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V3Si, a classical silicide superconductor with relatively high $T_{\mathrm{C}}$ T C (∼16 K), is promising for constructing silicon-based superconducting devices and hetero-structures. However, real space characterization on its surfaces and superconducting properties are still limited. Here we report the first low-temperature scanning tunnelling microscopy (STM) study on cleaned V3Si (111) single crystal surface. We observed a $\sqrt{3} \times \sqrt{3}$ 3 × 3 superstructure which displays mirror symmetry between adjacent terraces, indicating the surface is V-terminated and reconstructed. The tunneling spectrum shows full superconducting gap with double pairs of coherence peaks, but has a relatively small gap size with comparing to bulk $T_{\mathrm{C}}$ T C . Impurity induced in-gap state is absent on surface defects but present on introduced magnetic adatoms. Upon applying magnetic field, a hexagonal vortex lattice is visualized. Interestingly, the vortex size is found to be field dependent, and the coherence length measured from single vortex at low field is significantly larger than estimated value from bulk $H_{c2}$ H c 2 . These results reflect V3Si is a multi-band, s-wave superconductor.

The interface between magnetic material and superconductors has long been predicted to host unconventional superconductivity, such as spin-triplet pairing and topological nontrivial pairing state, particularly when spin-orbital coupling (SOC) is incorporated. To identify these novel pairing states, fabricating homogenous heterostructures which contain such various properties are preferred, but often challenging. Here we synthesized a tri-layer type van-der Waals heterostructure of MnTe/Bi2Te3/Fe(Te, Se), which combined s-wave superconductivity, thickness dependent magnetism and strong SOC. Via low-temperature scanning tunneling microscopy (STM), we observed robust zero-energy states with notably nontrivial properties and an enhanced superconducting gap size on single unit-cell (UC) MnTe surface. In contrast, no zero-energy state was observed on 2UC MnTe. First-principle calculations further suggest the 1UC MnTe has large interfacial Dzyaloshinskii-Moriya interaction (DMI) and a frustrated AFM state, which could promote non-collinear spin textures. It thus provides a promising platform for exploring topological nontrivial superconductivity., 33 pages, supplementary materials included

The interface between magnetic material and superconductors has long been predicted to host unconventional superconductivity, such as spin-triplet pairing and topological nontrivial pairing state, particularly when spin-orbital coupling (SOC) is incorporated. To identify these unconventional pairing states, fabricating homogenous heterostructures that contain such various properties are preferred but often challenging. Here, we synthesized a trilayer-type van der Waals heterostructure of MnTe/Bi

Description, Addressing design of future quantum devices, the study uncovers electronic structure of a superconductor-semiconductor interface., The electronic structure of heterointerfaces is a pivotal factor for their device functionality. We use soft x-ray angle-resolved photoelectron spectroscopy to directly measure the momentum-resolved electronic band structures on both sides of the Schottky heterointerface formed by epitaxial films of the superconducting NbN on semiconducting GaN, and determine their momentum-dependent interfacial band offset as well as the band-bending profile. We find, in particular, that the Fermi states in NbN are well separated in energy and momentum from the states in GaN, excluding any notable electronic cross-talk of the superconducting states in NbN to GaN. We support the experimental findings with first-principles calculations for bulk NbN and GaN. The Schottky barrier height obtained from photoemission is corroborated by electronic transport and optical measurements. The momentum-resolved understanding of electronic properties of interfaces elucidated in our work opens up new frontiers for the quantum materials where interfacial states play a defining role.

