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The J/Ψ pT spectrum and nuclear modification factor (RAA) are reported for pT < 5 GeV/c and |y| < 1 from 0% to 60% central Au + Au and Cu + Cu collisions at √SNN = 200 GeV at STAR. A significant suppression of pr-integrated J/Ψ production is observed in central Au + Au events. The Cu + Cu data are consistent with no suppression, although the precision is limited by the available statistics. RAA in Au + Au collisions exhibits a strong suppression at low transverse momentum and gradually increases with pT. The data are compared to high-pT STAR results and previously published BNL Relativistic Heavy Ion Collider results. Comparing with model calculations, it is found that the invariant yields at low pT are significantly above hydrodynamic flow predictions but are consistent with models that include color screening and regeneration. [ABSTRACT FROM AUTHOR]

We report the discovery of superconductivity at 2.3 K in Zr5Sb3, the first superconducting member in the large compound family of the Mn5Si3-type structure. Transport, magnetic, and calorimetric measurements clearly demonstrate bulk superconductivity for Zr5Sb3 and suggest it to be a possible phonon-mediated BCS superconductor with a relatively large density of states at the Fermi level associated with the d electrons of Zr and substantially larger electron-phonon coupling compared to the Sn counterpart compound Zr5Sn3 from band structure calculations. More superconductors with even higher transition temperatures are expected to be found in this family of compounds. [ABSTRACT FROM AUTHOR]

Severely localized deformation within shear bands can occur much more easily in a metal with nanoscale microstructures, such as nanograined and nanolayered materials. Based on atomic-scale observations, here we show that such locally large deformation (the continuous thinning of the layers) within the indentation-induced shear bands of the Cu/Au nanolayers is essentially attributed to the large shear stress component along the interface, which can refresh the capability of the interface to absorb incoming dislocations through unlocking the product of the dislocation-interface reaction. The results have implications for understanding the interface-mediated mechanisms of plastic deformation and for the engineering application of severe plastic deformation processing of metals at nanoscales. [ABSTRACT FROM AUTHOR]

A spin rectification effect due to the nonresonant magnetization motion has been found, which differs from the previously known spin rectification effect caused by the resonant magnetization precession at ferromagnetic resonance. A phase-resolved electrical detection technique has been used to measure the phase lag between the magnetization response and the driving microwave magnetic field, which demonstrates unambiguously the distinction between the resonant and nonresonant rectification effects. [ABSTRACT FROM AUTHOR]

The phase-resolved ferromagnetic resonance spectra has been electrically measured via a spintronic Michelson interferometer, where the nonlinear coupling between the microwave electric field and magnetic field results in time-independent fields and currents. Besides the microwave power the strength of the electrical signal strongly depends on the relative phase of microwave fields at the ferromagnetic material. Therefore, using a ferromagnetic sensor enables a quantification of microwave transmission in both the phase and amplitude, and demonstrates precision and feasibility in the determination of complex dielectric constants in both ideal dielectric materials and strongly absorbing materials. [ABSTRACT FROM AUTHOR]

The growth and geometric and electronic structures of Cr2O3 layers on the polar ZnO surfaces were characterized to determine how polar substrates can influence the properties of nonpolar films. X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), high resolution transmission electron microscopy (HRTEM), reflection high energy electron diffraction, low energy electron diffraction, x-ray diffraction (XRD), and x-ray reflectivity (XRR) were employed to characterize the growth mode, film quality, and interfacial electronic properties. Chromium oxide growth on ZnO (0001¯) and (0001) followed the same trends: two-dimensional growth with initial disorder followed by the formation of epitaxial Cr2O3 (0001). Despite the initial disorder, HRTEM and XRD/XRR measurements on thicker films revealed an abrupt interface with the Cr2O3 lattice extending all the way to the interface. This indicates that above a critical thickness of 10-15 Cr-O3-Cr repeat units, the entire film reorganizes into an ordered structure. It is postulated that the oxygen remained ordered throughout the growth but that the chromium initially filled interstices randomly in the oxygen sublattice, which allowed the film to eventually grow with a well-defined epitaxial relationship with the substrate. The polar interfaces showed a small band offset that decayed with increasing film thickness, suggesting that the compensating charges at the interface may partially migrate to the film surface. No evidence of formal changes in the Cr oxidation state at the interfaces was seen. On the other hand, statistical analyses of UPS valence band spectra revealed an enhanced density of states near the valence band edge for Cr2O3 on ZnO (0001), consistent with stabilization of the positive interface by filling surface electronic states. In contrast, no significant valence band differences were observed between bulk Cr2O3 and thin Cr2O3 layers on ZnO (0001¯), suggesting a different charge compensation mechanism on the negative surface. The potential impact of these findings on the surface properties of chromium oxide thin films is discussed. [ABSTRACT FROM AUTHOR]

