Superconductivity in topological insulator Sb2Te3 induced by pressure

Creation, stabilization, and investigation at ambient pressure of pressure-induced superconductivity in Bi0.5Sb1.5Te3

In light of breakthroughs in superconductivity under high pressure, and considering that record critical temperatures (Tcs) across various systems have been achieved under high pressure, the primary challenge for higher Tc should no longer solely be to increase Tc under extreme conditions but also to reduce, or ideally eliminate, the need for applied pressure in retaining pressure-induced or -enhanced superconductivity. The topological semiconductor Bi0.5Sb1.5Te3 (BST) was chosen to demonstrate our approach to addressing this challenge and exploring its intriguing physics. Under pressures up to ~50 GPa, three superconducting phases (BST-I, -II, and -III) were observed. A superconducting phase in BST-I appears at ~4 GPa, without a structural transition, suggesting the possible topological nature of this phase. Using the pressure-quench protocol (PQP) recently developed by us, we successfully retained this pressure-induced phase at ambient pressure and revealed the bulk nature of the state. Significantly, this demonstrates recovery of a pressure-quenched sample from a diamond anvil cell at room temperature with the pressure-induced phase retained at ambient pressure. Other superconducting phases were retained in BST-II and -III at ambient pressure and subjected to thermal and temporal stability testing. Superconductivity was also found in BST with Tc up to 10.2 K, the record for this compound series. While PQP maintains superconducting phases in BST at ambient pressure, both depressurization and PQP enhance its Tc, possibly due to microstructures formed during these processes, offering an added avenue to raise Tc. These findings are supported by our density-functional theory calculations.

Manipulating Topological Phases in Magnetic Topological Insulators

Magnetic topological insulators (MTIs) are a group of materials that feature topological band structures with concurrent magnetism, which can offer new opportunities for technological advancements in various applications, such as spintronics and quantum computing. The combination of topology and magnetism introduces a rich spectrum of topological phases in MTIs, which can be controllably manipulated by tuning material parameters such as doping profiles, interfacial proximity effect, or external conditions such as pressure and electric field. In this paper, we first review the mainstream MTI material platforms where the quantum anomalous Hall effect can be achieved, along with other exotic topological phases in MTIs. We then focus on highlighting recent developments in modulating topological properties in MTI with finite-size limit, pressure, electric field, and magnetic proximity effect. The manipulation of topological phases in MTIs provides an exciting avenue for advancing both fundamental research and practical applications. As this field continues to develop, further investigations into the interplay between topology and magnetism in MTIs will undoubtedly pave the way for innovative breakthroughs in the fundamental understanding of topological physics as well as practical applications.

Pressure-induced physical properties in topological semi-metal TaM2 (M = As, Sb)

In this study, DFT based first principles calculations are used for measuring the structural, elastic, mechanical, electronic, optical and thermodynamic features of topological semimetal TaM2 (M = As, Sb) under various pressures. We conducted the first investigation into the physical properties of the topological semimetal TaM2 (M = As, Sb) under pressure. Formation energy and Born stability criteria justify the compound's thermodynamic and mechanical stability. We used elastic constants, elastic moduli, Kleinman parameter, machinability index, and Vickers hardness to investigate the mechanical properties of topological semimetal TaM2. Poisson's and Pugh's ratios reveal that both compounds change from brittle to ductile in response to pressure. The increasing nature of elastic moduli suggests that TaM2 becomes stiffer under stress. The pressure has a significant effect on the anisotropy factor for both materials. Band structure analysis shows that both compounds are Weyl semi-metals and the d orbital contributes significantly to the formation of the Fermi level, as shown by the density of states (DOS) analysis. Investigation of electronic characteristics provides important support for dissecting optical performance. Both the reflectivity and absorption spectra shift upwards in energy when pressure is increased. The refractive index value decreases and becomes flat in the higher energy region. Based on their refractive indices, both of these materials demonstrate as a high-density optical data storage medium when exposed to the right light source. The thermodynamic properties including sound velocity, and Debye temperature all exhibit an increasing nature with applied pressure. Due to their high Debye temperatures, the components under study have a rather high melting point.

