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The discoveries of intrinsic ferromagnetism in atomically-thin van der Waals crystals have opened up a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. To date, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating 'originally ferromagnetic (FM)' van der Waals crystals, while bottom-up approach by thin film growth technique has demonstrated emergent 2D ferromagnetism in a variety of 'originally non-FM' van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy (MBE) exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is 'originally antiferromagnetic (AFM)'. Moreover, thickness-dependence measurements reveal that this newly-developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.

GeTe is a chemically simple IV-VI semiconductor which bears a rich plethora of different physical properties induced by doping and external stimuli. These include, among others, ferromagnetism, ferroelectricity, phase-change memory functionality, and comparably large thermoelectric figure of merits. Here we report a superconductor - semiconductor - superconductor transition controlled by finely-tuned In doping. Our results moreover show the existence of a critical doping concentration around $x = 0.12$ in Ge$_{1-x}$In$_{x}$Te, where various properties take either an extremum or change their characters: The structure changes from polarly-rhombohedral to cubic, the resistivity sharply increases by orders of magnitude, the type of charge carriers changes from holes to electrons, and the density of states diminishes at the dawn of an emerging superconducting phase. By core-level photoemission spectroscopy we find indications of a change in the In-valence state from In$^{3+}$ to In$^{1+}$ with increasing $x$, suggesting that this system is a new promising playground to probe valence fluctuations and their possible impact on superconductivity., Comment: Main text (4 Figures) + Supplement(9 Figures)

Ultrahigh-repetition-rate (1.1 GHz), deep-ultraviolet coherent light at 208.8 nm is generated by applying an external Fabry–Perot cavity for repetition-rate multiplication to the fourth harmonics of a 10-ps, mode-locked Ti:sapphire laser. Its small pulse energy minimizes the unwanted space charge effect, while its high repetition rate drastically reduces the acquisition time in high-energy resolution angle-resolved photoemission spectroscopy using hemispherical electron analyzers. The absence of the space charge effect in the photoemission spectrum near the Fermi edge of polycrystalline Au at 8 K demonstrates this idea.

The chiral crystal is characterized by a lack of mirror symmetry and an inversion center, resulting in the inequivalent right- and left-handed structures. In the noncentrosymmetric crystal structure, the spin and momentum of electrons are locked in the reciprocal space with the help of the spin-orbit interaction. To reveal the spin textures of chiral crystals, here we investigate the spin and electronic structure in p-type semiconductor elemental tellurium with a chiral crystal structure by using spin- and angle-resolved photoemission spectroscopy. Our data demonstrate that the highest valence band crossing the Fermi level has a spin component parallel to the electron momentum around the BZ corners. Significantly, we have also confirmed that the spin polarization is reversed in the crystal with the opposite chirality. The results indicate that the spin textures of the right- and left-handed chiral crystals are hedgehog-like, leading to unconventional magnetoelectric effects and nonreciprocal phenomena., Comment: 15 pages, 4 figures

In this paper, we investigate the femtosecond-optical-pulse-induced strain dynamics in relatively thin (100 nm) and thick (10 000 nm) silicon plates based on finite-element simulations. In the thin sample, almost spatially homogeneous excitation by the optical pulse predominantly generates a standing wave of the lowest-order acoustic resonance mode along the out-of-plane direction. At the same time, laterally propagating plate waves are emitted at the sample edge through the open edge deformation. Fourier transformation analysis reveals that the plate waves in the thin sample are mainly composed of two symmetric Lamb waves, reflecting the spatially uniform photoexcitation. In the thick sample, on the other hand, only the near surface region is photo-excited and thus a strain pulse that propagates along the out-of-plane direction is generated, accompanying the laterally propagating pulse-like strain dynamics through the edge deformation. These lateral strain pulses consist of multiple Lamb waves, including asymmetric and higher-order symmetric modes. Our simulations quantitatively demonstrate the out-of-plane and in-plane photoinduced strain dynamics in realistic silicon plates, ranging from the plate wave form to pulse trains, depending on material parameters such as sample thickness, optical penetration depth, and sound velocity.

Thermoelectric properties of chromium-modified Mn2Sb are systematically investigated to evaluate the power factors. Single-crystalline samples of Mn2−xCrxSb with 0 ≤ x ≤ 0.1 were successfully grown...