The superconductivity of a kagome superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ is studied by scanning tunneling microscopy and spectroscopy at ultralow temperature with high resolution. Two kinds of superconducting gaps with multiple sets of coherent peaks and residual zero-energy density of states (DOS) are observed on both half-Cs and Sb surfaces, implying multiband superconductivity. In addition, in-gap states can be induced by magnetic impurities but not by nonmagnetic impurities, suggesting a sign-preserving or $s$-wave superconducting order parameter. Moreover, the interplay between charge density waves (CDW) and superconductivity differs on various bands, resulting in different density-of-states distributions. Our results suggest that the superconducting gap is likely isotropic on the sections of Fermi surface that play little roles in CDW, and the superconducting gaps on the sections of Fermi surface with anisotropic CDW gaps are likely anisotropic as well. The residual spectral weights at zero energy are attributed to the extremely small superconducting gap on the tiny oval Fermi pockets. Our study provides critical clues for further understanding the superconductivity and its relation to CDW in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$.

At the interface between monolayer FeSe films and SrTiO3 substrates the superconducting transition temperature (Tc) is unexpectedly high, triggering a surge of excitement. The mechanism for the Tc enhancement has been the central question, as it may present a new strategy for seeking out higher Tc materials. To reveal this enigmatic mechanism, by combining advances in high quality interface growth, 16O $$\leftrightarrow$$ ↔ 18O isotope substitution, and extensive data from angle resolved photoemission spectroscopy, we provide striking evidence that the high Tc in FeSe/SrTiO3 is the cooperative effect of the intrinsic pairing mechanism in the FeSe and interactions between the FeSe electrons and SrTiO3 phonons. Furthermore, our results point to the promising prospect that similar cooperation between different Cooper pairing channels may be a general framework to understand and design high-temperature superconductors.

${\mathrm{CeOs}}_{4}{\mathrm{Sb}}_{12}$ (COS) and ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ (POS) are two representative compounds that provide the ideal vantage point to systematically study the physics of multi-$f$-electron systems. COS with Ce $4{f}^{1}$, and POS with Pr $4{f}^{2}$ configurations show distinct properties of Kondo insulating and heavy fermion superconductivity, respectively. We unveiled the underlying microscopic origin by angle-resolved photoemission spectroscopy studies. Their eV-scale band structure matches well, representing the common characters of conduction electrons in $R{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ systems ($R=\mathrm{rare}\text{ }\mathrm{earth}$). However, $f$ electrons interact differently with conduction electrons in COS and POS. Strong hybridization between conduction electrons and $f$ electrons is observed in COS with band dependent hybridization gaps, and the development of a Kondo insulating state is directly revealed. Although the ground state of POS is a singlet, finite but incoherent hybridization exists, which can be explained by the Kondo scattering with the thermally excited triplet crystalline electric field state. Our results help us to understand the intriguing properties in COS and POS, and provide a clean demonstration of the microscopic differences in heavy fermion systems with $4{f}^{1}$ and $4{f}^{2}$ configurations.

Scanning tunneling microscopy/spectroscopy is applied herein to study the pristine and potassium (K)-doped single-layer p-quaterphenyl (P4P) films grown on the Au(111) substrate at the molecular level. Abundant complex structural and electronic phases are induced by various K doping. The Fermi-level pinning effect is observed at a low doping level. On the contrary, K3P4P exhibits intriguing versatile phases and properties because charge carriers are effectively doped in. For example, two kinds of molecular vibration modes with energies below 100 meV are observed, indicating a possible strong electron-phonon coupling. The splitting of the lowest unoccupied molecular orbital state in K3P4P illustrates an electronic correlation effect, and its strength varies for four different K3P4P phases with different structures. In addition, the appearance of a Kondo resonance on the molecular vacancy/impurity implies a local molecular magnetic moment. Our results demonstrate that the complex electronic properties of an alkali metal-doped P4P/Au film stem from the existence of many competing interactions, such as electron-electron correlations and electron-vibration coupling, which can be effectively tuned via variable carrier doping and molecular structure. Our work also opens new routes toward engineering novel molecular devices and creating new electronic phases in strongly correlated molecular materials.