Topological Nodal Surface and Quadratic Dirac Semimetal States and van Hove Singularities in ScH3 and LuH3 Superconductors

The coexistence of non-trivial topology and superconductivity in a material may induce a novel physical phenomenon known as topological superconductivity. Topological superconductors have been the subject of intense research, yet there are severe limitations in their application due to a lack of suitable materials. Topological nodal surface semimetals with nearly flat nodal surfaces near the Fermi level can be promising materials to achieve topological superconductivity. Here, we use first-principles calculations to examine the topological electronic characteristics of two new superconductors, ScH₃ and LuH₃, at both ambient and high pressures. Our studies show that both ScH₃ and LuH₃ have van Hove singularities, which confirms their superconductivity. Interestingly, both materials host topological nodal surface states under the protection of time reversal and spatial inversion symmetries in the absence of spin-orbit coupling (SOC). These nodal surfaces are distinguished by a pair of unique drum-head-like surface states not previously observed in nodal surface semimetals. Moreover, the nodal surfaces transform into essential spin-orbit quadratic Dirac points when SOC is included. Our findings demonstrate that ScH₃ and LuH₃ are good candidates to investigate the exotic properties of both nodal surface semimetals (NSSMs) and quadratic Dirac semimetal states and also provide a platform to explore the coexistence of topology and superconductivity in NSSMs with promising applications in high-speed electronics and topological quantum computing.

Pressure-Induced Electronic Topological Transition and Superconductivity in Topological Insulator Bi2Te2.1Se0.9

One approach to discovering topological superconductors is establishing superconductivity based on well-identified topological insulators. However, the coexistence of superconductivity and a topological state is always arcane. In this paper, we report how pressure tunes the crystal structure, electronic structure, and superconductivity in topological insulator Bi₂Te_2.1Se_0.9. At ∼2.5 GPa, the abnormal changes in c/a and the full width at half-maximum of the A_1g¹ mode indicate the occurrence of an electronic topological transition. The pressure-induced superconductivity in Bi₂Te_2.1Se_0.9 pinned with an electronic topological transition presents at 2.4 GPa, which is far below the structural phase transition pressure of 8.4 GPa. These results suggest that the appearance of an electronic topological transition is closely correlated with superconductivity in the initial phase, where the topological surface state persists. Our work clarifies the complex electronic structure of Bi₂Te_2.1Se_0.9 and sheds light on the mechanism for superconductivity in topological insulators.

Pressured-induced superconducting phase with large upper critical field and concomitant enhancement of antiferromagnetic transition in EuTe2

We report an unusual pressure-induced superconducting state that coexists with an antiferromagnetic ordering of Eu²⁺ moments and shows a large upper critical field comparable to the Pauli paramagnetic limit in EuTe₂. In concomitant with the emergence of superconductivity with T_c ≈ 3–5 K above P_c ≈ 6 GPa, the antiferromagnetic transition temperature T_N(P) experiences a quicker rise with the slope increased dramatically from dT_N/dP = 0.85(14) K/GPa for P ≤ P_c to 3.7(2) K/GPa for P ≥ P_c. Moreover, the superconducting state can survive in the spin-flop state with a net ferromagnetic component of the Eu²⁺ sublattice under moderate magnetic fields μ₀H ≥ 2 T. Our findings establish the pressurized EuTe₂ as a rare magnetic superconductor possessing an intimated interplay between magnetism and superconductivity.

Here, the authors report pressure-induced superconductivity with concomitant enhancement of antiferromagnetic transition in layered EuTe₂. The superconductivity is distinctly characterized by the high upper critical fields exceeding the Pauli limit among binary tellurides, a prerequisite of the coexistence of ferromagnetism with superconductivity.

Superconductivity in In2Te3 under Compression Induced by Electronic and Structural Phase Transitions

Indium telluride (In₂Te₃) is a typical layered material among III-IV families that are extremely sensitive to pressure and strain. Here, we use a combination of high-pressure electric transport, Raman, XRD, and first-principles calculations to study the electronic properties and structural evolution characteristics of In₂Te₃ under high pressure. Our results reveal the evidence of isostructure electronic transitions. First-principle calculations demonstrate that the evolution of phonon modes is associated with the transition from semiconductor to metal due to the increase in the density of states near the Fermi level. The pressure-induced metalization as a precursor monitors the structural phase transition, and then the superconductivity is produced. Further, in decompression, T_c slightly increased and remained at 3.0 GPa, and then the disorder is present and the superconductivity is suppressed. Our work not only perfects the superconducting phase of the In-Te system under pressure but also provides a reference for further superconducting research and applications.

