Gap-like Feature Research Articles

Iron Vacancy Tunable Superconductor-Insulator Transition in FeSe/SrTiO_{3} Monolayer.

The Fe_{4}Se_{5} with a sqrt[5]×sqrt[5] Fe vacancy order is suggested to be a Mott insulator and the parent state of bulk FeSe superconductor. The iron vacancy ordered state has been considered as a Mott insulator and the parent compound of bulk FeSe-based superconductors. However, for the superconducting FeSe/SrTiO_{3} monolayer (FeSe/STO) with an interface-enhanced high transition temperature (T_{c}), the electronic evolution from its Fe vacancy ordered parent phase to the superconducting state, has not been explored due to the challenge to realize an Fe vacancy order in the FeSe/STO monolayer, even though important to the understanding of superconductivity mechanism. In this study, we developed a new method to generate Fe vacancies within the FeSe/STO monolayer in a tunable fashion, with the assistance of atomic hydrogen. As a consequence, an insulating sqrt[5]×sqrt[5] Fe vacancy ordered monolayer is realized as the parent state. By using scanning tunneling microscopy and scanning tunneling spectroscopy, the spectral evolution from superconductivity to insulator is fully characterized. Surprisingly, a prominent spectral weight transfer occurs, thus implying a strong electron correlation effect. Moreover, the Fe vacancy induced insulating gap exhibits no Mott gap-like features. This work provides new insights in understanding the high-T_{c} superconductivity in FeSe/STO monolayer.

Probing Hidden Mott Gap and Incommensurate Charge Modulation on the Polar Surfaces of PdCrO_{2}.

Here we report a combined study of low-temperature scanning tunneling microscopy and dynamical mean-field theory on PdCrO_{2}, a delafossite metal with an antiferromagnetic order below ∼37.5 K. First, on the CrO_{2}-terminated polar surface we detect a gaplike feature both below and above the Néel temperature. The dynamical mean-field theory calculations indicate that this gap is opened due to the strong correlations of Cr-3d electrons, suggesting the hidden Mott nature of the gap. Then, we observe two kinds of Pd-terminated polar surfaces. One is a well-ordered Pd surface with the Fermi-surface-nesting-induced incommensurate charge modulation, while the other one is a reconstructed Pd surface with the individual nanoscale nonperiodic domain structures. On the well-ordered Pd surface, the interference between the incommensurate charge modulation and the atomic lattice forms the periodic moiré pattern. Our results provide important microscopic information for fully understanding the correlated electronic properties of this class of materials.

Characterization and dynamics of the peculiar stream Jhelum

Context. Stellar streams are a promising tool to study the Milky Way’s dark matter subhalo population, as interactions with subhalos are expected to leave visible imprints in the streams in the form of substructure. However, there may be other causes of substructure. Aims. Here we studied the kinematics and the unusual morphology of the stellar stream Jhelum. Methods. Using a combination of ground-based photometry and Gaia EDR3 astrometry, we characterized the morphology of Jhelum. We combined this new data with radial velocities from the literature to perform orbit integrations of the stream in static Galactic potentials. We also carried out N-body simulations in the presence of the Sagittarius dwarf galaxy. Results. The new data reveal a previously unreported tertiary component in the stream, as well as several gaps and a kink-like feature in its narrow component. We find that for a range of realistic Galactic potentials, no single orbit is able to reproduce Jhelum’s radial velocity data entirely. A generic property of the orbital solutions is that they share a similar orbital plane to Sagittarius and this leads to repeated encounters with the stream. Using N-body simulations that include a massive Sagittarius, we explored its effect on Jhelum, and we show that these encounters are able to qualitatively reproduce the narrow and broad components in Jhelum, as well as create a tertiary component in some cases. We also find evidence that such encounters can result in an apparent increase in the velocity dispersion of the stream by a factor up to four due to overlapping narrow and broad components. Conclusions. Our findings suggest that the Jhelum stream is even more complex than once thought; however, its morphology and kinematics can tentatively be explained via the interactions with Sagittarius. In this scenario, the formation of Jhelum’s narrow and broad components occurs naturally, yet some of the smaller gap-like features remain to be explained.