We study the effect of hydrostatic pressure on the magnetotransport properties of zirconium pentatelluride. The magnitude of resistivity anomaly gets enhanced with increasing pressure, but the transition temperature T* is insensitive to it up to 2.5 GPa. In the case of H∥b, the quasilinear magnetoresistance decreases drastically from 3300% (9 T) at ambient pressure to 230% (9 T) at 2.5 GPa. Besides, the change of the quantum oscillation phase from topological nontrivial to trivial is revealed around 2 GPa. Both demonstrate that the pressure breaks the accidental Dirac node in ZrTe5. For H∥c, in contrast, subtle changes can be seen in the magnetoresistance and quantum oscillations. In the presence of pressure, ZrTe5 evolves from a highly anisotropic to a nearly isotropic electronic system, which accompanies the disruption of the accidental Dirac semimetal state. It supports the assumption that ZrTe5 is a semi-3D Dirac system with linear dispersion along two directions and a quadratic one along the third. [ABSTRACT FROM AUTHOR]

How an electron-hole pair escapes the Coulomb potential at a donor-acceptor interface has been a key issue in organic photovoltaic research. Recent evidence suggests that long-distance charge separation can occur on ultrafast time scales, yet the underlying mechanism remains unclear. Here we use charge transfer excitons (CTEs) across an organic semiconductor-vacuum interface as a model and show that nascent hot CTEs can spontaneously climb up the Coulomb potential within 100 fs. This process is driven by entropic gain due to the rapid rise in density of states with increasing electron-hole separation. In contrast, the lowest CTE cannot delocalize, but undergoes self-trapping and recombination. [ABSTRACT FROM AUTHOR]

An antiferromagnetically coupled FM/NM/FM (FM = ferromagnet, NM = normal metal) trilayer structure responds to an external magnetic field by the formation of a magnetic-moment spring within the FM layers. We show that the exchange stiffness (Aex) of an FM layer can be determined by fitting the field-dependent magnetization, M(H), of the FM/NM/FM trilayer to a micromagnetic model. Using this method, we have measured the exchange stiffness of thin-film Co alloyed with Cr, Fe, Ni, Pd, Pt, and Ru. The results show that the rate at which a substituent element reduces the exchange stiffness is not directly related to its effect on the magnetization of the alloy. The observed trends have been understood by material-specific modeling based on density functional theory within the local density approximation. The stiffness measurements are in agreement with Brillouin light scattering carried out on thicker Co films. [ABSTRACT FROM AUTHOR]

The band structure and intra- and interband scattering processes of the electrons at the surface of a bismuth bilayer on Bi2Se3 have been experimentally investigated by low-temperature Fourier-transform scanning tunneling spectroscopy. The observed complex quasiparticle interference patterns are compared to a simulation based on the spin-dependent joint density of states approach using the surface-localized spectral function calculated from first principles as the only input. Thereby, the origin of the quasiparticle interferences can be traced back to intraband scattering in the bismuth-bilayer valence band and Bi2Se3 conduction band and to interband scattering between the two-dimensional topological state and the bismuth-bilayer valence band. The investigation reveals that the bilayer band gap, which is predicted to host one-dimensional topological states at the edges of the bilayer, is pushed several hundred meV above the Fermi level. This result is rationalized by an electron transfer from the bilayer to Bi2Se3 which also leads to a two-dimensional electron state in the Bi2Se3 conduction band with a strong Rashba spin splitting, coexisting with the topological state and bilayer valence band. [ABSTRACT FROM AUTHOR]