Discovery of Dome-Shaped Superconducting Phase and Anisotropic Transport in a van der Waals Layered Candidate NbIrTe4 under Pressure

The unique electronic structure and crystal structure driven by external pressure in transition metal tellurides (TMTs) can host unconventional quantum states. Here, the discovery of pressure-induced phase transition at ≈2 GPa, and dome-shaped superconducting phase emerged in van der Waals layered NbIrTe4 is reported. The highest critical temperature (Tc ) is ≈5.8 K at pressure of ≈16 GPa, where the interlayered Te-Te covalent bonds form simultaneously derived from the synchrotron diffraction data, indicating the hosting structure of superconducting evolved from low-pressure two-dimensional (2D) phase to three-dimensional (3D) structure with pressure higher than 30 GPa. Strikingly, the authors have found an anisotropic transport in the vicinity of the superconducting state, suggesting the emergence of a "stripe"-like phase. The dome-shaped superconducting phase and anisotropic transport are possibly due to the spatial modulation of interlayer Josephson coupling .

Structural, electrical and optical properties of bilayer SiX (X = N, P, As and Sb)

In this work, the structural, electrical, and optical properties of bilayer SiX (X= N, P, As, and Sb) are studied using density functional theory. Five different stacking orders are considered for every compound and their structural properties are presented. The band structure of these materials demonstrates that they are indirect semiconductors. The out-of-plane strain has been applied to tune the bandgap and its electrical properties. The bandgap increases with tensile strain, whereas, compressive strain leads to semiconductor-to-metal transition. The sensitivity of the bandgap to the pressure is investigated and bilayer SiSb demonstrates the highest bandgap sensitivity to the pressure. These structures exhibit Mexican hat-like valence band dispersion that can be approved by a singularity in the density of states (DOS). The Mexican-hat coefficient can be tuned by out-of-plane strain. Optical absorption of these compounds shows that the second and lower valence bands due to the high DOS display a higher contribution to optical transitions.

Local insight to the structural phase transition sequence of Bi2Se3under quasi-hydrostatic and nonhydrostatic pressure

High-pressure behaviors of Bi₂Se₃, as one of layered 3D topological insulators, has attracted tremendous interest recent years. However, the phase transition sequence of Bi₂Se₃remain controversial. In this work, we explore the structural phase sequence of topological insulator Bi₂Se₃using high-pressure x-ray absorption fine structure (XAFS) spectroscopy under quasi-hydrostatic and nonhydrostatic pressure up to 42 GPa. By examining the XAFS features, we find that the appearance ofC2/cphase of Bi₂Se₃is strongly dependent on pressure condition,C2/cphase of Bi₂Se₃only exists under quasi-hydrostatic pressure condition. The phonon dispersion calculations also show thatC2/cphase is dynamic unstable. Furthermore, we confirm that Bi₂Se₃possessesI4/mmmphase rather thanIm-3mand 9/10-foldC2/mphase at high pressure. Combining the experimental and theoretical results, we determine the structural phase transition sequence for Bi₂Se₃ofR-3m→C2/m→C2/c→I4/mmmphase under quasi-hydrostatic pressure condition andR-3m→C2/m→I4/mmmphase under nonhydrostatic pressure condition. The dynamic unstability and pressure condition sensitivity ofC2/cphase may be account for the absence ofC2/cphase in the phase transition sequence under nonhydrostatic pressure condition. Our findings obtain the high-pressure phase transition sequences of Bi₂Se₃under hydrostatic and nonhydrostatic pressure condition, which can facilitate researchers to explore the novel properties in layered 3D topological insulators.