The energy and spatial distributions of vortex bound state in superconductors carry important information about superconducting pairing and the electronic structure. Although discrete vortex states, and sometimes a zero energy mode, had been observed in several iron-based superconductors, their spatial properties are rarely explored. In this study, we used low-temperature scanning tunneling microscopy to measure the vortex state of (Li,Fe)OHFeSe with high spatial resolution. We found that the nonzero energy states display clear spatial oscillations with a period corresponding to bulk Fermi wavelength; while in contrast, the zero energy mode does not show such oscillation, which suggests its distinct electronic origin. Furthermore, the oscillations of positive and negative energy states near E_{F} are found to be clearly out of phase. Based on a two-band model calculation, we show that our observation is more consistent with an s_{++} wave pairing in the bulk of (Li, Fe)OHFeSe, and superconducting topological states on the surface.

The energy and spatial distributions of vortex bound state in superconductors carry important information about superconducting pairing and the electronic structure. Although discrete vortex states, and sometimes a zero energy mode, had been observed in several iron-based superconductors, their spatial properties are rarely explored. In this study, we used low-temperature scanning tunneling microscopy to measure the vortex state of (Li,Fe)OHFeSe with high spatial resolution. We found that the nonzero energy states display clear spatial oscillations with a period corresponding to bulk Fermi wavelength; while in contrast, the zero energy mode does not show such oscillation, which suggests its distinct electronic origin. Furthermore, the oscillations of positive and negative energy states near E_{F} are found to be clearly out of phase. Based on a two-band model calculation, we show that our observation is more consistent with an s_{++} wave pairing in the bulk of (Li, Fe)OHFeSe, and superconducting topological states on the surface.

Symmetry principles play a critical role in formulating the fundamental laws of nature, with a large number of symmetry-protected topological states identified in recent studies of quantum materials. As compelling examples, massless Dirac fermions are jointly protected by the space inversion symmetry $P$ and time reversal symmetry $T$ supplemented by additional crystalline symmetry, while evolving into Weyl fermions when either $P$ or $T$ is broken. Here, based on first-principles calculations, we reveal that massless Dirac fermions are present in a layered FeSn crystal containing antiferromagnetically coupled ferromagnetic Fe kagome layers, where each of the $P$ and $T$ symmetries is individually broken but the combined $PT$ symmetry is preserved. These stable Dirac fermions protected by the combined $PT$ symmetry with additional non-symmorphic $S_{\rm{2z}}$ symmetry can be transformed to either massless/massive Weyl or massive Dirac fermions by breaking the $PT$ or $S_{\rm{2z}}$ symmetry. Our angle-resolved photoemission spectroscopy experiments indeed observed the Dirac states in the bulk and two-dimensional Weyl-like states at the surface. The present study substantially enriches our fundamental understanding of the intricate connections between symmetries and topologies of matter, especially with the spin degree of freedom playing a vital role., Comment: 6 pages, 4 figures

In ultrathin films of FeSe grown on SrTiO3 (FeSe/STO), the superconducting transition temperature Tc is increased by almost an order of magnitude, raising questions on the pairing mechanism. As in other superconductors, antiferromagnetic spin fluctuations have been proposed to mediate SC making it essential to study the evolution of the spin dynamics of FeSe from the bulk to the ultrathin limit. Here, we investigate the spin excitations in bulk and monolayer FeSe/STO using resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to its bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems revealing a remarkable reconfiguration of spin excitations in FeSe/STO, essential to understand the role of spin fluctuations in the pairing mechanism., Here, Pelliciari et al. present resonant inelastic X-ray scattering on monolayer samples of unconventional superconductor FeSe, finding evidence for gapped and dispersionless spin excitations. These experiments are very difficult due to the extremely small scattering volume of the FeSe monolayer.

We use an ultrafast optical pump-probe spectroscopy to study quasiparticle (QP) dynamics in a topological insulator LaBi. Temperature-dependent optical measurements have been carried out, by which we observed nearly constant fast component (with a lifetime of 0.15 ps) and slow component (with a lifetime of 1.5 ps) for the whole range from 10 K to 295 K. The laser fluence dependence result shows that there is no saturation for the QP dynamics up to 3.3 mJ /cm2. Moreover, an E g mode transverse optical (TO) coherent phonon has also been observed, with a frequency of 2.8 THz. Our results provide for the first time the ultrafast dynamics information of both the QPs and coherent phonons in a nodal line topological material.

