrated lattice with many short Mn-Mn bonds, competition of the exchange interactions is expected. We here investigate the magnetic structure and physical properties including pressure effect in single crystals of this material, and reveal a complex yet well-ordered helimagnetic structure. The itinerant character of this materials is strong, and the ordered state with small magnetic moments is easily suppressed under pressure, exhibiting a quantum critical point at ∼1.6 GPa. The remarkable mass renormalization, even in the ordered state, and an incoherent-coherent crossover in the low-temperature region, characterize an unusual electronic state in Mn_{3}P, which is most likely effected by the underlying frustration effect.

Excitons in InP, GaP, and GaxIn1−xP quantum dots: Insights from time-dependent density functional theory

Colloidal quantum dots (QDs) of group III-V are considered as promising candidates for next-generation environmentally friendly light emitting devices, yet there appears to be only limited understanding of the underlying electronic and excitonic properties. Using large-scale density functional theory with the hybrid B3LYP functional solving the single-particle states and time-dependent density functional theory accounting for the many-body excitonic effects, we have identified the structural, electronic and excitonic optical properties of InP, GaP and GaInP QDs containing up to a thousand atoms or more. The calculated optical gap of InP QD appears in excellent agreement with available experiments, and it scales nearly linearly with the inverse diameter. The radiative exciton decay lifetime is found to increase surprisingly linearly with increasing the dot size. For GaP QDs, we predict an unusual electronic state crossover at diameter around 1.5 nm whereby the nature of the lowest unoccupied molecular orbital (LUMO) state switches its symmetry from $\Gamma_{5}$-like at larger diameter to $\Gamma_{1}$-like at smaller diameter. After the crossover, the absorption intensity of the band-edge exciton states is significantly enhanced. Finally, we find that Vegard's law holds very well for GaInP random alloyed quantum dots down to ultra-small sizes with less than a hundred atoms. The obtained energy gap bowing parameter of this common-cation compound in QD regime appears positive, size-dependent and much smaller than its bulk parentage. The volume deformation, dominating over the charge exchange and structure relaxation effects, is mainly responsible for the QD energy gap bowing. The present work provides a road map for a variety of electronic and optical properties of colloidal QDs in group III-V that can guide spectroscopic studies.

Indication of Ferromagnetic Quantum Critical Point in Kondo Lattice CeRh6Ge4

We report resistivity measurements under pressure for Kondo-lattice ferromagnet CeRh$_6$Ge$_4$, and present that a quantum ferromagnetic (FM) phase transition is easily achieved. In most clean metallic ferromagnets, a quantum critical point (QCP) at zero field is avoided by changing the FM transition to a discontinuous transition or to an antiferromagnetic transition. In CeRh$_6$Ge$_4$, to the contrary, the Curie temperature of 2.5 K decreases continuously as increasing pressure without any clear signature that the transition changes to first order. The obvious non Fermi liquid behavior is observed in the vicinity of the quantum FM phase transition. The experimental data do not contradict a picture in which CeRh$_6$Ge$_4$ shows the FM QCP at zero field. Band structure calculation suggests the unusual electronic state of CeRh$_6$Ge$_4$ among Ce-based Kondo lattices. CeRh$_6$Ge$_4$ deserves further investigations and will be a key material to understand the matter of the FM QCP.

Robust Corrole-Based Metal-Organic Frameworks with Rare 9-Connected Zr/Hf-Oxo Clusters.

The corrole unit from the porphyrinoid family represents one of the most important ligands in the field of coordination chemistry, which creates a unique environment allowing for the observation of unusual electronic states of bound metal cations and has shown great promise in various applications. Nevertheless, studies that directly and systematically introduce these motifs in porous crystalline materials for targeting further functionalizations are still lacking. Herein, we report for the first time the construction of two robust corrole-based metal-organic frameworks (MOFs), M6(μ3-O)4(μ3-OH)4(OH)3(H2O)3(H3TCPC)3 (M = Zr for Corrole-MOF-1 and M = Hf for Corrole-MOF-2, H3TCPC = 5,10,15-tris(p-carboxylphenyl)corrole), which are assembled by a custom-designed C2ν-symmetric corrolic tricarboxylate ligand and the unprecedented D3d-symmetric 9-connected Zr6/Hf6 clusters. The resultant frameworks feature a rare (3,9)-connected gfy net and exhibit high chemical stability in aqueous solutions within a wide range of pH values. Furthermore, we successfully prepared the cationic Corrole-MOF-1(Fe) from the iron corrole ligand, which can serve as an efficient heterogeneous catalyst for [4 + 2] hetero-Diels-Alder reactions between unactivated aldehydes and a simple diene, outperforming both the homogeneous counterpart and the porphyrinic MOF counterpart.

