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In the cubic chiral magnet Cu_{2}OSeO_{3} a low-temperature skyrmion state (LTS) and a concomitant tilted conical state are observed for magnetic fields parallel to ⟨100⟩. Here, we report on the dynamic resonances of these novel magnetic states. After promoting the nucleation of the LTS by means of field cycling, we apply broadband microwave spectroscopy in two experimental geometries that provide either predominantly in-plane or out-of-plane excitation. By comparing the results to linear spin-wave theory, we clearly identify resonant modes associated with the tilted conical state, the gyrational and breathing modes associated with the LTS, as well as the hybridization of the breathing mode with a dark octupole gyration mode mediated by the magnetocrystalline anisotropies. Most intriguingly, our findings suggest that under decreasing fields the hexagonal skyrmion lattice becomes unstable with respect to an oblique deformation, reflected in the formation of elongated skyrmions.

In the cubic chiral magnet Cu_{2}OSeO_{3} a low-temperature skyrmion state (LTS) and a concomitant tilted conical state are observed for magnetic fields parallel to ⟨100⟩. Here, we report on the dynamic resonances of these novel magnetic states. After promoting the nucleation of the LTS by means of field cycling, we apply broadband microwave spectroscopy in two experimental geometries that provide either predominantly in-plane or out-of-plane excitation. By comparing the results to linear spin-wave theory, we clearly identify resonant modes associated with the tilted conical state, the gyrational and breathing modes associated with the LTS, as well as the hybridization of the breathing mode with a dark octupole gyration mode mediated by the magnetocrystalline anisotropies. Most intriguingly, our findings suggest that under decreasing fields the hexagonal skyrmion lattice becomes unstable with respect to an oblique deformation, reflected in the formation of elongated skyrmions.

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite ${\mathrm{Cu}}_{3}\mathrm{Bi}({\mathrm{SeO}}_{3}{)}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$ at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at high temperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic x-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this soft-mode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behavior over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu_{3}Bi(SeO_{3})_{2}O_{2}Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at high temperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic x-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this soft-mode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behavior over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.

Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. Th

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of T_{d}-MoTe_{2}. We found that on-site Coulomb interaction leads to a Lifshitz transition, which is essential for a precise description of the electronic structure. A hybrid Weyl semimetal state with a pair of energy bands touching at both type-I and type-II Weyl nodes is indicated by comparing the experimental data with theoretical calculations. Unveiling the importance of Coulomb interaction opens up a new route to comprehend the unique properties of MoTe_{2}, and is significant for understanding the interplay between correlation effects, strong spin-orbit coupling and superconductivity in this van der Waals material.

High-resolution angle-resolved photoemission spectroscopy (ARPES) data reveal evidence of a crossover from one-dimensional (1D) to three-dimensional (3D) behavior in the prototypical charge density wave (CDW) material NbSe3. In the low-temperature 3D regime, gaps in the electronic structure are observed due to two incommensurate CDWs, in agreement with x-ray diffraction and electronic-structure calculations. At higher temperatures we observe a spectral weight depletion that approaches the power-law behavior expected in 1D. From the warping of the quasi-1D Fermi surface at low temperatures, we extract the energy scale of the dimensional crossover. This is corroborated by a detailed analysis of the density of states, which reveals a change in dimensional behavior dependent on binding energy. Our results offer an important insight into the dimensionality of excitations in quasi-1D materials., Main text: 5 pages, 4 figures; Supplemental Material: 5 pages, 8 figures

We investigate the polaronic ground state of anatase TiO2 by bulk-sensitive resonant inelastic x-ray spectroscopy (RIXS) at the Ti L3 edge. We find that the formation of the polaron cloud involves a single 95 meV phonon along the c axis, in addition to the 108 meV ab-plane mode previously identified by photoemission. The coupling strength to both modes is the same within error bars, and it is unaffected by the carrier density. These data establish RIXS as a directional bulk-sensitive probe of electron-phonon coupling in solids.