Energy-gap driven low-temperature magnetic and transport properties inCr1/3MS2(M= Nb, Ta)

The helimagnets Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) have attracted renewed attention due to the discovery of a chiral soliton lattice (CSL) stabilized in an applied magnetic field, but reports of unusual low-temperature transport and magnetic properties in this system lack a unifying explanation. Here we present electronic structure calculations that demonstrate the materials are half-metals. There is also a gap-like feature (width in range 40-100 meV) in the density of states of one spin channel. This electronic structure explains the low-temperature electronic and magnetic properties of Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta), with the gap-like feature particularly important for explaining the magnetic behavior. Our magnetometry measurements confirm the existence of this gap. Dynamic spin fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with AC susceptibility and muon-spin relaxation ($\mu^+$SR) measurements. We show further how effects due to the CSL in Cr$_{1/3}$NbS$_2$, as detected with $\mu^+$SR, dominate over the gap-driven magnetism when the CSL is stabilized as the majority phase.

Evolution of the electronic structure in Ta2NiSe5 across the structural transition revealed by resonant inelastic x-ray scattering

We utilized high-energy-resolution resonant inelastic X-ray scattering (RIXS) at both the Ta and Ni $L_3$-edges to map out element-specific particle-hole excitations in Ta$_2$NiSe$_5$ across the phase transition. Our results reveal a momentum dependent gap-like feature in the low energy spectrum, which agrees well with the band gap in element-specific joint density of states calculations based on ab initio estimates of the electronic structure in both the low temperature monoclinic and the high temperature orthorhombic structure. Below $T_c$, the RIXS energy-momentum map shows a minimal gap at the Brillouin zone center ($\sim$ 0.16 eV), confirming that Ta$_2$NiSe$_5$ possesses a direct band gap in its low temperature ground state. However, inside the gap, no signature of anticipated collective modes with an energy scale comparable to the gap size can be identified. Upon increasing the temperature to above $T_c$, whereas the gap at the zone center closes, the RIXS map at finite momenta still possesses the gross features of the low temperature map, suggesting a substantial mixing between the Ta and Ni orbits in the conduction and valence bands, which does not change substantially across the phase transition. Our experimental observations and comparison to the theoretical calculations lend further support that the phase transition and the corresponding gap opening in Ta$_2$NiSe$_5$ is largely structural by nature with possible minor contribution from the putative exciton condensate.

Spectroscopic evidence for the direct involvement of local moments in the pairing process of the heavy-fermion superconductor CeCoIn5

The microscopic mechanism for electron pairing in heavy-fermion superconductors remains a major challenge in quantum materials. Some form of magnetic mediation is widely accepted with spin fluctuations as a prime candidate. A novel mechanism, 'composite pairing' based on the cooperative two-channel Kondo effect directly involving the f-electron moments has also been proposed for some heavy fermion compounds including CeCoIn$_5$. The origin of the spin resonance peak observed in neutron scattering measurements on CeCoIn$_5$ is still controversial and the corresponding hump-dip structure in the tunneling conductance is missing. This is in contrast to the cuprate and Fe-based high-temperature superconductors, where both characteristic signatures are observed, indicating spin fluctuations are likely involved in the pairing process. Here, we report results from planar tunneling spectroscopy along three major crystallographic orientations of CeCoIn5 over wide ranges of temperature and magnetic field. The pairing gap opens at T$_p$ ~ 5 K, well above the bulk T$_c$ = 2.3 K, and its directional dependence is consistent with d$_{x^2-y^2}$ symmetry. With increasing magnetic field, this pairing gap is suppressed as expected but, intriguingly, a gaplike structure emerges smoothly, increasing linearly up to the highest field applied. This field-induced gaplike feature is only observed below T$_p$. The concomitant appearance of the pairing gap and the field-induced gaplike feature, along with its linear increase with field, indicates that the f-electron local moments are directly involved in the pairing process in CeCoIn$_5$.

The transport properties of ultrathin 2H–NbSe2

The competing orders arising in a two-dimensional limit are the key to understanding unconventional superconductivity. The monolayer NbSe2 possessing a charge density wave (CDW) and superconducting orders provides a playground to study unconventional superconductivity. Here we fabricate an ultrathin 2H–NbSe2 device based on the mechanical exfoliation method and our electrode transfer technique. Detailed four-lead and two-lead transport measurements are performed in the ultrathin sample. The superconducting transition ( T c ) at 4.3 K and CDW transition at 70 K are found in the sample, consistent with the literature report for ultrathin 2H–NbSe2. The superconducting gap (Δ) of ultrathin 2H–NbSe2 is estimated by fitting the two-lead transport spectroscopy with the Blonder–Tinkham–Klapwijk (BTK) theory. We found that Δ( T ) exhibits a BCS-like temperature dependence with Δ 1.1 meV at 0 K and . Meanwhile the gap-like features persist in the normal state and become unresolvable with the increasing temperature and magnetic field, suggesting a possible precursor superconductivity similar to the cuprate superconductors. Our results provide a new insight into superconductivity in ultrathin 2H–NbSe2.

