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Excitonic insulator (EI) was proposed in the 1960s as a distinct insulating state originating from pure electronic interaction, but its material realization has been elusive with extremely few material candidates and with only limited evidence such as anomalies in transport properties, band dispersions, or optical transitions. We investigate the real-space electronic states of the low-temperature phase in ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ with atomic resolution to clearly identify the quasiparticle energy gap together with the strong electron-hole band renormalization using scanning tunneling microscopy and spectroscopy (STS). These results are in good agreement with the EI transition scenario in ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$. Our spatially resolved STS data and theoretical calculations reveal further the orbital inversion at band edges, which indicates the exciton condensation close to the Bardeen-Cooper-Schrieffer regime.

The upper critical field (H c2 ) was determined by applying a magnetic field along the ab plane and c axis for two single crystals of BaFe 2-x Ru x As 2 (x=0.48 and 0.75). The anisotropy of the H c2 (0), γ(0)=H c2ab (0)/H c2c (0), was ~1.6 for x=0.48and ~2.3 for x=0.75. The angle-depen dent resistance measured below T c allowed perfect scaling features based on anisotropic Ginzburg -Landau theory, leading to consistent anisotropy values. Because only one fitting parameter γ is used in the scaling for each temperature, the validity of the abγvalue was compared with that determined from =H c2 /H c2c γ. The γ obtained at a temperature close to T c was 3.0 and decreased to 2.0 at low temperatures. Comparing to the anis otropy determined for electron- or hole -doped BaFe 2 As 2 using the same method, the present results point to consistent anisotropy in Ru-doped BaFe 2 As 2 with other electron- or hole-doped BaFe 2 As 2 . Keywords : Fe based superconductor, superconducting anisotropy, upper critical field

The band width control through external stress has been demonstrated as a useful knob to modulate metal-insulator transition (MIT) in RNiO3 as a prototype correlated materials. In particular, lattice mismatch strain using different substrates have been widely utilized to investigate the effect of strain on transition temperature so far but the results were inconsistent in the previous literatures. Here, we demonstrate dynamic modulation of MIT based on electric field-controlled pure strain in high-quality NdNiO3 (NNO) thin films utilizing converse-piezoelectric effect of (001)-cut "Equation missing" - "Equation missing" (PMN-PT) single crystal substrates. Despite the difficulty in the NNO growth on rough PMN-PT substrates, the structural quality of NNO thin films has been significantly improved by inserting SrTiO3 (STO) buffer layers. Interestingly, the MIT temperature in NNO is downward shifted by ~3.3 K in response of 0.25% in-plane compressive strain, which indicates less effective TMI modulation of field-induced strain than substrate-induced strain. This study provides not only scientific insights on band-width control of correlated materials using pure strain but also potentials for energy-efficient electronic devices.

We report the valley-selective interlayer conduction of ${\mathrm{SrMnBi}}_{2}$ under in-plane magnetic fields. The $c$-axis resistivity of ${\mathrm{SrMnBi}}_{2}$ shows clear angular magnetoresistance oscillations indicating coherent interlayer conduction. Strong fourfold variation of the coherent peak in the $c$-axis resistivity reveals that the contribution of each Dirac valley is significantly modulated by the in-plane field orientation. This originates from anisotropic Dirac Fermi surfaces with strong disparity in the momentum-dependent interlayer coupling. Furthermore, we found a signature of broken valley symmetry at high magnetic fields. These findings demonstrate that a quasi-two-dimensional anisotropic Dirac system can host a valley-polarized interlayer current through magnetic valley control.