The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi2Se3. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic ab initio spin- resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin- polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.

The recent discovery of Skyrmions in Cu2OSeO3 has established a new platform to create and manipulate Skyrmionic spin textures. We use high-field electron spin resonance with a terahertz free-electron laser and pulsed magnetic fields up to 64 T to probe and quantify its microscopic spin-spin interactions. In addition to the previously observed long-wavelength Goldstone mode, this technique probes also the high-energy part of the excitation spectrum which is inaccessible by standard low-frequency electron spin resonance. Fitting the behavior of the observed modes in magnetic field to a theoretical framework establishes experimentally that the fundamental magnetic building blocks of this Skyrmionic magnet are rigid, highly entangled and weakly coupled tetrahedra.

Uniquely in Cu2OSeO3, the Skyrmions, which are topologically protected magnetic spin vortexlike objects, display a magnetoelectric coupling and can be manipulated by externally applied electric (E) fields. Here, we explore the E-field coupling to the magnetoelectric Skyrmion lattice phase, and study the response using neutron scattering. Giant E-field induced rotations of the Skyrmion lattice are achieved that span a range of ∼25°. Supporting calculations show that an E-field-induced Skyrmion distortion lies behind the lattice rotation. Overall, we present a new approach to Skyrmion control that makes no use of spin-transfer torques due to currents of either electrons or magnons.

On the basis of angle-scanned photoemission data recorded using unpolarized radiation, with high $(E,\mathbf{k})$ resolution, and an extremely dense sampling of $\mathbf{k}$ space, we resolve the current controversy regarding the normal state Fermi surface (FS) in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. The true picture is simple, self-consistent, and robust: the FS is holelike, with the form of rounded tubes centered on the corners of the Brillouin zone. Two further types of features are also clearly observed: shadow FSs, which are most likely to be due to short range antiferromagnetic spin correlations, and diffraction replicas of the main FS caused by passage of the photoelectrons through the modulated Bi-O planes.

Optical and photoemission experiments reveal unexpected spectral signatures of one-dimensional band insulators. In the model compound (NbSe4)(3)I the optical conductivity decays as a power law sigma(1)(omega) similar to omega(-4.25) above a sharp gap edge. Photoemission observes both the valence and a shadow band, produced w by a commensurate superstructure. We identify an optical and photoemission band gap consistent with other measurements but much smaller than the energy scale defined by the dispersion of the band peak in the photoemission spectra.

We report high resolution angle-scanned photoemission and Fermi surface (FS) mapping experiments on the layered transition-metal dichalcogenide 1T-TaS2 in the quasicommensurate (metallic) and the commensurate (insulating) charge-density-wave (CDW) phase. Instead of a nesting induced partially removed FS in the CDW phase we find a pseudogap over large portions of the FS. This remnant FS exhibits the symmetry of the one-particle normal state FS. Possibly, this Mott localization induced transition represents the underlying instability responsible fur the pseudogapped FS.

We investigated the effect of disorder on the spectral properties of the high temperature superconductor Bi2Sr2CaCu2O8+x. We find that small defect densities, in the low 10(-3) range, already suppress the characteristic spectral signature of the superconducting state, while new excitations appear within the gap. We conclude that, due to defect-induced pair breaking, superconducting pairs and normal carriers coexist below T-c. At higher levels of disorder the normal state is also strongly affected, and the quasiparticle features progressively smeared out.

Random defects destroy long-range phase coherence, and inhibit the metal-insulator transition in the charge density wave (CDW) system 1T-TaS2. Photoelectron spectroscopy with high resolution reveals corresponding changes in the electronic structure. Core level spectra indicate a temperature-dependent limited growth of commensurate CDW domains. Screening by free carriers from the domain boundaries reduces the Coulomb repulsion in the Ta d band by similar to 1/3, compared to pure samples, yielding a finite spectral weight at the Fermi level.