Coulomb blockade effects in a topological insulator grown on a high-Tc cuprate superconductor

The evidence for proximity-induced superconductivity in heterostructures of topological insulators and high-Tc cuprates has been intensely debated. We use molecular-beam epitaxy to grow thin films of topological insulator Bi2Te3 on a cuprate Bi2Sr2CaCu2O8+x, and study the surface of Bi2Te3 using low-temperature scanning tunneling microscopy and spectroscopy. In few unit-cell thick Bi2Te3 films, we find a V-shaped gap-like feature at the Fermi energy in dI/dV spectra. By reducing the coverage of Bi2Te3 films to create nanoscale islands, we discover that this spectral feature dramatically evolves into a much larger hard gap, which can be understood as a Coulomb blockade gap. This conclusion is supported by the evolution of dI/dV spectra with the lateral size of Bi2Te3 islands, as well as by topographic measurements that show an additional barrier separating Bi2Te3 and Bi2Sr2CaCu2O8+x. We conclude that the prominent gap-like feature in dI/dV spectra in Bi2Te3 films is not a proximity-induced superconducting gap. Instead, it can be explained by Coulomb blockade effects, which take into account additional resistive and capacitive coupling at the interface. Our experiments provide a fresh insight into the tunneling measurements of complex heterostructures with buried interfaces.

Clarifying the intrinsic nature of the phonon-induced gaps of graphite in the spectra of scanning tunneling microscopy/spectroscopy

We report our investigation on the electron-phonon coupling in graphite using scanning tunneling microscopy/spectroscopy (STM/STS), in combination with angle-resolved photoemission spectroscopy (ARPES). The exhibition of a gaplike feature near the Fermi level both in the STS and ARPES spectra confirms its intrinsic origination because of the electron-phonon coupling in graphite, where the phonon modes at $\ifmmode\pm\else\textpm\fi{}65$ and $\ifmmode\pm\else\textpm\fi{}160$ meV are highly resolvable in the clean samples. Under our controlled experiment using W, Pt/Ir, and Ag tips, we find that the presence of hydrogen at the tip apex or in the tip-substrate gap is responsible for smearing out the gaps in the STS spectra by providing additional tunneling channels. Due to its small adsorption energy of hydrogen for Ag among these tip materials, our results suggest that the use of Ag tips can be more suitable for resolving the intrinsic phonon-induced property in carbon-based materials.

Melting of charge density wave and Mott gap collapse on 1T−TaS2 induced by interfacial water

Microscopically revealing the interactions between interfacial water and the quantum states of matter is an important task from both the materials science and the physics points of view. Here we report a low-temperature scanning tunneling microscopy (STM) and spectroscopy study of water adsorption on the charge density wave compound $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which has a Mott-insulating ground state. Interfacial water forms monolayer islands with $6\ifmmode\times\else\texttimes\fi{}6$ superstructures on the surface of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, and the charge order under water islands can be directly imaged in STM topographies taken with negative bias voltages. Compared with the original $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ charge order in $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, the charge order under water islands becomes significantly disordered and denser. A $\mathsf{V}$-shaped gaplike feature emerges in water-covered $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which may be due to the enhanced dielectric constant of interfacial water, which reduces short-range Coulomb repulsion and induces Mott gap collapse. Our observations open the way to microscopically understanding the interactions between interfacial water and the correlated quantum states of matter.

Point-contact spectroscopy on antiferromagnetic Kondo semiconductors CeT2Al10 (T = Ru and Os)**Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0303101 and 2016FYA0300402), the National Natural Science Foundation of China (Grant Nos. 11674279, 11774404, and 11374257), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LR18A04001), and the Japan Society for the Promotion of

We have carried out point-contact spectroscopy (PCS) measurements on one family of antiferromagnetic Kondo semiconductor CeT2Al10 (T = Ru and Os) with a Nèel temperature TN ∼ 27.5 and 28.5 K, respectively. Their PCS conductance curves both exhibit a characteristic coherent double-peak-structure at temperatures below TN, signaling an AFM gap around the Fermi surface. The temperature dependent AFM gap Δ1 follows a Bardeen–Cooper–Schrieffer (BCS)-like mean-field behavior with a moderate gap anisotropy for PCS along different crystal axes. Another asymmetric gap-like feature is observed for both compounds at temperatures far below TN, which is consistent with opening of a new hybridization gap Δh inside the long-range ordered AFM state. Our results suggest a common itinerant nature of the anomalous AFM ordering, constraining theoretical models to explain the AFM origin in CeRu2Al10 and CeOs2Al10.