We report a de Haas-van Alphen (dHvA) oscillation study on IrTe2 single crystals showing complex dimer formations. By comparing the angle dependence of dHvA oscillations with band structure calculations, we show distinct Fermi surface reconstruction induced by a 1/5-type and a 1/8-type dimerizations. This verifies that an intriguing quasi-two-dimensional conducting plane across the layers is induced by dimerization in both cases. A phase transition to the 1/8 phase with higher dimer density reveals that local instabilities associated with intra- and interdimer couplings are the main driving force for complex dimer formations in IrTe2., Comment: 5 pages, 4 figures, accepted in Phys. Rev. Lett

We use (75)As nuclear magnetic resonance to investigate the local electronic properties of Ba(Fe(1-x)Ru(x))(2)As(2) (x = 0.23). We find two phase transitions: to antiferromagnetism at T(N) ≈ 60 K and to superconductivity at T(C) ≈ 15 K. Below T(N), our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4μ(B)/Fe. The spin-lattice relaxation rate 1/(75)T(1) is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below T(C), 1/(75)T(1) on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.

Superconductivity arises from pairing of mobile electrons. The symmetry of the electron-pair wave function contains important clues about the mechanism underlying the pairing. A Chinese-Korean experimental collaboration carries out element-selective substitutional doping of BaFe${}_{2}$As${}_{2}$ and searches for the pairing symmetry of the resulting iron-based superconductors.

The effects of isovalent Ru substitution at the Fe sites of BaFe${}_{2\ensuremath{-}x}$Ru${}_{x}$As${}_{2}$ are investigated by measuring resistivity ($\ensuremath{\rho}$) and Hall coefficient (${R}_{H}$) on high-quality single crystals in a wide range of doping (0 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 1.4). Ru substitution weakens the antiferromagnetic (AFM) order, inducing superconductivity for relatively high doping level of 0.4 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 0.9. Near the AFM phase boundary, the transport properties show non-Fermi-liquid-like behavior with a linear-temperature dependence of $\ensuremath{\rho}$ and a strong temperature dependence of ${R}_{H}$ with a sign change. Upon higher doping, however, both $\ensuremath{\rho}$ and ${R}_{H}$ recover conventional Fermi-liquid behavior. Strong doping dependence of ${R}_{H}$ together with a small magnetoresistance suggest that the anomalous transport properties can be explained in terms of anisotropic charge carrier scattering due to interband AFM fluctuations rather than a conventional multiband scenario.

The phase diagrams of EuFe$_{2-x}$Co$_x$As$_2$ $(0 \leq x \leq 0.4)$ and EuFe$_2$As$_{2-y}$P$_y$ $(0 \leq y \leq 0.43)$ are investigated by Eu$^{2+}$ electron spin resonance (ESR) in single crystals. From the temperature dependence of the linewidth $��H(T)$ of the exchange narrowed ESR line the spin-density wave (SDW) $(T < T_{\rm SDW})$ and the normal metallic regime $(T > T_{\rm SDW})$ are clearly distinguished. At $T > T_{\rm SDW}$ the isotropic linear increase of the linewidth is driven by the Korringa relaxation which measures the conduction-electron density of states at the Fermi level. For $T < T_{\rm SDW}$ the anisotropy probes the local ligand field, while the coupling to the conduction electrons disappears. With increasing substitution $x$ or $y$ the transition temperature $T_{\rm SDW}$ decreases linearly accompanied by a linear decrease of the Korringa-relaxation rate from 8 Oe/K at $x=y=0$ down to 3 Oe/K at the onset of superconductivity at $x \approx 0.2$ or at $y \approx 0.3$, above which it remains nearly constant. Comparative ESR measurements on single crystals of the Eu diluted SDW compound Eu$_{0.2}$Sr$_{0.8}$Fe$_2$As$_2$ and superconducting (SC) Eu$_{0.22}$Sr$_{0.78}$Fe$_{1.72}$Co$_{0.28}$As$_2$ corroborate the leading influence of the ligand field on the Eu$^{2+}$ spin relaxation in the SDW regime as well as the Korringa relaxation in the normal metallic regime. Like in Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ a coherence peak is not detected in the latter compound at $T_{\rm c}=21$ K, which is in agreement with the expected complex anisotropic SC gap structure.

We report the highly anisotropic Dirac fermions in a Bi square net of SrMnBi2, based on a first principle calculation, angle resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the (SrBi)+ layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi2 is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of ~ 8. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provide a new platform that hosts highly anisotropic Dirac fermions., Comment: 5 pages, 4 figures