We report a study of dc transport properties up to 600 K and of the optical response over a broad spectral range of the two-dimensional (2D) dichalcogenide 2H-TaSe2. Our findings suggest a dynamics of the charge excitation spectrum dominated by two relevant features: a Drude contribution, which narrows with decreasing temperature, and a broad pseudogaplike feature in the midinfrared range, which is present at all temperatures below 300 K. The implications of these results with respect to the charge-density-wave phase transition at 122 K and more generally for the excitation spectrum of 2D correlated systems, e.g., the high temperature superconducting cuprates, will be addressed. [S0031-9007(98)06527-2]

Raman spectra of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}({\mathrm{Ca}}_{0.62}{\mathrm{Y}}_{0.38}){\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ with ${T}_{c}\ensuremath{\cong}{0.65T}_{c}^{\mathrm{max}}$ in the underdoped regime of the phase diagram are studied as a function of temperature and polarization. At ${B}_{2g}$ $(\mathrm{xy})$ symmetry a reduction of spectral weight by 10% for frequencies less than $700{\mathrm{cm}}^{\ensuremath{-}1}$ or ${15k}_{B}{T}_{c}$ is found below approximately 200 K. Below ${T}_{c}$, a superconducting gap opens up which closely resembles that observed at higher doping levels. It is compatible with ${d}_{{x}^{2}{\ensuremath{-}y}^{2}}$ pairing and its amplitude $2{\ensuremath{\Delta}}_{0}$ can be estimated to be ${8k}_{B}{T}_{c}$.

We report the observation of two signatures of a pressure-induced topological quantum phase transition in the polar semiconductor BiTeI using x-ray powder diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI remains in its ambient-pressure structure up to 8 GPa. The lattice parameter ratio c/a shows a minimum between 2.0-2.9 GPa, indicating an enhanced c-axis bonding through pz band crossing as expected during the transition. Over the same pressure range, the infrared spectra reveal a maximum in the optical spectral weight of the charge carriers, reflecting the closing and reopening of the semiconducting band gap. Both of these features are characteristics of a topological quantum phase transition, and are consistent with a recent theoretical proposal., revised final version

We report a comprehensive study of the paradigmatic quasi-1D compound (TaSe4)(2)I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in the nondistorted structure, with non-negligible interchain coupling. Theory and experiment support a Peierls-like scenario for the charge-density wave formation below T-CDW = 263 K, where the incommensurability is a direct consequence of the finite interchain coupling. The formation of small polarons, strongly suggested by the ARPES data, explains the puzzling semiconductor-to-semiconductor transition observed in transport at T-CDW.

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe2O5Br is surprisingly found to persist down to 53(3) mK (T/T_N ~ 1/200), according to neutron scattering and muon spin relaxation. In this state, finite spin fluctuations at T -> 0 are evidenced by muon depolarization, which is in agreement with specific-heat data indicating the presence of both gapless and gapped excitations. We thus show that the amplitude-modulated magnetic order intrinsically accommodates contradictory persistent spin dynamics and long-range order and can serve as a model structure to investigate their coexistence., 5 pages + supplementary

The dynamics of the photoinduced commensurate-to-incommensurate charge density wave (CDW) phase transition in 4H(b)-TaSe2 are investigated by femtosecond electron diffraction. In the perturbative regime, the CDW re-forms on a 150-ps time scale, which is two orders of magnitude slower than in other transition-metal dichalcogenides. We attribute this to a weak coupling between the CDW carrying T layers and thus demonstrate the importance of three-dimensionality for the existence of CDWs. With increasing optical excitation, the phase transition is achieved, showing a second-order character, in contrast to the first-order behavior in thermal equilibrium.

The magnetoelectric (ME) coupling on spin-wave resonances in single-crystal Cu2OSeO3 was studied by a novel technique using electron spin resonance combined with electric field modulation. An external electric field E induces a magnetic field component μ0H(i)=γE along the applied magnetic field H with γ=0.7(1) μT/(V/mm) at 10 K. The ME coupling strength γ is found to be temperature dependent and highly anisotropic. γ(T) nearly follows that of the spin susceptibility J(M)(T) and rapidly decreases above the Curie temperature T(c). The ratio γ/J(M) monotonically decreases with increasing temperature without an anomaly at T(c).