Inelastic Electron Tunneling in 2H-Ta_{x}Nb_{1-x}Se_{2} Evidenced by Scanning Tunneling Spectroscopy.

We report a detailed study of tunneling spectra measured on 2H-Ta_{x}Nb_{1-x}Se_{2} (x=0∼0.1) single crystals using a low-temperature scanning tunneling microscope. The prominent gaplike feature, which has not been understood for a long time, was found to be accompanied by some "in-gap" fine structures. By investigating the second-derivative spectra and their temperature and magnetic field dependencies, we were able to prove that inelastic electron tunneling is the origin of these features and obtain the Eliashberg function of 2H-Ta_{x}Nb_{1-x}Se_{2} at an atomic scale, providing a potential way to study the local Eliashberg function and the phonon spectra of the related transition-metal dichalcogenides.

Gap-like feature observed in the non-magnetic topological insulators

Non-magnetic gap at the Dirac point of topological insulators remains an open question in the field. Here, we present angle-resolved photoemission spectroscopy experiments performed on Cr-doped Bi2Se3 and showed that the Dirac point is progressively buried by the bulk bands and a low spectral weight region in the vicinity of the Dirac point appears. These two mechanisms lead to spectral weight suppression region being mistakenly identified as an energy gap in earlier studies. We further calculated the band structure and found that the original Dirac point splits into two nodes due to the impurity resonant states and the energy separation between the nodes is the low density of state region which appears to be like an energy gap in potoemission experiments. We supported our arguments by presenting photoemission experiments carried out with on- and off- resonant photon energies. Our observation resolves the widely debated questions of apparent energy gap opening at the Dirac point without long range ferromagnetic order in topological insulators.

Exchange-Correlation Effects and the Quasiparticle Properties in a Two-Dimensional Dipolar Fermi Liquid

We investigate the effects of exchange and correlation on the quasiparticle properties such as the self-energy, the many-body effective mass and the renormalization constant in a two-dimensional system of ultracold dipolar fermions with dipole moments aligned in the perpendicular direction to the plane. We use the G0W approximation along with the generalized random phase approximation, where the many-body effects have been incorporated in the effective interaction W through the Hubbard local-field factor. The many-body effective mass and the renormalization constant are reduced with the increase of coupling strength. We also study the effect of dipole-dipole interaction on the single-particle spectral function of the two dimensional dipolar Fermi liquid. We observe composite hole-zero sound excitation which is a bound state of quasiparticles with the collective mode (i.e. zero-sound) at intermediate and high coupling constants. These composite excitations are undamped at small wave vectors. Due to repulsion between quasiparticle and composite excitation resonances, we find a gap-like feature between quasiparticle and composite excitation dispersions at long wavelengths.

Angle-resolved photoemission spectroscopy of a Fermi–Hubbard system

Angle-resolved photoemission spectroscopy (ARPES) measures the single-particle excitations of a many-body quantum system with both energy and momentum resolution, providing detailed information about strongly interacting materials. ARPES is a direct probe of fermion pairing, and hence a natural technique to study the development of superconductivity in a variety of experimental systems ranging from high temperature superconductors to unitary Fermi gases. In these systems a remnant gap-like feature persists in the normal state, which is referred to as a pseudogap. A quantitative understanding of pseudogap regimes may elucidate details about the pairing mechanisms that lead to superconductivity, but developing this is difficult in real materials partly because the microscopic Hamiltonian is not known. Here we report on the development of ARPES to study strongly interacting fermions in an optical lattice using a quantum gas microscope. We benchmark the technique by measuring the occupied single-particle spectral function of an attractive Fermi-Hubbard system across the BCS-BEC crossover and comparing to quantum Monte Carlo calculations. We find evidence for a pseudogap in our system that opens well above the expected critical temperature for superfluidity. This technique may also be applied to the doped repulsive Hubbard model which is expected to exhibit a pseudogap at temperatures close to those achieved in recent experiments.