Monoclinic EuSn_{2}As_{2}: A Novel High-Pressure Network Structure

The layered crystal of EuSn_{2}As_{2} has a Bi_{2}Te_{3}-type structure in rhombohedral (R3[over ¯]m) symmetry and has been confirmed to be an intrinsic magnetic topological insulator at ambient conditions. Combining ab initio calculations and in situ x-ray diffraction measurements, we identify a new monoclinic EuSn_{2}As_{2} structure in C2/m symmetry above ∼14  GPa. It has a three-dimensional network made up of honeycomblike Sn sheets and zigzag As chains, transformed from the layered EuSn_{2}As_{2} via a two-stage reconstruction mechanism with the connecting of Sn-Sn and As-As atoms successively between the buckled SnAs layers. Its dynamic structural stability has been verified by phonon mode analysis. Electrical resistance measurements reveal an insulator-metal-superconductor transition at low temperature around 5 and 15 GPa, respectively, according to the structural conversion, and the superconductivity with a T_{C} value of ∼4  K is observed up to 30.8 GPa. These results establish a high-pressure EuSn_{2}As_{2} phase with intriguing structural and electronic properties and expand our understandings about the layered magnetic topological insulators.

The pressure-enhanced superconducting phase of Sr[Formula: see text]-Bi[Formula: see text]Se[Formula: see text] probed by hard point contact spectroscopy

The superconducting systems emerging from topological insulators upon metal ion intercalation or application of high pressure are ideal for investigation of possible topological superconductivity. In this context, Sr-intercalated Bi[Formula: see text]Se[Formula: see text] is specially interesting because it displays pressure induced re-entrant superconductivity where the high pressure phase shows almost two times higher [Formula: see text] than the ambient superconducting phase ( [Formula: see text] K). Interestingly, unlike the ambient phase, the pressure-induced superconducting phase shows strong indication of unconventional superconductivity. However, since the pressure-induced phase remains inaccessible to spectroscopic techniques, the detailed study of the phase remained an unattained goal. Here we show that the high-pressure phase can be realized under a mesoscopic point contact, where transport spectroscopy can be used to probe the spectroscopic properties of the pressure-induced phase. We find that the point contact junctions on the high-pressure phase show unusual response to magnetic field supporting the possibility of unconventional superconductivity.

A Combinatory Package for Diamond Anvil Cell Experiments

In this work, we introduce the Architecture Tech for High-Pressure Experiments Net Assembly (ATHENA) package based on diamond anvil cells, combining both the deposition of specimens as well as the detection of probes on anvils layer by layer. The specimens are typically ~1 μm in thickness and very hard to manipulate with traditional hand skills. ATHENA represents an all-in-one package by accurately synergizing chip-like networks prepared using magnetic sputtering methods and guaranteeing well-designed dimensions, positions and perfect electric contacts. We apply ATHENA successfully to the study of lanthanum metal above 60 GPa, showing very sharp pressure-enhanced superconductivity and parabolic critical temperature (Tc) evolution as a function of pressure with pressure-enhanced itinerant behavior at normal state.

Characterization and Decomposition of the Natural van der Waals SnSb2Te4 under Compression

High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb₂Te₄ into the high-pressure phases of its parent binary compounds (α-Sb₂Te₃ and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb₂Te₄ exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb₂Te₄ is framed within the extended orbital radii map of BA₂Te₄ compounds, so our results pave the way to understand the pressure behavior and stability ranges of other "natural van der Waals" compounds with similar stoichiometry.

Pressure-induced superconductivity in SnSb2Te4

Here we firstly report the pressure-induced superconductivity in phase change materials SnSb₂Te₄. Single crystals of SnSb₂Te₄ were grown using a conventional melting-down method. The resistance under pressure was measured using an originally designed diamond anvil cell with boron-doped diamond electrodes. The temperature dependence of the resistance under different pressures has been measured up to 32.6 GPa. The superconducting transition of SnSb₂Te₄ appeared at 2.1 K ([Formula: see text]) under 8.1 GPa, which was further increased with applied pressure to a maximum onset transition temperature 7.4 K under 32.6 GPa.