CrAs is a well-known helimagnet with the double-helix structure originating from the competition between the Dzyaloshinskii-Moriya interaction (DMI) and antiferromagnetic exchange interaction $J$. By resonant soft-x-ray scattering, we observe the magnetic peak (0 0 ${q}_{m}$) that emerges at the helical transition with ${T}_{S}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}267.5\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Intriguingly, the helimagnetic domains significantly shrink on cooling below $\ensuremath{\sim}255\phantom{\rule{4pt}{0ex}}\mathrm{K}$, opposite to the conventional thermal effect. The weakening of DMI on cooling is found to play a critical role here. It causes the helical wave vector to vary, ordered spins to rotate, and extra helimagnetic domain boundaries to form at local defects, thus leading to the anomalous shrinkage of helimagnetic domains. Our results indicate that the size of magnetic helical domains can be controlled by tuning DMI in certain helimagnets.

Quantum critical behavior occurs at low temperatures where quantum fluctuations become more important than thermal fluctuations. Of particular current interest is unconventional quantum criticality, which lacks a classical thermal counterpart. Angle resolved photoemission and scanning tunneling microscopy are experimental techniques that measure single-particle excitations, and recently been used to probe strongly correlated metals near quantum critical points Here, how the single-particle properties reflect the unconventional quantum criticality is discussed, and the prospect for further measurements to unearth new physical behavior.

Using low-temperature scanning tunneling microscopy (STM), we studied the vortex states of single-layer FeSe film on a ${\mathrm{SrTiO}}_{3}$ (100) substrate, and the local behaviors of superconductivity at sample boundaries. We clearly observed multiple discrete Caroli--de Gennes--Matricon states in the vortex core, and quantitative analysis shows their energies well follow the formula: $E{=\ensuremath{\mu}\mathrm{\ensuremath{\Delta}}}^{2}/{E}_{F}$, where $\ensuremath{\mu}$ is a half integer ($\ifmmode\pm\else\textpm\fi{}1/2,\ifmmode\pm\else\textpm\fi{}3/2,\ifmmode\pm\else\textpm\fi{}5/2\dots{}$) and $\mathrm{\ensuremath{\Delta}}$ is the mean superconducting gap over the Fermi surface. Meanwhile, a fully gapped spectrum without states near zero bias is observed at the ${[110]}_{\mathrm{Fe}}$ oriented boundary of 1 and 2 ML FeSe films, and atomic step edge of 1 ML FeSe. Accompanied with theoretical calculations, our results indicate an $s$-wave pairing without sign change in the high-${T}_{C}$ $\mathrm{FeSe}/{\mathrm{SrTiO}}_{3}$ superconductor.

${\mathrm{BaAg}}_{2}{\mathrm{As}}_{2}$, a sibling compound of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ with a nonmagnetic phase transition around 150 K, is studied by the comprehensive measurements of angle-resolved photoemission spectroscopy, synchrotron x-ray diffraction, and resonant soft x-ray scattering. The Fermi surfaces and electronic structure of ${\mathrm{BaAg}}_{2}{\mathrm{As}}_{2}$ are revealed, with strong ${k}_{z}$ dispersion, consistent with the strongly contracted $c/a$ ratio in ${\mathrm{BaAg}}_{2}{\mathrm{As}}_{2}$. Across the phase transition, splitting of [101] Bragg peak is observed, showing a structural distortion with the in-plane distortion magnitude $\ensuremath{\delta}=|a\ensuremath{-}b|/(a+b)=0.0052$. Although the nesting condition is satisfied in some parallel Fermi-surface sectors, there is no signature of charge density wave order at the nesting wave vector. Moreover, neither a charge density wave gap opening nor band reconstruction are observed across the phase transition. Instead, an enhancement on the spectral weight of dispersive bands is observed across the structural phase transition, which can explain the sharp drop of resistivity below the phase transition temperature. These studies could enrich the understanding of the variable and common features of the structural transition in transition metal pnictide layered materials.