High-temperature superconductivity in iron-pnictides/chalcogenides arises in balance with several electronic and lattice instabilities. Beside the antiferromagnetic order, the orbital anisotropy between Fe 3d xz and 3d yz occurs near the orthorhombic structural transition in several parent compounds. However, the extent of the survival of orbital anisotropy against the ion-substitution remains to be established. Here we report the composition (x) and temperature (T) dependences of the orbital anisotropy in the electronic structure of a BaFe2(As1−xP x )2 system by using angle-resolved photoemission spectroscopy. In the low-x regime, the orbital anisotropy starts to evolve on cooling from high temperatures above both antiferromagnetic and orthorhombic transitions. By increasing x, it is gradually suppressed and survives in the optimally doped regime. We find that the in-plane orbital anisotropy persists in a large area of the nonmagnetic phase, including the superconducting dome. These results suggest that the rotational symmetry-broken electronic state acts as the stage for superconductivity in BaFe2(As1−xP x )2.

Science advances 4(7), eaar3867 - (2018). doi:10.1126/sciadv.aar3867, The observation and control of interweaving spin, charge, orbital, and structural degrees of freedom in materials on ultrafast time scales reveal exotic quantum phenomena and enable new active forms of nanotechnology. Bonding is the prime example of the relation between electronic and nuclear degrees of freedom. We report direct evidence illustrating that photoexcitation can be used for ultrafast control of the breaking and recovery of bonds in solids on unprecedented time scales, near the limit for nuclear motions. We describe experimental and theoretical studies of IrTe%$_2$ using femtosecond electron diffraction and density functional theory to investigate bonding instability. Ir-Ir dimerization shows an unexpected fast dissociation and recovery due to the filling of the antibonding $d_{xy}$ orbital. Bond length changes of 20% in IrTe$_2$ are achieved by effectively addressing the bonds directly through this relaxation process. These results could pave the way to ultrafast switching between metastable structures by photoinduced manipulation of the relative degree of bonding in this manner., Published by AAAS, Washington, DC [u.a.]

Antiferroic electronic instability in Ba,KFe2As2 persists in the nonmagnetic phase covering the superconducting dome., A major problem in the field of high-transition temperature (Tc) superconductivity is the identification of the electronic instabilities near superconductivity. It is known that the iron-based superconductors exhibit antiferromagnetic order, which competes with the superconductivity. However, in the nonmagnetic state, there are many aspects of the electronic instabilities that remain unclarified, as represented by the orbital instability and several in-plane anisotropic physical properties. We report a new aspect of the electronic state of the optimally doped iron-based superconductors by using high–energy resolution angle-resolved photoemission spectroscopy. We find spectral evidence for the folded electronic structure suggestive of an antiferroic electronic instability, coexisting with the superconductivity in the nonmagnetic state of Ba1−xKxFe2As2. We further establish a phase diagram showing that the antiferroic electronic structure persists in a large portion of the nonmagnetic phase covering the superconducting dome. These results motivate consideration of a key unknown electronic instability, which is necessary for the achievement of high-Tc superconductivity in the iron-based superconductors.

We report the ultrafast dynamics of electrons and lattice in transition metal thin films (Au, Cu, and Mo) investigated by a combination of ultrafast electron diffraction (UED) and pump-probe optical methods. For a single-crystalline Au thin film, we observe the suppression of the diffraction intensity occuring in 10 ps, which direcly reflects the lattice thermalization via the electron-phonon interaction. By using the two-temperature model, the electron-phonon coupling constant (g) and the electron and lattice temperatures (Te, Tl) are evaluated from UED, with which we simulate the transient optical transmittance. The simulation well agrees with the experimentally obtained transmittance data, except for the slight deviations at the initial photoexcitation and the relaxed quasi-equilibrium state. We also present the results similarly obtained for polycrystalline Au, Cu, and Mo thin films and demonstrate the electron and lattice dynamics occurring in metals with different electron-phonon coupling strengths.

We investigate photo-induced lattice strains of a monoclinic VTe2 thin flake using ultrafast electron diffraction. After photoexcitation by a 190 fs pulse, we observe diffraction intensity oscillations with periods of 35 and 75 ps, which indicate coherent acoustic phonons of two distinct branches. The oscillations of and diffraction intensities have opposite signs, indicating the change of the diffraction angle due to the shear strain. By numerically simulating these diffraction intensities as a function of the monoclinic angle, we evaluate the amplitude of the photo-induced shear strain.