Highly dispersive magnons with spin-gap-like features in the frustrated ferromagneticS=12chain compoundCa2Y2Cu5O10detected by inelastic neutron scattering

We report inelastic neutron scattering experiments in Ca2Y2Cu5O10 and map out the full one magnon dispersion which extends up to a record value of 53 meV for frustrated ferromagnetic (FM) edge-sharing CuO2 chain (FFESC) cuprates. A homogeneous spin-1/2 chain model with a FM nearest-neighbor (NN), an antiferromagnetic (AFM) next-nearest-neighbor (NNN) inchain, and two diagonal AFM interchain couplings (ICs) analyzed within linear spin-wave theory (LSWT) reproduces well the observed strong dispersion along the chains and a weak one perpendicularly. The ratio R=|J_{a2}/J_{a1}| of the FM NN and the AFM NNN couplings is found as ~0.23, close to the critical point Rc=1/4 which separates ferromagnetically and antiferromagnetically correlated spiral magnetic ground states in single chains, whereas Rc>0.25 for coupled chains is considerably upshifted even for relatively weak IC. Although the measured dispersion can be described by homogeneous LSWT, the scattering intensity appears to be considerably reduced at ~11.5 and ~28 meV. The gap-like feature at 11.5 meV is attributed to magnon-phonon coupling whereas based on DMRG simulations of the dynamical structure factor the gap at 28 meV is considered to stem partly from quantum effects due to the AFM IC. Another contribution is ascribed to the intrinsic superstructure from the distorting incommensurate pattern of CaY cationic chains adjacent to the CuO2 ones. It gives rise to non-equivalent CuO4 units and Cu-O-Cu bond angles Phi and a resulting distribution of all exchange integrals. The J's fitted by homogeneous LSWT are regarded as average values. The record value of the FM NN integral J1=24 meV among FFESC cuprates can be explained by a non-universal Phi (not 90 deg.) and Cu-O bond length dependent anisotropic mean direct FM Cu-O exchange K_{pd}~120 meV. Enhanced K_{pd} values are also needed to compensate a significant AFM J_{dd} > ~6 meV.

Anomalous density fluctuations in a strange metal

A central mystery in high-temperature superconductivity is the origin of the so-called strange metal (i.e., the anomalous conductor from which superconductivity emerges at low temperature). Measuring the dynamic charge response of the copper oxides, [Formula: see text], would directly reveal the collective properties of the strange metal, but it has never been possible to measure this quantity with millielectronvolt resolution. Here, we present a measurement of [Formula: see text] for a cuprate, optimally doped Bi2.1Sr1.9CaCu2O8+x (Tc = 91 K), using momentum-resolved inelastic electron scattering. In the medium energy range 0.1-2 eV relevant to the strange metal, the spectra are dominated by a featureless, temperature- and momentum-independent continuum persisting to the electronvolt energy scale. This continuum displays a simple power-law form, exhibiting q2 behavior at low energy and q2/ω2 behavior at high energy. Measurements of an overdoped crystal (Tc = 50 K) showed the emergence of a gap-like feature at low temperature, indicating deviation from power law form outside the strange-metal regime. Our study suggests the strange metal exhibits a new type of charge dynamics in which excitations are local to such a degree that space and time axes are decoupled.

Terahertz-infrared electrodynamics of single-wall carbon nanotube films

Broad-band (4–20 000 cm−1) spectra of real and imaginary conductance of a set of high-quality pristine and AuCl3-doped single-walled carbon nanotube (SWCNT) films with different transparency are systematically measured. It is shown that while the high-energy (≥1 eV) response is determined by well-known interband transitions, the lower-energy electrodynamic properties of the films are fully dominated by unbound charge carriers. Their main spectral effect is seen as the free-carrier Drude-type contribution. Partial localization of these carriers leads to a weak plasmon resonance around 100 cm−1. At the lowest frequencies, below 10 cm−1, a gap-like feature is detected whose origin is associated with the energy barrier experienced by the carriers at the intersections between SWCNTs. It is assumed that these three mechanisms are universal and determine the low-frequency terahertz-infrared electrodynamics of SWCNT wafer-scale films.

Revealing correlation effect of Co 3d electrons in La3Co4Sn13 and Ce3Co4Sn13 by infrared spectroscopy study

We report resistivity, specific heat and optical spectroscopy measurements on single crystal samples of and . We observed clear temperature-induced spectral weight suppression below 4000 for both compounds in the conductivity spectra , indicating the progressive formation of gap-like features with decreasing temperature. The suppressed spectral weight transfers mostly to the higher energy region. This observation reflects the presence of the correlation effect in the compounds. We attribute the correlation effect to the Co 3d electrons.

Lattice distortion and charge density wave in Na2Ti2Sb2O revealed by scanning tunnelling microscopy

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.