The energy scales in rare-earth-based heavy-fermion compounds are relatively small, which can be easily tuned by applying pressure, magnetic field, or chemical doping. By substituting Yb for Ce on the rare-earth site, the ground state of superconductivity can be smoothly suppressed without the appearance of an apparent quantum critical point, and a number of remarkable phenomena have been observed. The slight changes in the electronic structure are supposed to dominate the underlying physics in these compounds. In the present study, we provide an electronic structure study of ${\mathrm{Ce}}_{0.85}{\mathrm{Yb}}_{0.15}{\mathrm{CoIn}}_{5}$ with a superconducting state but suppressed transition temperature by angle-resolved photoemission spectroscopy, and the results are compared with ${\mathrm{CeCoIn}}_{5}$. We find that the $f$ electrons in ${\mathrm{Ce}}_{0.85}{\mathrm{Yb}}_{0.15}{\mathrm{CoIn}}_{5}$ are itinerant, forming the weakly dispersive hybridized band at low temperature. More interestingly, the hybridization strength between the $f$ electrons and conduction electrons in ${\mathrm{Ce}}_{0.85}{\mathrm{Yb}}_{0.15}{\mathrm{CoIn}}_{5}$ is comparable with ${\mathrm{CeCoIn}}_{5}$. Further temperature-dependent measurements provide direct evidence of the localized-to-itinerant crossover behavior of the $4f$ electrons in this compound.

The interaction between superconductivity and magnetism in heavy fermion compounds has remained a challenge ever since the discovery of unconventional superconductivity in such systems. The heavy fermion $\mathrm{Ce}T\mathrm{In}{}_{5}$ ($T$ = Co,Rh,Ir) family offers a good platform to study the relationship between superconductivity and magnetism, whose ground state can be easily tuned by chemical doping, making the ambient-pressure experimental techniques like angle-resolved photoemission spectroscopy possible to address this old but engaging question. By Cd doping, the superconducting state of ${\mathrm{CeCoIn}}_{5}$ can be smoothly tuned to an antiferromagnetic ground state, which can be reversed by applying pressure. Here we present an electronic structure study of $\mathrm{CeCo}(\mathrm{In}{}_{0.85}\mathrm{Cd}{}_{0.15}{)}_{5}$ with an antiferromagnetic ground state and the results are compared with ${\mathrm{CeCoIn}}_{5}$. We found that the hybridization strength between the $f$ electrons and conduction electrons in ${\mathrm{CeCoIn}}_{5}$ has been suppressed by Cd doping, but still the Fermi surface includes a significant contribution from the Ce $f$ electrons. Our results find that the electronic tuning effect by Cd doping arises from the change in the hybridization strength between the Ce $f$ electrons and conduction electrons caused by the Cd dopant, and the subtle interplay between itinerancy and localization of the $f$ electrons determines the ground state.

Doped BaCoSO was recently predicted to be a high-temperature superconductor in a new class based on Co and Ni. Using a Co-S self flux method, we synthesized single crystals of the antiferromagnetic insulator BaCoSO. Our magnetic and specific heat measurements and neutron diffraction provide details of its magnetic anisotropy and order. Its band gap was determined to be about 1.3 eV by our measurements of its photoemission spectrum and infrared optical conductivity. Our results can pave the way to exploring the predicted superconductivity in this Co-based material., 6 pages, 6 figures and Supplemental Material

While the pure elements tend to exhibit Type-I rather than Type-II superconductivity, nearly all compound superconductors are Type-II, with only a few known exceptions. We report single crystal growth and physical characterization of the rhenium aluminide Al$_6$Re, which we conclude is a Type-I superconductor based on magnetization, ac-susceptibility, and specific-heat measurements. This detection of superconductivity, despite the strong similarity of Al$_6$Re to a family of W and Mo aluminides that do not superconduct, suggests that these aluminides are an ideal testbed for identifying the relative importance of valence electron count and inversion symmetry in determining whether a material will superconduct., 9 pages, 7 figures, CIF file as ancillary