We have performed temperature ( T )-dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi 2 B 2 C to study the electronic structure, especially the effect of AF ordering in T -dependent superconducting (SC) gap. To estimate the values of T -dependent SC gap, we fitted the experimental data by Dynes function having an anisotropic s -wave SC gap. From the fitting results, we find a sudden deviation from the BCS prediction just below T N . This observation can be well explained by the theoretical model, indicating that the origin of anomalous T -dependence is competition between rapid evolution of AF molecular field and SC condensation energy.

Transition metal compounds often undergo spin-charge-orbital ordering due to strong electron-electron correlations. In contrast, low-dimensional materials can exhibit a Peierls transition arising from low-energy electron-phonon-coupling-induced structural instabilities. We study the electronic structure of the tunnel framework compound K2Cr8O16, which exhibits a temperature-dependent (T-dependent) paramagnetic-to-ferromagnetic- metal transition at T-C = 180 K and transforms into a ferromagnetic insulator below T-MI = 95 K. We observe clear T-dependent dynamic valence (charge) fluctuations from above T-C to T-MI, which effectively get pinned to an average nominal valence of Cr+3.75 (Cr4+:Cr3+ states in a 3:1 ratio) in the ferromagnetic-insulating phase. High-resolution laser photoemission shows a T-dependent BCS-type energy gap, with 2G(0) similar to 3.5(k(B)T(MI)) similar to 35 meV. First-principles band-structure calculations, using the experimentally estimated on-site Coulomb energy of U similar to 4 eV, establish the necessity of strong correlations and finite structural distortions for driving the metal-insulator transition. In spite of the strong correlations, the nonintegral occupancy (2.25 d-electrons/Cr) and the half-metallic ferromagnetism in the t(2g) up-spin band favor a low-energy Peierls metal-insulator transition.

We have investigated the low-energy electronic state of boron-doped diamond thin film by the laser-excited photoemission spectroscopy. A clear Fermi-edge is observed for samples doped above the semiconductor–metal boundary, together with the characteristic structures at 150 � n meV possibly due to the strong electron–lattice coupling effect. In addition, for the superconducting sample, we observed a shift of the leading edge below T c indicative of a superconducting gap opening. We discuss the electron–lattice coupling and the superconductivity in doped diamond. r 2006 NIMS and Elsevier Ltd. All rights reserved.

Using time-resolved photoemission spectroscopy, we have investigated the nonequilibrium electronic structures of a ${\mathrm{VO}}_{2}$ thin film upon photoexcitation. We employed the high-harmonic generation method, which was crucial in obtaining the results. Irradiation by a 170-fs optical pulse at a fluence above $\ensuremath{\sim}$6 mJ/cm${}^{2}$ rapidly converts the insulating ${\mathrm{VO}}_{2}$ thin film into a metallic state, and we show that the transition is accompanied by a spectral-weight redistribution over a 1-eV scale. This observation provides direct spectroscopic evidence of an ultrafast insulator-to-metal transition. The transient metallic state has a unique spectrum that deviates from the static spectrum of the rutile metal phase. We also observe an anomalous spreading of spectral weight up to $\ensuremath{\sim}$0.4 eV above the Fermi level as soon as the transient metallic state emerges. The temporal evolution of the spectral weight near the Fermi level exhibits an ultrafast increase and a subsequent slower increase over $\ensuremath{\sim}$3 ps. We suggest that a broadening mechanism related to the excitation of phonon modes is responsible for the spreading of the spectral weight and that the slower increase of the spectral weight is associated with the expansion of the metallic region after photoinduced nucleation. In addition, the initial nucleation of the metallic state appears to be spatially inhomogeneous even near the surface. The results of this study highlight the uniqueness of the nonequilibrium metallic state in comparison to the equilibrium rutile metallic state from an electronic-state perspective.

We investigate the bulk superconducting electronic structure in optimally doped YBa 2 Cu 3 O 7 - δ (YBCO) using laser-excited angle-resolved photoemission spectroscopy. The band dispersions with the d x 2 - y 2 -wave like gap opening are observed without being hindered by surface state and CuO chain band due to bulk-sensitivity and/or matrix element effect at h ν = 6.994 eV . The momentum-dependence of the superconducting gap exhibits a nonzero minimum of ∼12 meV along the ( 0 , 0 ) – ( π , π ) line. In addition, the unusual suppression of the quasiparticle coherence is observed at T / T c > 0.5 – 0.7 .