We report a long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator Cu2OSeO3. The magnetic phase diagram associated with the helimagnetic modulation inferred from small-angle neutron scattering and magnetization measurements includes a skyrmion lattice phase and is strongly reminiscent of MnSi, FeGe, and Fe(1-x)Co(x)Si, i.e., binary isostructural siblings of Cu2OSeO3 that order helimagnetically. The temperature dependence of the specific heat of Cu2OSeO3 is characteristic of nearly critical spin fluctuations at the helimagnetic transition. This provides putative evidence for effective spin currents as the origin of enhancements of the magnetodielectric response instead of atomic displacements considered so far.

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular to) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular to) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure.

We use time- and angle-resolved photoemission spectroscopy with sub-30-fs extreme-ultraviolet pulses to map the time- and momentum-dependent electronic structure of photoexcited 1T-TaS2. This compound is a two-dimensional Mott insulator with charge-density wave ordering. Charge order, evidenced by splitting between occupied subbands at the Brillouin zone boundary, melts well before the lattice responds. This challenges the view of a charge-density wave caused by electron-phonon coupling and Fermi-surface nesting alone, and suggests that electronic correlations play a key role in driving charge order.

We measured the momentum dependence of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr2CuO2Cl2 (SCOC). We identify a single-spin-wave feature and a multimagnon continuum, with different polarization dependences. The spin waves display a large (70 meV) dispersion between the zone-boundary points (π, 0) and (π/2, π/2). Employing an extended t-t'-t''-U one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions, indicative of extended magnetic interactions. The spectral line shape at (π, 0) indicates sizable quantum effects in SCOC and probably more generally in the cuprates.

The interplay between superconductivity and the charge-density wave (CDW) state in pure \mbox 1T-TiSe_2 is examined through a high pressure study extending up to pressures of 10~GPa between sub-Kelvin and room temperatures. At a critical pressure of 2~GPa a superconducting phase sets in and persists up to pressures of 4~GPa. The maximum superconducting transition temperature is 1.8~K. These findings complement the recent discovery of superconductivity in copper-intercalated 1T-TiSe_2. In both cases the superconductivity is limited to a region where the CDW fluctuations are enhanced, with the superconducting dome surrounding the CDW quantum critical points. However, the comparisons of the normal state and superconducting properties of the two systems reveal the possibility that the emergent electronic state qualitatively depends on the manner in which the CDW state is destabilized, making this an unique example where two different superconducting domes are obtained by two different methods from the same parent compound.

The magnetic and ferroelectric properties of the layered geometrically frustrated cluster compound FeTe2O5Br were investigated with single-crystal neutron diffraction and dielectric measurements. An incommensurate transverse amplitude modulated magnetic order with the wave vector q=(1/2,0.463,0) develops below T(N)=10.6(2) K. Simultaneously, a ferroelectric order due to exchange striction involving polarizable Te4+ lone-pair electrons develops perpendicular to q and to Fe3+ magnetic moments. The observed magnetoelectric coupling is proposed to originate from the temperature dependent phase difference between neighboring amplitude modulation waves.

We present a calculation of the Hall coefficient in 2H-TaSe(2) and 2H-Cu(0.2)NbS(2) based on their electronic structure extracted from angle-resolved photoemission spectra. The well-known semiclassical approach, based on the solution of the Boltzmann equation, yields the correct value for the normal-state Hall coefficient. Entering the charge density wave state results in the opening of the pseudogap and redistribution of the spectral weight. Accounting for this allows us to reproduce the temperature dependence of the Hall coefficient, including the prominent sign change, with no adjustable parameters.