Unconventional superconductivity often intertwines with various forms of order, such as the "nematic" order which breaks the rotational symmetry of the lattice. Investigation of these ordered phases sheds crucial light on the superconductivity itself. Here we report a low-temperature scanning tunneling microscopy (STM) study on RbFe2As2, a heavily hole-doped Fe-based superconductor (FeSC). We observe significant symmetry breaking in its electronic structure and magnetic vortex which differentiates the ({\pi}, {\pi}) and ({\pi}, -{\pi}) directions of the unfolded Brillouin zone (BZ). It is thus a novel nematic state, distinct from the nematicity of undoped/lightly-doped FeSCs which breaks the ({\pi}, 0) / (0, {\pi}) equivalence. Moreover, we observe a clear "V"-shaped superconducting gap which can be well fitted with a nodal gap function. The gap is found to be suppressed on surface Rb vacancies and at step edges, and the suppression is particularly strong at the [110]-oriented edges. This is possibly due to a dx2-y2 like pairing component with nodes along the [110] directions. We further demonstrated that such ({\pi}, {\pi}) nematic state can be suppressed via surface electron doping to RbFe2As2, and the superconductivity is subsequently enhanced. Our results thus highlight the intimate connection between nematicity and superconducting pairing in iron-based superconductors., Comment: 19 pages, 7 figures, supplementary materials included

The localized-to-itinerant transition of $f$ electrons lies at the heart of heavy-fermion physics, but has only been directly observed in single-layer Ce-based materials. Here, we report a comprehensive study on the electronic structure and nature of the Ce $4f$ electrons in the heavy-fermion superconductor ${\mathrm{Ce}}_{2}{\mathrm{PdIn}}_{8}$, a typical $n=2 {\mathrm{Ce}}_{n}{\mathrm{M}}_{m}{\mathrm{In}}_{3n+2m}$ compound, using high-resolution and $4d\ensuremath{-}4f$ resonant photoemission spectroscopies. The electronic structure of this material has been studied over a wide temperature range, and hybridization between $f$ and conduction electrons can be clearly observed to form a Kondo resonance near the Fermi level at low temperatures. The characteristic temperature of the localized-to-itinerant transition is around 120 K, which is much higher than its coherence temperature ${T}_{\text{coh}}\ensuremath{\sim}30\phantom{\rule{0.28em}{0ex}}\mathrm{K}$.

The Majorana zero mode (MZM), which manifests as an exotic neutral excitation in superconductors, is the building block of topological quantum computing. It has recently been found in the vortices of several iron-based superconductors as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. In particular, a clean and robust MZM has been observed in the cores of free vortices in (Li0.84Fe0.16)OHFeSe. Here using scanning tunneling spectroscopy (STS), we demonstrate that Majorana-induced resonant Andreev reflection occurs between the STM tip and this zero-bias bound state, and consequently, the conductance at zero bias is quantized as 2e2/h. Our results present a hallmark signature of the MZM in the vortex of an intrinsic topological superconductor, together with its intriguing behavior., Comment: 10 pages, 7 figures, supplementary materials included

Na2Ti2Sb2O is a parent compound of the titanium-based oxypnictide superconductors. It possesses a phase transition at Ts = 115 K whose nature is still unclear. Here we report a scanning tunnelling microscopy/spectroscopy (STM/STS) study on a Na2Ti2Sb2O single crystal. We observe a lattice distortion on the Na-terminated surface below Ts, which displays a 2 × 2 modulation (superstructure). The modulation vectors q = (±0.5, 0) and (0, ±0.5) match well with the Fermi surface nesting vectors observed by angle resolved photoemission spectroscopy (ARPES), and also agree with recent X-ray diffraction. A partially opened gap-like feature is present in STS near EF. This evidences that the lattice distortion is driven by commensurate charge density wave order. Our results suggest the electron–phonon interaction is strong in Na2Ti2Sb2O and could be responsible for the superconductivity in titanium-based oxypnictides.

