Observation of Weyl Nodes in Robust Type-II Weyl Semimetal WP_{2}.

Distinct to type-I Weyl semimetals (WSMs) that host quasiparticles described by the Weyl equation, the energy dispersion of quasiparticles in type-II WSMs violates Lorentz invariance and the Weyl cones in the momentum space are tilted. Since it was proposed that type-II Weyl fermions could emerge from (W,Mo)Te_{2} and (W,Mo)P_{2} families of materials, a large number of experiments have been dedicated to unveiling the possible manifestation of type-II WSMs, e.g., surface-state Fermi arcs. However, the interpretations of the experimental results are very controversial. Here, using angle-resolved photoemission spectroscopy supported by the first-principles calculations, we probe the tilted Weyl cone bands in the bulk electronic structure of WP_{2} directly, which are at the origin of Fermi arcs at the surfaces and transport properties related to the chiral anomaly in type-II WSMs. Our results ascertain that, due to the spin-orbit coupling, the Weyl nodes originate from the splitting of fourfold degenerate band-crossing points with Chern numbers C=±2 induced by the crystal symmetries of WP_{2}, which is unique among all the discovered WSMs. Our finding also provides a guiding line to observe the chiral anomaly that could manifest in novel transport properties.

Time-Reversal Symmetry Breaking in Re-Based Superconductors.

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation and relaxation (µSR) studies of superconducting noncentrosymmetric Re_{0.82}Nb_{0.18} (T_{c}=8.8 K) and centrosymmetric Re (T_{c}=2.7 K). In Re_{0.82}Nb_{0.18}, the low-temperature superfluid density and the electronic specific heat evidence a fully gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both Re_{0.82}Nb_{0.18} and pure Re, the spontaneous magnetic fields revealed by zero-field µSR below T_{c} indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric ReT (T=transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg_{10}Ir_{19}B_{16} and Nb_{0.5}Os_{0.5}, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and ReT. We discuss the superconducting order parameter symmetries that are compatible with the experimental observations.

Evidence of a Coulomb-Interaction-Induced Lifshitz Transition and Robust Hybrid Weyl Semimetal in T_{d}-MoTe_{2}.

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.

Atomically Precise Lateral Modulation of a Two-Dimensional Electron Liquid in Anatase TiO2 Thin Films.

Engineering the electronic band structure of two-dimensional electron liquids (2DELs) confined at the surface or interface of transition metal oxides is key to unlocking their full potential. Here we describe a new approach to tailoring the electronic structure of an oxide surface 2DEL demonstrating the lateral modulation of electronic states with atomic scale precision on an unprecedented length scale comparable to the Fermi wavelength. To this end, we use pulsed laser deposition to grow anatase TiO2 films terminated by a (1 × 4) in-plane surface reconstruction. Employing photostimulated chemical surface doping we induce 2DELs with tunable carrier densities that are confined within a few TiO2 layers below the surface. Subsequent in situ angle-resolved photoemission experiments demonstrate that the (1 × 4) surface reconstruction provides a periodic lateral perturbation of the electron liquid. This causes strong backfolding of the electronic bands, opening of unidirectional gaps and a saddle point singularity in the density of states near the chemical potential.

Tailoring the nature and strength of electron-phonon interactions in the SrTiO3(001) 2D electron liquid.

Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/SrTiO3 interface, our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3-based 2DELs.

Selective Probing of Hidden Spin-Polarized States in Inversion-Symmetric Bulk MoS_{2}.

Spin- and angle-resolved photoemission spectroscopy is used to reveal that a large spin polarization is observable in the bulk centrosymmetric transition metal dichalcogenide MoS_{2}. It is found that the measured spin polarization can be reversed by changing the handedness of incident circularly polarized light. Calculations based on a three-step model of photoemission show that the valley and layer-locked spin-polarized electronic states can be selectively addressed by circularly polarized light, therefore providing a novel route to probe these hidden spin-polarized states in inversion-symmetric systems as predicted by Zhang et al. [Nat. Phys. 10, 387 (2014).NPAHAX1745-247310.1038/nphys2933].

Distinct Evolutions of Weyl Fermion Quasiparticles and Fermi Arcs with Bulk Band Topology in Weyl Semimetals.