Using high-resolution angle-resolved photoemission spectroscopy we have studied the momentum and photon energy dependence of the anomalous high-energy dispersion, termed waterfalls, between the Fermi level and 1 eV binding energy in several high-${T}_{c}$ superconductors. We observe strong changes of the dispersion between different Brillouin zones and a strong dependence on the photon energy around 75 eV, which we associate with the resonant photoemission at the $\mathrm{Cu}3p\ensuremath{\rightarrow}3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ edge. We conclude that the high-energy ``waterfall'' dispersion results from a strong suppression of the photoemission intensity at the center of the Brillouin zone due to matrix element effects and is, therefore, not an intrinsic feature of the spectral function. This indicates that the new high-energy scale in the electronic structure of cuprates derived from the waterfall-like dispersion may be incorrect.

We present a new high-resolution angle-resolved photoemission study of 1\textit{T}-TiSe$_{2}$ in both, its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalisations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A theoretical calculation of the spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favour of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1\textit{T}-TiSe$_{2}$., Submitted to PRL

The field dependence of the electron spin resonance in a helimagnet ${\mathrm{LiCu}}_{2}{\mathrm{O}}_{2}$ was investigated for the first time. In the paramagnetic state, a broad resonance line was observed corresponding to a $g$ factor of 2.3. In the critical regime, around the paramagnetic to helimagnetic phase transition the resonance broadens and shifts to higher frequencies. A narrow signal is recovered at a low temperature, corresponding to a spin gap of 1.4 meV in zero field. A comprehensive model of the magnons is presented, using exchange parameters from neutron scattering [T. Masuda et al. Phys. Rev. B 72, 014405 (2005)] and the spin anisotropy determined here. The role of the quantum fluctuations is discussed.

Femtosecond time-resolved photoemission is used to investigate the time evolution of electronic structure in the Mott insulator 1T-TaS2. A collapse of the electronic gap is observed within 100 femtoseconds after optical excitation. The photoemission spectra and the spectral function calculated by dynamical mean field theory show that this insulator-metal transition is driven solely by hot electrons. A coherently excited lattice displacement results in a periodic shift of the spectra lasting for 20 ps without perturbing the insulating phase. This capability to disentangle electronic and phononic excitations opens new directions to study electron correlation in solids.

Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in detail the formation of the quasiparticle spectrum along the nodal direction of high-Tc cuprates. The heavily discussed ‘‘70 meV kink’’ on the renormalized dispersion exhibits a strong temperature and doping dependence when purified from structural effects such as bilayer splitting, diffraction replicas, etc. This dependence is well understood in terms of fermionic and bosonic constituents of the self-energy. The latter follows the evolution of the spin-fluctuation spectrum, emerging below some doping dependent temperature and sharpening below Tc, and is mainly responsible for the formation of the kink in question. The nodal direction is thought to be the simplest place in the Brillouin zone of high-Tc cuprates where electron renormalization effects can be most easily understood. However, since the discovery of an energy scale in the experimental dispersion [1–3], the so-called ‘‘70 meV kink’’, its origin remains a matter of extensive debate [4 – 14], which now has converged into a vital dilemma: phonon versus spin fluctuations [15]. Historically, the kink has been associated with a coupling to the magnetic resonance mode because of its energy and doping dependence [2], its seemingly smooth evolution into a spectral dip when moving to the antinodal region [3,4], and its temperature dependence (emerging below Tc )[ 5]. At the same time, the persistence of the effect above Tc reported by another group [2] was taken as an argument against the resonance mode scenario. Moreover, a visual ‘‘ubiquity’’ of the kink for a number of families of cuprates in a wide range of doping and temperature [6] and recently found similarity between a fine structure seen in dispersion to an expected phonon spectrum [8] have made a strong claim in favor of phonon scenario. However, we have also recently reported a careful investigation of the scattering rate kink [10], which is a simple consequence of the Kramers-Kronig (KK) relation between the real and imaginary parts of the electron self-energy [12]. The observed strong dependence of this kink on doping and temperature (xT dependence) questions seriously the phonon scenario. Moreover, the odd parity [13] and strong dependence on Zn impurities [14] of the nodal scattering form solid arguments for the magnetic scenario. Thus, from a number of arguments from both sides, it seems that studies on the nature of the nodal kink have brought us to a stalemate, and an evident way to resolve it is to turn from a qualitative consideration of the kink effect to its quantitative analysis to derive the parameters of the bosonic spectrum that will enable unambiguous identification of its origin. Recently we have developed a KK-consistent procedure [12] which allows extraction of both the self-energy and the underlying bare dispersion from the photoemission data, and, thus, to place the kink problem into a quantitative domain. Subsequently, we have applied this self