New experiments find that Majorana zero modes---quasiparticles with potential quantum computing applications---can persist in topological superconductors, despite previous difficulties in doing so.

Cu$_x$Bi$_2$Se$_3$ hosts both topological surface states and bulk superconductivity. It has been identified recently as a topological superconductor (TSC) with an extraordinary nematic, i. e. C2-symmetric, superconducting state and odd-parity pairing. Here, using scanning tunneling microscopy (STM), we directly examine the response of the superconductivity of CuxBi2Se3 to magnetic field. Under out-of-plane fields, we discover elongated magnetic vortices hosting zero-bias conductance peaks consistent with the Majorana bound states expected in a TSC. Under in-plane fields, the average superconducting gap exhibits two-fold symmetry with field orientation; the long C2 symmetry axes are pinned to the dihedral mirror planes under B//=0.5 T but rotate slightly under B//=1.0 T. Moreover, a nodeless {\Delta}4x gap structure is semi-quantitatively determined for the first time. Our data paint a microscopic picture of the nematic superconductivity in CuxBi2Se3 and pose strong constraints on theory.

Surface potassium dosing has been proven to be an effective method in tuning the electron doping and enhancing the superconducting transition temperatures in both iron chalcogenides and electron doped iron pnictides. However, it is not clear how surface potassium dosing affects the hole doping and superconductivity in hole doped Fe-based superconductors. Here we performed K-dosing on Ba

The mechanism of high superconducting transition temperatures (T_{c}) in bismuthates remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba_{0.51}K_{0.49}BiO_{3} reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions-the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics.Furthermore, we observe an isotropic superconducting gap with 2Δ_{0}/k_{B}T_{c}=3.51±0.05, and strong electron-phonon interactions with a coupling constant λ∼1.3±0.2. These findings solve a long-standing mystery-Ba_{0.51}K_{0.49}BiO_{3} is an extraordinary Bardeen-Cooper-Schrieffer superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high T_{c}. Such effects will also be critical for finding new superconductors.

van der Waals heterostructures (VDWHs) exhibit rich properties and thus has potential for applications, and charge transfer between different layers in a heterostructure often dominates its properties and device performance. It is thus critical to reveal and understand the charge transfer effects in VDWHs, for which electronic structure measurements have proven to be effective. Using angle-resolved photoemission spectroscopy, we studied the electronic structures of $(\mathrm{PbSe}{)}_{1.16}({\mathrm{TiSe}}_{2}{)}_{m}$ ($m=1$, 2), which are naturally occurring VDWHs, and discovered several striking charge transfer effects. When the thickness of the ${\mathrm{TiSe}}_{2}$ layers is halved from $m=2$ to $m=1$, the amount of charge transferred increases unexpectedly by more than 250%. This is accompanied by a dramatic drop in the electron-phonon interaction strength far beyond the prediction by first-principles calculations and, consequently, superconductivity only exists in the $m=2$ compound with strong electron-phonon interaction, albeit with lower carrier density. Furthermore, we found that the amount of charge transferred in both compounds is nearly halved when warmed from below 10 K to room temperature, due to the different thermal expansion coefficients of the constituent layers of these misfit compounds. These unprecedentedly large charge transfer effects might widely exist in VDWHs composed of metal-semiconductor contacts; thus, our results provide important insights for further understanding and applications of VDWHs.