The Weyl semimetal phase is a recently discovered topological quantum state of matter characterized by the presence of topologically protected degeneracies near the Fermi level. These degeneracies are the source of exotic phenomena, including the realization of chiral Weyl fermions as quasiparticles in the bulk and the formation of Fermi arc states on the surfaces. Here, we demonstrate that these two key signatures show distinct evolutions with the bulk band topology by performing angle-resolved photoemission spectroscopy, supported by first-principles calculations, on transition-metal monophosphides. While Weyl fermion quasiparticles exist only when the chemical potential is located between two saddle points of the Weyl cone features, the Fermi arc states extend in a larger energy scale and are robust across the bulk Lifshitz transitions associated with the recombination of two nontrivial Fermi surfaces enclosing one Weyl point into a single trivial Fermi surface enclosing two Weyl points of opposite chirality. Therefore, in some systems (e.g., NbP), topological Fermi arc states are preserved even if Weyl fermion quasiparticles are absent in the bulk. Our findings not only provide insight into the relationship between the exotic physical phenomena and the intrinsic bulk band topology in Weyl semimetals, but also resolve the apparent puzzle of the different magnetotransport properties observed in TaAs, TaP, and NbP, where the Fermi arc states are similar.

NaFe_{0.56}Cu_{0.44}As: A Pnictide Insulating Phase Induced by On-Site Coulomb Interaction.

In the studies of iron pnictides, a key question is whether their bad-metal state from which the superconductivity emerges lies in close proximity with a magnetically ordered insulating phase. Recently, it was found that at low temperatures, the heavily Cu-doped NaFe_{1-x}Cu_{x}As (x>0.3) iron pnictide is an insulator with long-range antiferromagnetic order, similar to the parent compound of cuprates but distinct from all other iron pnictides. Using angle-resolved photoemission spectroscopy, we determined the momentum-resolved electronic structure of NaFe_{1-x}Cu_{x}As (x=0.44) and identified that its ground state is a narrow-gap insulator. Combining the experimental results with density functional theory (DFT) and DFT+U calculations, our analysis reveals that the on-site Coulombic (Hubbard) and Hund's coupling energies play crucial roles in the formation of the band gap about the chemical potential. We propose that at finite temperatures, charge carriers are thermally excited from the Cu-As-like valence band into the conduction band, which is of Fe 3d-like character. With increasing temperature, the number of electrons in the conduction band becomes larger and the hopping energy between Fe sites increases, and finally the long-range antiferromagnetic order is destroyed at T>T_{N}. Our study provides a basis for investigating the evolution of the electronic structure of a Mott insulator transforming into a bad metallic phase and eventually forming a superconducting state in iron pnictides.

Momentum-Resolved Electronic Structure of the High-T_{c} Superconductor Parent Compound BaBiO_{3}.

We investigate the band structure of BaBiO_{3}, an insulating parent compound of doped high-T_{c} superconductors, using in situ angle-resolved photoemission spectroscopy on thin films. The data compare favorably overall with density functional theory calculations within the local density approximation, demonstrating that electron correlations are weak. The bands exhibit Brillouin zone folding consistent with known BiO_{6} breathing distortions. Though the distortions are often thought to coincide with Bi^{3+}/Bi^{5+} charge ordering, core level spectra show that bismuth is monovalent. We further demonstrate that the bands closest to the Fermi level are primarily oxygen derived, while the bismuth 6s states mostly contribute to dispersive bands at deeper binding energy. The results support a model of Bi-O charge transfer in which hole pairs are localized on combinations of the O 2p orbitals.

Observation of Fermi-Arc Spin Texture in TaAs.

We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin polarized. The measured spin texture fulfills the requirement of mirror and time-reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first-principles calculations.

Correlated decay of triplet excitations in the Shastry-Sutherland compound SrCu2(BO3)2.

The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu(2)(BO(3))(2) is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature, while the integrated spectral weight can be explained by an isolated dimer model up to 10 K. Analyzing this anomalous spectral line shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long lived if no thermally populated triplons are nearby but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry-Sutherland lattice.

Mixed dimensionality of confined conducting electrons in the surface region of SrTiO3.

Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO(3) consists of nondegenerate t(2g) states with different dimensional characters. While the d(xy) bands have quasi-2D dispersions with weak k(z) dependence, the lifted d(xz)/d(yz) bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the d(xz)/d(yz) Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO(3) in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O 1s core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-E(F) electrons in the surface region.

Evolution from a nodeless gap to d(x(2)-y(2))-wave in underdoped La(2-x)Sr(x)CuO4.

Using angle-resolved photoemission spectroscopy (ARPES), it is revealed that the low-energy electronic excitation spectra of highly underdoped superconducting and nonsuperconducting La(2-x)Sr(x)CuO(4) cuprates are gapped along the entire underlying Fermi surface at low temperatures. We show how the gap function evolves to a d(x(2)-y(2)) form with increasing temperature or doping, consistent with the vast majority of ARPES studies of cuprates. Our results provide essential information for uncovering the symmetry of the order parameter(s) in strongly underdoped cuprates, which is a prerequisite for understanding the pairing mechanism and how superconductivity emerges from a Mott insulator.

Field-induced quantum soliton lattice in a frustrated two-leg spin-1/2 ladder.