The correlation-driven metal-insulator transition (MIT) of BaVS$_3$ was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: The quasi 1D metallic transport of $A_{1g}$ electrons is superimposed on the isotropic hopping conduction of localized $E_g$ electrons. The "bad-metal" character and the weak anisotropy are the consequences of the large effective mass $m_{eff}\approx7m_e$ and scattering rate $\Gamma\geq160$ meV of the quasi-particles in the $A_{1g}$ band. There is a pseudo-gap above $T_{MI}=69$ K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with $\Delta_{ch}\approx42$ meV in the ground state. The MIT is described in terms of a weakly coupled two-band model., Comment: 5 pages, 4 figures. submitted to PRL

By combining surprising new results from a full polarization analysis of nodal angle-resolved photoemission data from pristine and modulation-free ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ with structural information from LEED and ab initio one-step photoemission simulations, we prove that the shadow Fermi surface in these systems is of structural origin, being due to orthorhombic distortions from tetragonal symmetry present both in surface and bulk. Consequently, one of the longest standing open issues in the investigation of the Fermi surface of these widely studied systems finally meets its resolution.

We report the observation of a novel effect in the bilayer $\mathrm{Pb}\mathrm{\text{\ensuremath{-}}}{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ (Pb-Bi2212) high-${T}_{c}$ superconductor by means of angle-resolved photoemission with circularly polarized excitation. Different scattering rates, determined as a function of energy separately for the bonding and antibonding copper-oxygen bands, strongly imply that the dominating scattering channel is odd with respect to layer exchange within a bilayer. This is inconsistent with a phonon-mediated scattering and favors the participation of the odd collective spin excitations in the scattering mechanism in near-nodal regions of the $\mathbf{k}$ space, suggesting a magnetic nature of the pairing mediator.

The field dependence of the electron spin resonance in a helimagnet LiCu2O2 was investigated for the first time. In the paramagnetic state, a broad resonance line was observed corresponding to a g factor of 2.3. In the critical regime, around the paramagnetic to helimagnetic phase transition the resonance broadens and shifts to higher frequencies. A narrow signal is recovered at a low temperature, corresponding to a spin gap of 1.4 meV in zero field. A comprehensive model of the magnons is presented, using exchange parameters from neutron scattering [T. Masuda Phys. Rev. B 72, 014405 (2005)10.1103/PhysRevB.72.014405] and the spin anisotropy determined here. The role of the quantum fluctuations is discussed.

We present a combined high-energy x-ray diffraction and local-density approximation study of the sodium ordering in ${\mathrm{Na}}_{0.75}{\mathrm{CoO}}_{2}$. The obtained results rule out previously proposed Na-ordering models and provide strong evidence for the formation of sodium-density stripes in this material. The local-density approximation calculations prove that the sodium-density stripes lead to a sizable dip in the density of the Co states at the Fermi level, pointing to band structure effects as a driving force for the stripe formation. This indicates that the sodium ordering is connected to stripelike charge correlations within the ${\mathrm{CoO}}_{2}$ layers, leading to an astonishing similarity between the doped cuprates and the ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$ compounds.