We present the data from ultrahigh-resolution laser-excited photoemission spectroscopy on a newly discovered noncentrosymmetric superconcluctor Mg(10)Ir(19)B(16). We observed the evidences for the opening of superconducting gap where numerical fittings show that isotropic s-wave model can satisfactorily explain the behavior of spectra near the Fermi level. We also present the valence band photoemission spectrum and B K fluorescence spectrum to discuss the basic electronic structure of Mg(10)Ir(19)B(16). (C) 2009 Elsevier B.V. All rights reserved.

The authors have investigated the temperature variation of optical spectra comparatively for an oxygenated nonsuperconducting and a reduced superconducting crystal of Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4+y}. Anomalous gap-like structure as large as 0.3 eV has been observed in the optical conductivity spectrum of the oxygenated crystal with lowering temperature from 340 K, while little temperature-variation has been observed for the reduced crystal. Furthermore, anomalies in lattice dynamics have been observed only for the oxygenated crystal in far-infrared spectra. These spectral signatures have shown parallel temperature-dependence, signaling a possible charge ordering in the oxygenated nonsuperconducting crystal.

Time-resolved photoelectron spectroscopy (trPES) can directly detect transient electronic structure, thus bringing out its promising potential to clarify nonequilibrium processes arising in condensed matters. Here we report the result of core-level trPES on 1$T$-TaS${}_{2}$, realized by developing a high-intensity 60-eV laser obtained by high-order harmonic generation. Ta$4f$ core-level trPES offers the transient amplitude of the charge-density wave (CDW), via the site-selective and real-time observation of Ta electrons. The present result indicates an ultrafast photoinduced melting and recovery of CDW amplitude, followed by a peculiar long-life oscillation (i.e., collective amplitudon excitation) accompanying the transfer of electrons among adjacent Ta atoms. core-level trPES offers a broad range of opportunities for investigating the ultrafast atom-specific electron dynamics in photorelated phenomena of interest.

Laser angle-resolved photoemission spectroscopy (ARPES) is employed to investigate the temperature (T) dependence of the electronic structure in BaFe2As2 across the magnetostructural transition at T{N} approximately 140 K. A drastic transformation in Fermi surface (FS) shape across T{N} is observed, as expected by first-principles band calculations. Polarization-dependent ARPES and band calculations consistently indicate that the observed FSs at k{z} approximately pi in the low-T antiferromagnetic state are dominated by the Fe3d{zx} orbital, leading to the twofold electronic structure. These results indicate that magnetostructural transition in BaFe2As2 accompanies orbital-dependent modifications in the electronic structure.

We study electron dynamics in graphite with time resolved photoemission spectroscopy (TRPES). Using the fourth harmonic (5.95 eV) and fundamental of a Ti:sapphire laser as a pump-probe, respectively, we observe a biexponential decay of excited electrons

Laser-excited photoemission spectroscopy is used to show that the doped carriers in metallic or superconducting diamond couple strongly to the lattice via high-energy ($\ensuremath{\sim}150\text{ }\text{ }\mathrm{meV}$) optical phonons, with direct observations of localized Franck-Condon multiphonon sidebands appearing as Fermi-edge replicas. It exhibits a temperature-dependent spectral weight transfer from higher to lower energy sidebands and zero-phonon Fermi-edge states. The quantified coupling strength shows a systematic increase on lowering temperature, implicating its relation to the normal state transport and superconductivity.

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi 2 B 2 C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below T N . This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

We have performed soft x-ray and ultrahigh-resolution laser-excited photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band structure calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self-energy correction. We have also performed ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling., Comment: 6 pages, 3 figures

The Nd-moment order in the layered nickelate Nd2−xSrxNiO4 (x = 0.7) has been investigated by performing a neutron diffraction experiment using a single crystal sample. First, the checkerboard (CB)-type charge order was confirmed by observing the temperature dependence of the nuclear superlattice peak at \(\boldsymbol{{Q}} = (5,0,0)\) between 1.9 and 300 K, which indicates that the transition temperature of the CB-type charge order is above 300 K. Magnetic superlattice peaks with the propagation vector \(\boldsymbol{{k}} = (1 - \varepsilon ,0,1)\) appear below 67 K, and the value of e was determined to be 0.455 in good agreement with previous studies. The intensity of the magnetic superlattice peaks appearing below 67 K shows a sharp increase below ≈20 K. This behavior indicates that the Nd moments freeze under the influence of the Ni ordering. The CB-type antiferromagnetic (AFM) Ni order in the NiO2 layers is stacked antiferromagnetically in the c-axis direction, while the Nd moments in the Nd/SrO2 layers...