NdO0.5F0.5BiS2 is a new layered superconductor. We have studied the low-lying electronic structure of the single-crystalline NdO0.5F0.5BiS2 superconductor, whose superconducting transition temperature is 4.83 K, with angle-resolved photoemission spectroscopy. The Fermi surface consists of two small electron pockets around the X point and shows little warping along the kz direction. Our results demonstrate the multiband and two-dimensional nature of the electronic structure. The comparison between the photoemission data and the band calculations gives the renormalization factor of ∼1.14, indicating the rather weak electron correlations in this material. Moreover, we found that the actual electron doping level and Fermi surface size are much smaller than what are expected from the nominal composition, which could be largely explained by the bismuth deficiency. The small Fermi pocket size and the weak electron correlations found here put strong constraints on theory, and suggest that the BiS2-based superconductors could be conventional BCS superconductors mediated by the electron-phonon coupling. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of Bi6Fe2Ti3O18 and Bi6FeCrTi3O18 are investigated. The samples can be indexed by an orthorhombic lattice with the space group B2cb and the valence state of Fe and Cr ions is suggested to be +3 based on the x-ray photoelectron spectroscopy spectra. The sample Bi6Fe2Ti3O18 is paramagnetic with the presence of a short-range antiferromagnetic ordering; whereas Bi6FeCrTi3O18 undergoes a short-range ferrimagnetic-like transition at 106 K originating from the spin canting of the antiferromagnetic coupling of Fe-based and Cr-based sublattices via the Dzyaloshinskii-Moriya interaction due to the distorted crystal structure of Bi6Fe2Ti3O18, which are in agreement with the EPR results. The magnetic structure and EPR spectra of Bi6Fe2Ti3O18 and Bi6FeCrTi3O18 can be understood by the presence of a small amount of isolated Fe3+ ions in an octahedral ligand field with a low symmetry and the coupling between the random occupancy of Fe3+ and/or Cr3+ ions in other two octahedral sites in the layered structure of Aurivilius phase. [ABSTRACT FROM AUTHOR]

Charge separated interlayer excitons in transition metal dichalcogenide heterobilayers are being explored for moiré exciton lattices and exciton condensates. The presence of permanent dipole moments and the poorly screened Coulomb interaction make many-body interactions particularly strong for interlayer excitons. Here we reveal two distinct phase transitions for interlayer excitons in the MoSe2/WSe2 heterobilayer using time and spatially resolved photoluminescence imaging: from trapped excitons in the moiré potential to the modestly mobile exciton gas as exciton density increases to nex∼1011 cm-2 and from the exciton gas to the highly mobile charge separated electron-hole plasma for nex>1012 cm-2. The latter is the Mott transition and is confirmed in photoconductivity measurements. These findings set fundamental limits for achieving quantum states of interlayer excitons. [ABSTRACT FROM AUTHOR]

The nodal-line semimetals have attracted immense interest due to the unique electronic structures such as the linear dispersion and the vanishing density of states as the Fermi energy approaching the nodes. Here, we report temperature-dependent transport and scanning tunneling microscopy (spectroscopy) [STM(S)] measurements on nodal-line semimetal ZrSiSe. Our experimental results and theoretical analyses consistently demonstrate that the temperature induces Lifshitz transitions at 80 and 106 K in ZrSiSe, which results in the transport anomalies at the same temperatures. More strikingly, we observe a V-shaped dip structure around Fermi energy from the STS spectrum at low temperature, which can be attributed to co-effect of the spin-orbit coupling and excitonic instability. Our observations indicate the correlation interaction may play an important role in ZrSiSe, which owns the quasi-two-dimensional electronic structures. [ABSTRACT FROM AUTHOR]

The elliptic flow (v2) event-by-event fluctuations in PbPb collisions at 5.02 TeV are analyzed within the HYDJET++ model. Using the multiparticle, so-called Q-cumulant method, v2{2}, v2{4}, v2{6}, and v2{8} are calculated and used to study their ratios and to construct skewness (γ1exp) as a measure of the asymmetry of the elliptic flow distribution. Additionally, in order to check if there is a hydrodynamics nature in the elliptic collectivity generated by the HYDJET++ model, the ratio of v2{6}-v2{8} and v2{4}-v2{6} distributions is calculated. The analysis is performed as a function of the collision centrality. In order to check the HYDJET++ model responses, the results of this analysis are compared to the corresponding experimental measurements from the ALICE, ATLAS, and CMS experiments. A rather good qualitative agreement is found. [ABSTRACT FROM AUTHOR]

Correlations between azimuthal anisotropy Fourier harmonics vn (n=2,3,4) are studied using the events from PbPb collisions at vsNN=2.76 TeV generated by the HYDJET++ and multiphase transport (AMPT) models, and compared to the corresponding experimental results obtained by the ATLAS Collaboration. The Fourier harmonics vn are measured over a wide centrality range using the two-particle azimuthal correlation method. The slopes of the v2-v3 correlation from both models are in a good agreement with the ATLAS data. The HYDJET++ model predicts a stronger slope for the v2-v4 and v3-v4 correlations than the ones experimentally measured, while the results from the AMPT model are in a rather good agreement with the experimental results. In contrast to the HYDJET++ predictions, the AMPT model predicts a boomeranglike shape in the structure of the correlations as found in the experimental data. [ABSTRACT FROM AUTHOR]