Symmetry of Order Parameters in Multiband Superconductors LaRu_{4}As_{12} and PrOs_{4}Sb_{12} Probed by Local Magnetization Measurements

The temperature dependencies of the lower critical field H_{c1}(T) of several filled-skutterudite superconductors were investigated by local magnetization measurements. While LaOs_{4}As_{12} and PrRu_{4}As_{12} exhibit the H_{c1}(T) dependencies consistent with the single-band BCS prediction, for LaRu_{4}As_{12} (the superconducting temperature T_{c}=10.4  K) with a similar three-dimensional Fermi surface, we observe a sudden increase in H_{c1}(T) deep in a superconducting state below about 0.32T_{c}. Remarkably, a rapid rise of H_{c1}(T) at approximately the same reduced temperature 0.27T_{c} is also found for the heavy-fermion compound PrOs_{4}Sb_{12} (T_{c}≃1.78  K), in fair accord with the earlier macroscopic study. We attribute the unusual H_{c1}(T) dependencies of LaRu_{4}As_{12} and PrOs_{4}Sb_{12} to a kink structure in their superfluid densities due to different contributions from two nearly decoupled bands. Whereas LaRu_{4}As_{12} is established as a two-band isotropic s-wave superconductor, nonsaturating behavior of H_{c1}(T) is observed for PrOs_{4}Sb_{12}, indicative of an anisotropic structure of a smaller gap. For this superconductor with broken time-reversal symmetry, our findings suggest a superconducting state with multiple symmetries of the order parameters.

Tip-induced superconductivity coexisting with preserved topological properties in line-nodal semimetal ZrSiS

ZrSiS was recently shown to be a new material with topologically non-trivial band structure that exhibits multiple Dirac nodes and a robust linear band dispersion up to an unusually high energy of 2 eV. Such a robust linear dispersion makes the topological properties of ZrSiS insensitive to perturbations like carrier doping or lattice distortion. Here, we show that a novel superconducting phase with a remarkably high [Formula: see text] of 7.5 K can be induced in single crystals of ZrSiS by a non-superconducting metallic tip of Ag. From first-principles calculations, we show that the observed superconducting phase might originate from a dramatic enhancement of density of states due to the presence of a metallic tip on ZrSiS. Our calculations also show that the emerging tip-induced superconducting phase co-exists with the well preserved topological properties of ZrSiS.

Structural phase transitions of (Bi1-xSbx)2(Te1-ySey)3 compounds under high pressure and the influence of the atomic radius on the compression processes of tetradymites

Recently, A₂B₃-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi₂Te₂Se, BiSbTeSe₂, and Sb₂Te₂Se tetradymites under high pressure. Bi₂Te₂Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi₂Te₃. Thus, the compression behavior of Bi₂Te₂Se is the same as that of Bi₂Se₃, which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe₂ and Sb₂Te₂Se undergo similar structural phase transitions to Bi₂Te₂Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A₂B₃-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. The influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.

Pressure induced superconductivity in the antiferromagnetic Dirac material BaMnBi2

The so-called Dirac materials such as graphene and topological insulators are a new class of matter different from conventional metals and (doped) semiconductors. Superconductivity induced by doing or applying pressure in these systems may be unconventional, or host mysterious Majorana fermions. Here, we report a successfully observation of pressure-induced superconductivity in an antiferromagnetic Dirac material BaMnBi₂ with T_c of ~4 K at 2.6 GPa. Both the higher upper critical field, μ₀H_c2(0) ~ 7 Tesla, and the measured current independent of T_c precludes that superconductivity is ascribed to the Bi impurity. The similarity in ρ_ab(B) linear behavior at high magnetic fields measured at 2 K both at ambient pressure (non-superconductivity) and 2.6 GPa (superconductivity, but at the normal state), as well as the smooth and similar change of resistivity with pressure measured at 7 K and 300 K in zero field, suggests that there may be no structure transition occurred below 2.6 GPa, and superconductivity observed here may emerge in the same phase with Dirac fermions. Our findings imply that BaMnBi₂ may provide another platform for studying SC mechanism in the system with Dirac fermions.

