Spin fluctuation induced Weyl semimetal state in the paramagnetic phase of EuCd2As2.

Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetically induced WSMs is scarce. Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd2As2. We attribute this effect to the itinerant electrons experiencing quasi-static and quasi-long-range ferromagnetic fluctuations. Moreover, the spin-nondegenerate band structure harbors a pair of ideal Weyl nodes near the Fermi level. Hence, we show that long-range magnetic order and the spontaneous breaking of time-reversal symmetry are not essential requirements for WSM states in centrosymmetric systems and that WSM states can emerge in a wider range of condensed matter systems than previously thought.

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.

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.

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.

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.

High-T c superconductivity in undoped ThFeAsN.

Unlike the widely studied ReFeAsO series, the newly discovered iron-based superconductor ThFeAsN exhibits a remarkably high critical temperature of 30 K, without chemical doping or external pressure. Here we investigate in detail its magnetic and superconducting properties via muon-spin rotation/relaxation and nuclear magnetic resonance techniques and show that ThFeAsN exhibits strong magnetic fluctuations, suppressed below ~35 K, but no magnetic order. This contrasts strongly with the ReFeAsO series, where stoichiometric parent materials order antiferromagnetically and superconductivity appears only upon doping. The ThFeAsN case indicates that Fermi-surface modifications due to structural distortions and correlation effects are as important as doping in inducing superconductivity. The direct competition between antiferromagnetism and superconductivity, which in ThFeAsN (as in LiFeAs) occurs at already zero doping, may indicate a significant deviation of the s-wave superconducting gap in this compound from the standard s ± scenario.Exploring the interplay between the superconducting gap and the antiferromagnetic phase in Fe-based superconductors remains an open issue. Here, the authors show that Fermi-surface modifications by means of structural distortions and correlation effects are as important as doping in inducing superconductivity in undoped ThFeAsN.

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.

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].

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.

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.

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.

Observation of Weyl nodes and Fermi arcs in tantalum phosphide.

A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes, connected by topological surface arcs. The low-energy excitations near the crossing points behave the same as massless Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport properties dominated by Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and enclosed by pairs of individual Fermi surfaces with non-zero Fermi Chern numbers. Combining angle-resolved photoemission spectroscopy and first-principles calculation, here we show that TaP is a Weyl semimetal with only a single type of Weyl fermions, topologically distinguished from TaAs where two types of Weyl fermions contribute to the low-energy physical properties. The simple Weyl fermions in TaP are not only of fundamental interests but also of great potential for future applications. Fermi arcs on the Ta-terminated surface are observed, which appear in a different pattern from that on the As-termination in TaAs and NbAs.

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.

A firmware-defined digital direct-sampling NMR spectrometer for condensed matter physics.

We report on the design and implementation of a new digital, broad-band nuclear magnetic resonance (NMR) spectrometer suitable for probing condensed matter. The spectrometer uses direct sampling in both transmission and reception. It relies on a single, commercially-available signal processing device with a user-accessible field-programmable gate array (FPGA). Its functions are defined exclusively by the FPGA firmware and the application software. Besides allowing for fast replication, flexibility, and extensibility, our software-based solution preserves the option to reuse the components for other projects. The device operates up to 400 MHz without, and up to 800 MHz with undersampling, respectively. Digital down-conversion with ±10 MHz passband is provided on the receiver side. The system supports high repetition rates and has virtually no intrinsic dead time. We describe briefly how the spectrometer integrates into the experimental setup and present test data which demonstrates that its performance is competitive with that of conventional designs.

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.

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.

Direct observation of the spin texture in SmB6 as evidence of the topological Kondo insulator.

Topological Kondo insulators have been proposed as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to strong spin-orbit coupling. In contrast to other three-dimensional topological insulators, a topological Kondo insulator is truly bulk insulating. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. By applying spin- and angle-resolved photoemission spectroscopy, here we show that the surface states of SmB6 are spin polarized. The spin is locked to the crystal momentum, fulfilling time reversal and crystal symmetries. Our results provide strong evidence that SmB6 can host topological surface states in a bulk insulating gap stemming from the Kondo effect, which can serve as an ideal platform for investigating of the interplay between novel topological quantum states with emergent effects and competing orders induced by strongly correlated electrons.

Anisotropic breakdown of Fermi liquid quasiparticle excitations in overdoped La2-xSrxCuO4.

High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La1.77Sr0.23CuO4 is highly anisotropic in momentum space. The quasiparticle scattering and residue behave differently along the Fermi surface and hence the Kadowaki-Wood's relation is not obeyed. This kind of Fermi liquid breakdown may apply to a wide range of strongly correlated metal systems where spin fluctuations are present.

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.