In this Letter we report the observation, by scanning tunneling microscopy, of a Mott metal to insulator transition at the surface of 1T-TaSe2. Our spectroscopic data compare considerably well with previous angle-resolved photoemission spectroscopy measurements and confirm the presence of a large hysteresis related to a first order process. The local character of the tunneling spectroscopy technique allows a direct visualization of the surface symmetry and provides spectroscopic measurements on the defect-free region of the sample. It follows that the electronic localization is driven purely by the enhancement of the charge density wave amplitude which drives a bandwidth controlled metal-insulator transition.

We present the results of the first neutron powder and single crystal diffraction studies of the coupled spin tetrahedra systems Cu2Te2O5X2 (X = Cl,Br). Incommensurate antiferromagnetic order with the propagation vectors kCl approximately [0.150,0.422,1/2], kBr approximately [0.158,0.354,1/2] sets in below TN = 18 K for X = Cl and 11 K for X = Br. No simple collinear antiferromagnetic or ferromagnetic spin arrangements within Cu2+ tetrahedra fit these observations. Fitting the diffraction data to more complex but physically reasonable models with multiple helices leads to a moment of 0.67(1)microB/Cu2+ at 1.5 K for the Cl compound. The reason for such a complex ground state may be geometrical frustration of the spins due to the intratetrahedral and intertetrahedral couplings having similar strengths. In neither compound has any evidence for a structural transition accompanying the magnetic ordering been found.

In this Letter we report the observation, by scanning tunneling microscopy, of a Mott metal to insulator transition at the surface of 1T-TaSe2. Our spectroscopic data compare considerably well with previous angle-resolved photoemission spectroscopy measurements and confirm the presence of a large hysteresis related to a first order process. The local character of the tunneling spectroscopy technique allows a direct visualization of the surface symmetry and provides spectroscopic measurements on the defect-free region of the sample. It follows that the electronic localization is driven purely by the enhancement of the charge density wave amplitude which drives a bandwidth controlled metal-insulator transition.

We use angle-resolved photoemission with circularly polarized excitation to demonstrate that in the 5x1 superstructure-free Pb-Bi2212 material there are no signatures of time-reversal symmetry breaking in the sense of the criteria developed earlier (Kaminski et al. Nature {\bf 416}, 610 (2002)). In addition to the existing technique, we suggest and apply an independent experimental approach to prove the absence of the effect in the studied compounds. The dichroic signal retains reflection antisymmetry as a function of temperature and doping and in all mirror planes, precisely defined by the experimental dispersion at low energies. The obtained results demonstrate that the signatures of time-reversal symmetry violation in pristine Bi2212, as determined by ARPES, are not a universal feature of all cuprate superconductors., 4.5 RevTEX pages, 4 figures, after referees

We have measured the resonant inelastic x-ray scattering (RIXS) spectra at the Cu L3 edge in a variety of cuprates. Exploiting a considerably improved energy resolution (0.8 eV) we recorded significant dependencies on the sample composition and orientation, on the scattering geometry, and on the incident photon polarization. The RIXS final states correspond to two families of electronic excitations, having local (dd excitations) and nonlocal (charge-transfer) character. The dd energy splitting can be estimated with a simple crystal field model. The RIXS at the L3 edge demonstrates here a great potential, thanks to the resonance strength and to the large 2p spin-orbit splitting.

High-resolution photoemission has been used to study the electronic structure of the charge-density wave (CDW) and superconducting dichalcogenide, 2H-NbSe2. From the extracted self-energies, important components of the quasiparticle interactions have been identified. In contrast to previously studied TaSe2, the CDW transition does not affect the electronic properties significantly. The electron-phonon coupling is identified as a dominant contribution to the quasiparticle self-energy and is shown to be very anisotropic (k dependent) and much stronger than in TaSe2.

The thermal conductivity kappa in the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c axis, at several temperatures below T(c). With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H(c2), an unexpected hysteretic behavior of kappa(H) with all the generic features of a first order phase transition. The transition is not manifest in the kappa(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.