Superconductivity in HfTe5 across weak to strong topological insulator transition induced via pressures

Recently, theoretical studies show that layered HfTe₅ is at the boundary of weak & strong topological insulator (TI) and might crossover to a Dirac semimetal state by changing lattice parameters. The topological properties of 3D stacked HfTe₅ are expected hence to be sensitive to pressures tuning. Here, we report pressure induced phase evolution in both electronic & crystal structures for HfTe₅ with a culmination of pressure induced superconductivity. Our experiments indicated that the temperature for anomaly resistance peak (Tp) due to Lifshitz transition decreases first before climbs up to a maximum with pressure while the Tp minimum corresponds to the transition from a weak TI to strong TI. The HfTe₅ crystal becomes superconductive above ~5.5 GPa where the Tp reaches maximum. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. Crystal structure studies indicate that HfTe₅ transforms from a Cmcm phase across a monoclinic C2/m phase then to a P-1 phase with increasing pressure. Based on transport, structure studies a comprehensive phase diagram of HfTe₅ is constructed as function of pressure. The work provides valuable experimental insights into the evolution on how to proceed from a weak TI precursor across a strong TI to superconductors.

Superconductivity Bordering Rashba Type Topological Transition

Strong spin orbital interaction (SOI) can induce unique quantum phenomena such as topological insulators, the Rashba effect, or p-wave superconductivity. Combining these three quantum phenomena into a single compound has important scientific implications. Here we report experimental observations of consecutive quantum phase transitions from a Rashba type topological trivial phase to topological insulator state then further proceeding to superconductivity in a SOI compound BiTeI tuned via pressures. The electrical resistivity measurement with V shape change signals the transition from a Rashba type topological trivial to a topological insulator phase at 2 GPa, which is caused by an energy gap close then reopen with band inverse. Superconducting transition appears at 8 GPa with a critical temperature T_C of 5.3 K. Structure refinements indicate that the consecutive phase transitions are correlated to the changes in the Bi–Te bond and bond angle as function of pressures. The Hall Effect measurements reveal an intimate relationship between superconductivity and the unusual change in carrier density that points to possible unconventional superconductivity.

Suppression of superconductivity and structural phase transitions under pressure in tetragonal FeS

Pressure is a powerful tool to study iron-based superconductors. Here, we report systematic high-pressure transport and structural characterizations of the newly discovered superconductor FeS. It is found that superconductor FeS (tetragonal) partly transforms to a hexagonal structure at 0.4 GPa, and then completely transforms to an orthorhombic phase at 7.4 GPa and finally to a monoclinic phase above 9.0 GPa. The superconducting transition temperature of tetragonal FeS was gradually depressed by pressure, different from the case in tetragonal FeSe. With pressure increasing, the S-Fe-S angles only slightly change but the anion height deviates farther from 1.38 Å. This change of anion height, together with the structural instability under pressure, should be closely related to the suppression of superconductivity. We also observed an anomalous metal-semiconductor transition at 6.0 GPa and an unusual increased resistance with further compression above 9.6 GPa. The former can be ascribed to the tetragonal-orthorhombic structural phase transition, and the latter to the electronic structure changes of the high-pressure monoclinic phase. Finally, a phase diagram of tetragonal FeS as functions of pressure and temperature was mapped out for the first time, which will shed new light on understanding of the structure and physics of the superconducting FeS.

Nanoscale arrays of antimony telluride single crystals by selective chemical vapor deposition

Arrays of individual single nanocrystals of Sb₂Te₃ have been formed using selective chemical vapor deposition (CVD) from a single source precursor. Crystals are self-assembled reproducibly in confined spaces of 100 nm diameter with pitch down to 500 nm. The distribution of crystallite sizes across the arrays is very narrow (standard deviation of 15%) and is affected by both the hole diameter and the array pitch. The preferred growth of the crystals in the <1 1 0> orientation along the diagonal of the square holes strongly indicates that the diffusion of adatoms results in a near thermodynamic equilibrium growth mechanism of the nuclei. A clear relationship between electrical resistivity and selectivity is established across a range of metal selenides and tellurides, showing that conductive materials result in more selective growth and suggesting that electron donation is of critical importance for selective deposition.