Dirac nodal lines protected against spin-orbit interaction in IrO2

Dirac semimetals have electrons that propagate in the solid as if they had no mass. In rutile iridium oxide IrO${}_{2}$, these unusual electronic states form lines in reciprocal space, which are protected by crystal symmetry against interactions such as spin-orbit coupling. In this work, the authors synthesized epitaxial films of IrO${}_{2}$ along two crystal orientations and used angle-resolved photoemission spectroscopy to reveal the existence of two Dirac nodal lines. These cross the Fermi level and therefore directly impact the low-energy properties of the material.

Half-Excitonic Insulator: A Single-Spin Bose-Einstein Condensate.

First-principles calculations reveal an unusual electronic state (dubbed as half excitonic insulator) in monolayer 1T-MX_{2} (M=Co, Ni and X=Cl, Br). Its one spin channel has a many-body ground state due to excitonic instability, while the other is characterized by a conventional band insulator gap. This disparity arises from a competition between the band gap and exciton binding energy, which exhibits a spin dependence due to different orbital occupations. Such a state can be identified by optical absorption measurements and angle-resolved photoemission spectroscopy. Our theory not only provides new insights for the study of exciton condensation in magnetic materials but also suggests that strongly correlated materials could be fertile candidates for excitonic insulators.

Real-space observation of charge ordering in epitaxial La2\u2212xSrxCuO4 films

The cuprate superconductors exhibit ubiquitous instabilities toward charge-ordered states. These unusual electronic states break the spatial symmetries of the host crystal, and have been widely appreciated as essential ingredients for constructing a theory for high-temperature superconductivity in cuprates. Here, we report real-space imaging of the doping-dependent charge orders in the epitaxial thin films of a canonical cuprate compound La2–xSrxCuO4 using scanning tunneling microscopy. As the films are moderately doped, we observe a crossover from incommensurate to commensurate (4a0, where a0 is the Cu–O–Cu distance) stripes. Furthermore, at lower and higher doping levels, the charge orders occur in the form of distorted Wigner crystal and grid phase of crossed vertical and horizontal stripes. We discuss how the charge orders are stabilized, and their interplay with superconductivity.

A gap-protected zero-Hall effect state in the quantum limit of the non-symmorphic metal KHgSb.

A recurring theme in topological matter is the protection of unusual electronic states by symmetry, for example, protection of the surface states in Z2 topological insulators by time-reversal symmetry1-3. Recently, interest has turned to unusual surface states in the large class of non-symmorphic materials4-12. In particular, KHgSb is predicted to exhibit double quantum spin Hall states10. Here we report measurements of the Hall conductivity in KHgSb in a strong magnetic field B. In the quantum limit, the Hall conductivity is observed to fall exponentially to zero, but the diagonal conductivity is finite. A large gap protects this unusual zero-Hall state. We theoretically propose that, in this quantum limit, the chemical potential drops into the bulk gap, intersecting equal numbers of right- and left-moving quantum spin Hall surface modes to produce the zero-Hall state. The zero-Hall state illustrates how topological protection in a non-symmorphic material with glide symmetry may lead to highly unusual transport phenomena.

Unusual Electronic States and Superconducting Proximity Effect of Bi Films Modulated by a NbSe2 Substrate.

Heterostructures of two-dimensional layered materials can be functionalized with exotic phenomena that are unpresented with each constituting component. The interface effect plays a key role in determining the electronic properties of the heterostructure, whose characterization requires a correlation with the morphology with atomic-scale precision. Here, we report an investigation on the electronic properties of few-layer Bi(110) films mediated by a NbSe2 substrate. By utilizing scanning tunneling microscopy and spectroscopy, we show a significant variation of the density of states at different Bi film thicknesses, resulting in an unusual superconducting proximity effect that deviates from the conventional monotonous decay behavior. Moreover, the electronic states of the Bi films are also prominently modulated by the Moiré pattern spatially. With first-principles calculations, we illuminate these findings as the results of covalent-like quasi-bonds formed at the Bi/NbSe2 interface, which profoundly alter the charge distributions in the Bi films. Our study indicates a viable way of modulating the electronic properties of ultrathin films by quasi-covalent interfacial couplings beyond conventional van der Waals interactions.