A key issue in heavy fermion research is how subtle changes in the hybridization between the $4f$ ($5f$) and conduction electrons can result in fundamentally different ground states. ${\mathrm{CeRhIn}}_{5}$ stands out as a particularly notable example: when replacing Rh with either Co or Ir, antiferromagnetism gives way to superconductivity. In this photoemission study of ${\mathrm{CeRhIn}}_{5}$, we demonstrate that the use of resonant angle-resolved photoemission spectroscopy with polarized light allows us to extract detailed information on the $4f$ crystal field states and details on the $4f$ and conduction electron hybridization, which together determine the ground state. We directly observe weakly dispersive Kondo resonances of $f$ electrons and identify two of the three Ce $4{f}_{5/2}^{1}$ crystal-electric-field levels and band-dependent hybridization, which signals that the hybridization occurs primarily between the Ce $4f$ states in the ${\mathrm{CeIn}}_{3}$ layer and two more three-dimensional bands composed of the Rh $4d$ and In $5p$ orbitals in the ${\mathrm{RhIn}}_{2}$ layer. Our results allow us to connect the properties observed at elevated temperatures with the unusual low-temperature properties of this enigmatic heavy fermion compound.

A lack of spatial inversion symmetry gives rise to a variety of unconventional physics, from noncollinear order and Skyrmion lattice phases in magnetic materials to topologically-protected surface states in certain band insulators, to mixed-parity pairing states in superconductors. The search for exotic physics in such materials is largely limited by a lack of candidate materials, and often by difficulty in obtaining crystals. Here, we report the single crystal growth and physical properties of the noncentrosymmetric tungsten aluminide cage compounds Al$_4$W and Al$_5$W, alongside related molybdenum aluminides in which spin-orbit coupling should be significantly weaker. All compounds are nonmagnetic metals. Their high conductivities suggest the opportunity to find superconductivity at lower temperatures, while the limits we can place on their transition temperatures suggest that any superconductivity may be expected to exhibit significant parity mixing., Comment: 11 pages, 6 figures, including Supplementary. CIF files available in source

Crystal electric field states in rare earth intermetallics show an intricate entanglement with the many-body physics that occurs in these systems and that is known to lead to a plethora of electronic phases. Here we attempt to trace different contributions to the crystal electric field (CEF) splittings in ${\mathrm{CeIrIn}}_{5}$, a heavy-fermion compound and member of the $\mathrm{Ce}M{\mathrm{In}}_{5}$ ($M=$ Co, Rh, Ir) family. To this end, we utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) and present a spectroscopic study of the electronic structure of this unconventional superconductor over a wide temperature range. As a result, we show how ARPES can be used in combination with thermodynamic measurements or neutron scattering to disentangle different contributions to the CEF splitting in rare earth intermetallics. We also find that the hybridization is stronger in ${\mathrm{CeIrIn}}_{5}$ than ${\mathrm{CeCoIn}}_{5}$ and the effects of the hybridization on the Fermi volume increase is much smaller than predicted. By providing experimental evidence for $4{f}_{7/2}^{1}$ splittings which, in ${\mathrm{CeIrIn}}_{5}$, split the octet into four doublets, we clearly demonstrate the many-body origin of the so-called $4{f}_{7/2}^{1}$ state.

Unconventional superconductors are often characterized by numerous competing and even intertwined orders in their phase diagrams. In particular, the electronic nematic phases, which spontaneously break rotational symmetry and often simultaneously involve spin, charge and/or orbital orders, appear conspicuously in both the cuprate and iron-based superconductors. The fluctuations associated with these phases may provide the exotic pairing glue that underlies their high-temperature superconductivity. Helimagnet MnP, the first Mn-based superconductor under pressure, lacks high rotational symmetry. However our resonant soft X-ray scattering (RSXS) experiment discovers novel helical orbital density wave (ODW) orders in this three-dimensional, low-symmetry system, and reveals intertwined ordering phenomena in unprecedented detail. In particular, a ODW forms with half the period of the spin order and fully develops slightly above the spin ordering temperature, their domains develop simultaneously, yet the spin order domains are larger than those of the ODW, and they cooperatively produce another ODW with 1/3 the period of the spin order. These observations provide a comprehensive picture of the intricate interplay between spin and orbital orders in correlated materials, and they suggest that nematic-like physics ubiquitously exists beyond two-dimensional and high-symmetry systems, and the superconducting mechanism of MnP is likely analogous to those of cuprate and iron-based superconductors.