Structural properties of Sb2S3 under pressure: evidence of an electronic topological transition

High-pressure Raman spectroscopy and x-ray diffraction of Sb₂S₃ up to 53 GPa reveals two phase transitions at 5 GPa and 15 GPa. The first transition is evidenced by noticeable compressibility changes in distinct Raman-active modes, in the lattice parameter axial ratios, the unit cell volume, as well as in specific interatomic bond lengths and bond angles. By taking into account relevant results from the literature, we assign these effects to a second-order isostructural transition arising from an electronic topological transition in Sb₂S₃ near 5 GPa. Close comparison between Sb₂S₃ and Sb₂Se₃ up to 10 GPa reveals a slightly diverse structural behavior for these two compounds after the isostructural transition pressure. This structural diversity appears to account for the different pressure-induced electronic behavior of Sb₂S₃ and Sb₂Se₃ up to 10 GPa, i.e. the absence of an insulator-metal transition in Sb₂S₃ up to that pressure. Finally, the second high-pressure modification appearing above 15 GPa appears to trigger a structural disorder at ~20 GPa; full decompression from 53 GPa leads to the recovery of an amorphous state.

Topologically protected surface states in a centrosymmetric superconductor β-PdBi2

The topological aspects of electrons in solids can emerge in real materials, as represented by topological insulators. In theory, they show a variety of new magneto-electric phenomena, and especially the ones hosting superconductivity are strongly desired as candidates for topological superconductors. While efforts have been made to develop possible topological superconductors by introducing carriers into topological insulators, those exhibiting indisputable superconductivity free from inhomogeneity are very few. Here we report on the observation of topologically protected surface states in a centrosymmetric layered superconductor, β-PdBi₂, by utilizing spin- and angle-resolved photoemission spectroscopy. Besides the bulk bands, several surface bands are clearly observed with symmetrically allowed in-plane spin polarizations, some of which crossing the Fermi level. These surface states are precisely evaluated to be topological, based on the Z₂ invariant analysis in analogy to three-dimensional strong topological insulators. β-PdBi₂ may offer a solid stage to investigate the topological aspect in the superconducting condensate.

Materials possessing topologically non-trivial electronic surface states are predicted to host exotic Majorana fermion excitations in the superconducting state. Here, the authors demonstrate the existence of topologically-protected surface states in the centrosymmetric layered superconductor β-PdBi2.

Electronic Topological Transition in Ag2Te at High-pressure

Recently, Ag₂Te was experimentally confirmed to be a 3D topological insulator (TI) at ambient pressure. However, the high-pressure behaviors and properties of Ag₂Te were rarely reported. Here, a pressure-induced electronic topological transition (ETT) is firstly found in Ag₂Te at 1.8 GPa. Before ETT, the positive pressure coefficient of bulk band-gap, which is firstly found in TIs family, is found by both first-principle calculations and in situ high-pressure resistivity measurements. The electrical resistivity obtained at room temperature shows a maximum at 1.8 GPa, which is nearly 3.3 times to that at ambient pressure. This result indicates that the best bulk insulating character and topological nature in Ag₂Te can be obtained at this pressure. Furthermore, the high-pressure structural behavior of Ag₂Te has been investigated by in situ high-pressure synchrotron powder X-ray diffraction technique up to 33.0 GPa. The accurate pressure-induced phase transition sequence is firstly determined as P2₁/c → Cmca → Pnma. It is worth noting that the reported isostructural P2₁/c phase is not existed, and the reported structure of Cmca phase is corrected by CALYPSO methodology. The second high-pressure structure, a long puzzle to previous reports, is determined as Pnma phase. A pressure-induced metallization in Ag₂Te is confirmed by the results of temperature-dependent resistivity measurements.

Emergent surface superconductivity in the topological insulator Sb2Te3

Surfaces of three-dimensional topological insulators have emerged as one of the most remarkable states of condensed quantum matter where exotic electronic phases of Dirac particles should arise. Here we report on superconductivity in the topological insulator Sb2Te3 with transition to zero resistance induced through a minor tuning of growth chemistry that depletes bulk conduction channels. The depletion shifts Fermi energy towards the Dirac point as witnessed by a factor of 300 reduction of bulk carrier density and by the largest carrier mobility (≳25,000 cm(2) V(-1) s(-1)) found in any topological material. Direct evidence from transport, the unprecedentedly large diamagnetic screening, and the presence of ∼25 meV gaps detected by scanning tunnelling spectroscopy reveal the superconducting condensate to emerge first in surface puddles, with the onset of global phase coherence at ∼9 K. The rich structure of this state lends itself to manipulation via growth conditions and the material parameters such as Fermi velocity and mean free path.