We have investigated the low energy electronic state of a boron-doped diamond thin film by the ultrahigh resolution laser-excited photoemission spectroscopy. We observed a clear shift of the leading edge below 11 K indicative of a superconducting gap opening ($\Delta \sim 0.78$ meV at 4.5 K). The gap feature is significantly broad and the well-defined quasiparticle peak is not recognizable even at the lowest temperature of measurement 4.5 K. We discuss our result in terms of possible disorder effect on superconductivity in this system., Comment: 12 pages, 4 figures

We have investigated temperature dependence of optical conductivity spectra comparatively for oxygenated (antiferromagnetic) and reduced (superconducting) crystals of Nd1.85Ce0.15CuO4+y. In the spectra of the oxygenated crystal, anomalous pseudogap structure, accompanied by phonon anomalies, evolves with decreasing temperature from 340 K, while these features almost disappear in the spectra of the reduced crystal. These anomalies are proposed to be due to charge ordering instability induced by a minute amount of interstitial apical-oxygen, which seems to suppress the superconductivity in the oxygenated (or as-grown) crystal.

We have investigated the electronic structures of antiferromagnetic superconductor ErNi 2 B 2 C by using laser-photoemission spectroscopy. The observed spectra below the superconducting transition temperature ( T c = 9.0 K) show a very broad coherent peak and finite density of states at the Fermi level due to strong magnetic pair breaking effects. In addition, we find a dip structure in the temperature dependence of the superconducting order parameter just below the antiferromagnetic ordering temperature ( T N = 6.0 K). This behavior can be well interpreted as a result of competition between the rapid evolution of the antiferromagnetic molecular field and the increase of the superconducting condensation energy with decreasing temperature.

We report the results of first photoemission spectroscopy on a newly discovered noncentrosymmetric superconductor Mg 10 Ir 19 B 16 . We measured the valence band structure of Mg 10 Ir 19 B 16 using soft X-ray photoemission technique and found that the first principal calculation explains our observation well. In the bulk-sensitive laser-excited photoemission study, we observed that the intensity at the Fermi level decreases as we lower the sample temperature, indicative of the opening of superconducting energy gap. Using Dynes' function, we performed fitting analysis on the spectra of superconducting state.

Noncentrosymmetric superconductors Li 2 Pd 3 B and Li 2 Pt 3 B are studied by high-resolution photoemission spectroscopy. The valence band spectra of Li 2 Pd 3 B and Li 2 Pt 3 B are well explained by first-principles band calculations. The sub-meV photoemission result shows that the size of the superconducting gap of Li 2 Pd 3 B is about 1.1 meV at 5.4 K with a reduced gap value (2Δ 0 / k B T c ) of about 4.5, which is greater than the Bardeen–Cooper–Schrieffer (BCS) value of 3.52. The obtained superconducting gap can be explained more clearly with an s -wave superconducting gap than with p - and d -wave superconducting gaps. This result indicates that Li 2 Pd 3 B is a conventional strong-coupling superconductor.

We have developed a low temperature ultrahigh resolution system for polarization dependent angle-resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet (vuv) laser (hnu=6.994 eV) as a photon source. With the aim of addressing low energy physics, we show the system performance with angle-integrated PES at the highest energy resolution of 360 mueV and the lowest temperature of 2.9 K. We describe the importance of a multiple-thermal-shield design for achieving the low temperature, which allows a clear measurement of the superconducting gap of tantalum metal with a T(c)=4.5 K. The unique specifications and quality of the laser source (narrow linewidth of 260 mueV, high photon flux), combined with a half-wave plate, facilitates ultrahigh energy and momentum resolution polarization dependent ARPES. We demonstrate the use of s- and p-polarized laser-ARPESs in studying the superconducting gap on bilayer-split bands of a high T(c) cuprate. The unique features of the quasi-continuous-wave vuv laser and low temperature enables ultrahigh-energy and -momentum resolution studies of the spectral function of a solid with large escape depth. We hope the present work helps in defining polarization dependent laser excited angle-resolved photoemission spectroscopy as a frontier tool for the study of electronic structure and properties of materials at the sub-meV energy scale.