Based on high-field (31)P nuclear magnetic resonance experiments and accompanying numerical calculations, it is argued that in the frustrated S=1/2 ladder compound BiCu(2)PO(6) a field-induced soliton lattice develops above a critical field of µ(0)H(c1)=20.96(7) T. Solitons result from the fractionalization of the S=1, bosonlike triplet excitations, which in other quantum antiferromagnets are commonly known to experience Bose-Einstein condensation or to crystallize in a superstructure. Unlike in spin-Peierls systems, these field-induced quantum domain walls do not arise from a state with broken translational symmetry and are triggered exclusively by magnetic frustration. Our model predicts yet another second-order phase transition at H(c2)>H(c1), driven by soliton-soliton interactions, most likely corresponding to the one observed in recent magnetocaloric and other bulk measurements.

Bulk electronic structure of superconducting LaRu2P2 single crystals measured by soft-X-ray angle-resolved photoemission spectroscopy.

We present a soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the stoichiometric pnictide superconductor LaRu(2)P(2). The observed electronic structure is in good agreement with density functional theory (DFT) calculations. However, it is significantly different from its counterpart in high-temperature superconducting Fe pnictides. In particular, the bandwidth renormalization present in the Fe pnictides (~2-3) is negligible in LaRu(2)P(2) even though the mass enhancement is similar in both systems. Our results suggest that the superconductivity in LaRu(2) P(2) has a different origin with respect to the iron pnictides. Finally, we demonstrate that the increased probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is essential in determining the bulk electronic structure in the experiment.

Distribution of NMR relaxations in a random Heisenberg chain.

NMR measurements of the (29)Si spin-lattice relaxation time T(1) were used to probe the spin-1/2 random Heisenberg chain compound BaCu(2)(Si(1-x)Ge(x))(2)O(7). Remarkable differences between the pure (x=0) and the fully random (x=0.5) cases are observed, indicating that randomness generates a distribution of local magnetic relaxations. This distribution, which is reflected in a stretched exponential NMR relaxation, exhibits a progressive broadening with decreasing temperature, caused by a growing inequivalence of magnetic sites. Compelling independent evidence for the influence of randomness is also obtained from magnetization data and Monte Carlo calculations. These results suggest the formation of random-singlet states in this class of materials, as previously predicted by theory.

Two-dimensional orbital-like magnetic order in the high-temperature La(2-x)Sr(x)CuO4 superconductor.

In high-temperature copper oxide superconductors, a novel magnetic order associated with the pseudogap phase has been identified in two different cuprate families over a wide region of temperature and doping. We report here the observation below 120 K of a similar magnetic ordering in the archetypal cuprate La(2-x)Sr(x)CuO4 (LSCO) system for x=0.085. In contrast with the previous reports, the magnetic ordering in LSCO is only short range with an in-plane correlation length of ∼10 A and is bidimensional (2D). Such a less pronounced order suggests an interaction with other electronic instabilities. In particular, LSCO also exhibits a strong tendency towards stripes ordering at the expense of the superconducting state.

Direct observation of impurity-induced magnetism in a spin-(1/2) antiferromagnetic Heisenberg two-leg spin ladder.

Nuclear magnetic resonance and magnetization measurements were used to probe the magnetic features of single-crystalline Bi(Cu(1-x)Zn(x))(2)PO(6) with 00 and we present clear evidence for a temperature-dependent variation of the local magnetization close to the Zn sites. The generic nature of this observation is indicated by results of model calculations on appropriate spin systems of limited size employing quantum Monte Carlo methods.

Anomalies in the Fermi surface and band dispersion of quasi-one-dimensional CuO chains in the high-temperature superconductor YBa2Cu4O8.

We have investigated the electronic states in quasi-one-dimensional CuO chains by microprobe angle resolved photoemission spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale (∼40 meV), temperature dependence, and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high-temperature superconductors, indicating they have a common origin.

Design optimization through thermomechanical finite-element analysis of a hybrid piston-clamped anvil cell for nuclear magnetic resonance experiments.

The investigation of materials under extreme pressure conditions requires high-performance cells whose design invariably involves trade-offs between the maximum achievable pressure, the allowed sample volume, and the possibility of real-time pressure monitoring. With a newly conceived hybrid piston-clamped anvil cell, we offer a relatively simple and versatile system, suitable for nuclear magnetic resonance experiments up to 4.4 GPa. Finite-element models, taking into account mechanical and thermal conditions, were used to optimize and validate the design prior to the realization of the device. Cell body and gaskets were made of beryllium-copper alloy and the pistons and pusher were made of tungsten carbide, while the anvils consist of zirconium dioxide. The low-temperature pressure cell performance was tested by monitoring in situ the pressure-dependent 63Cu nuclear-quadrupole-resonance signal of Cu2O.