New developments in micro-X-ray diffraction and X-ray absorption spectroscopy for high-pressure research at 16-BM-D at the Advanced Photon Source

The monochromator and focusing mirrors of the 16-BM-D beamline, which is dedicated to high-pressure research with micro-X-ray diffraction (micro-XRD) and X-ray absorption near edge structure (XANES) (6-45 keV) spectroscopy, have been recently upgraded. Monochromatic X-rays are selected by a Si (111) double-crystal monochromator operated in an artificial channel-cut mode and focused to 5 μm × 5 μm (FWHM) by table-top Kirkpatrick-Baez type mirrors located near the sample stage. The typical X-ray flux is ∼5 × 10(8) photons/s at 30 keV. The instrumental resolution, Δq/qmax, reaches to 2 × 10(-3) and is tunable through adjustments of the detector distance and X-ray energy. The setup is stable and reproducible, which allows versatile application to various types of experiments including resistive heating and cryogenic cooling as well as ambient temperature compression. Transmission XANES is readily combined with micro-XRD utilizing the fixed-exit feature of the monochromator, which allows combined XRD-XANES measurements at a given sample condition.

Superconductivity in strong spin orbital coupling compound Sb₂Se₃

Recently, A₂B₃ type strong spin orbital coupling compounds such as Bi₂Te₃, Bi₂Se₃ and Sb₂Te₃ were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb₂Se₃ on the other hand was found to be topological trivial, but further theoretical studies indicated that the pressure might induce Sb₂Se₃ into a topological nontrivial state. Here, we report on the discovery of superconductivity in Sb₂Se₃ single crystal induced via pressure. Our experiments indicated that Sb₂Se₃ became superconductive at high pressures above 10 GPa proceeded by a pressure induced insulator to metal like transition at ~3 GPa which should be related to the topological quantum transition. The superconducting transition temperature (T_C) increased to around 8.0 K with pressure up to 40 GPa while it keeps ambient structure. High pressure Raman revealed that new modes appeared around 10 GPa and 20 GPa, respectively, which correspond to occurrence of superconductivity and to the change of T_C slop as the function of high pressure in conjunction with the evolutions of structural parameters at high pressures.

The d-p band-inversion topological insulator in bismuth-based skutterudites

Skutterudites, a class of materials with cage-like crystal structure which have received considerable research interest in recent years, are the breeding ground of several unusual phenomena such as heavy fermion superconductivity, exciton-mediated superconducting state and Weyl fermions. Here, we predict a new topological insulator in bismuth-based skutterudites, in which the bands involved in the topological band-inversion process are d- and p-orbitals, which is distinctive with usual topological insulators, for instance in Bi2Se3 and BiTeI the bands involved in the topological band-inversion process are only p-orbitals. Due to the present of large d-electronic states, the electronic interaction in this topological insulator is much stronger than that in other conventional topological insulators. The stability of the new material is verified by binding energy calculation, phonon modes analysis, and the finite temperature molecular dynamics simulations. This new material can provide nearly zero-resistivity signal current for devices and is expected to be applied in spintronics devices.

Sb₂Se₃ under pressure

Selected members of the A₂B₃ (A = Sb, Bi; B = Se, Te) family are topological insulators. The Sb₂Se₃ compound does not exhibit any topological properties at ambient conditions; a recent high-pressure study, however, indicated that pressure transforms Sb₂Se₃ from a band insulator into a topological insulator above ~2 GPa; in addition, three structural transitions were proposed to occur up to 25 GPa. Partly motivated by these results, we have performed x-ray diffraction and Raman spectroscopy investigations on Sb₂Se₃ under pressure up to 65 GPa. We have identified only one reversible structural transition: the initial Pnma structure transforms into a disordered cubic bcc alloy above 51 GPa. On the other hand, our high-pressure Raman study did not reproduce the previous results; we attribute the discrepancies to the effects of the different pressure transmitting media used in the high-pressure experiments. We discuss the structural behavior of Sb₂Se₃ within the A₂B₃ (A = Sb, Bi; B = Se, Te) series.