Thermoelectric effects are more sensitive and promising probes to topological properties of emergent materials, but much less addressed compared to other physical properties. We study the thermoelectric effects of ZrTe5 in a magnetic field. The presence of the nontrivial electrons leads to the anomalous Nernst effect and quasilinear field dependence of thermopower below the quantum limit. In the strong-field quantum limit, both the thermopower and Nernst signal exhibit exotic peaks. At higher magnetic fields, the Nernst signal has a sign reversal at a critical field where the thermopower approaches zero. We propose that these anomalous behaviors can be attributed to the gap closing of the zeroth Landau bands in topological materials with the band inversion. Our understanding to the anomalous thermoelectric properties in ZrTe5 opens a new avenue for exploring Dirac physics in topological materials. [ABSTRACT FROM AUTHOR]

In heavy-fermion compounds, the dual character of f electrons underlies their rich and often exotic properties like fragile heavy quasiparticles, a variety of magnetic orders and unconventional superconductivity. 5f-electron actinide materials provide a rich setting to elucidate the larger and outstanding issue of the competition between magnetic order and Kondo entanglement and, more generally, the interplay among different channels of interactions in correlated electron systems. Here, by using angle-resolved photoemission spectroscopy, we present the detailed electronic structure of USb2 and observe two different kinds of nearly flat bands in the antiferromagnetic state of USb2. Polarization-dependent measurements show that these electronic states are derived from 5f orbitals with different characters; in addition, further temperature-dependent measurements reveal that one of them is driven by the Kondo correlations between the 5f electrons and conduction electrons, while the other reflects the dominant role of the magnetic order. Our results on the low-energy electronic excitations of USb2 implicate orbital selectivity as an important new ingredient for the competition between Kondo correlations and magnetic order and, by extension, in the rich landscape of quantum phases for strongly correlated f electron systems. [ABSTRACT FROM AUTHOR]

A pair of type-II Dirac cones in PdTe2 was recently predicted by theories and confirmed in experiments, making PdTe2 the first material that processes both superconductivity and type-II Dirac fermions. In this paper, we study the evolution of Dirac cones in PdTe2 under hydrostatic pressure by first-principles calculations. Our results show that the pair of type-II Dirac points disappears at 6.1 GPa. Interestingly, a new pair of type-I Dirac points from the same two bands emerges at 4.7 GPa. Due to the distinctive band structures compared with those of PtSe2 and PtTe2, the two types of Dirac points can coexist in PdTe2 under proper pressure (4.7-6.1 GPa). The emergence of type-I Dirac cones and the disappearance of type-II Dirac ones are attributed to an increase/decrease of the energy of the states at the Γ and A points, which have antibonding/bonding characters of the interlayer Te-Te atoms. On the other hand, we find that the superconductivity of PdTe2 slightly decreases with pressure. The pressure-induced types of Dirac cones combined with superconductivity may open a promising way to investigate the complex interactions between Dirac fermions and superconducting quasiparticles. [ABSTRACT FROM AUTHOR]

CrGeTe3 recently emerges as a new two-dimensional (2D) ferromagnetic semiconductor that is promising for spintronic device applications. Unlike CrSiTe3 whose magnetism can be understood using the 2D-Ising model, CrGeTe3 exhibits a smaller van der Waals gap and larger cleavage energy, which could lead to a transition of magnetic mechanism from 2D to 3D. To confirm this speculation, we investigate the critical behavior of CrGeTe3 around the second-order paramagnetic-ferromagnetic phase transition. We obtain the critical exponents estimated by several common experimental techniques including the modified Arrott plot, Kouvel-Fisher method, and critical isotherm analysis, which show that the magnetism of CrGeTe3 follows the tricritical mean-field model with the critical exponents β, γ, and δ of 0.240±0.006, 1.000±0.005, and 5.070±0.006, respectively, at the Curie temperature of 67.9 K. We therefore suggest that the magnetic phase transition from 2D to 3D for CrGeTe3 should locate near a tricritical point. Our experiment provides a direct demonstration of the applicability of the tricritical mean-field model to a 2D ferromagnetic semiconductor. [ABSTRACT FROM AUTHOR]

The radiationless recombination of electron-hole pairs in semiconductors is detrimental to optoelectronic technologies. A prominent mechanism is Auger recombination, in which nonradiative recombination occurs efficiently by transferring the released energy-momentum to a third charge carrier. Here we use femtosecond photoemission to directly detect Auger electrons as they scatter into energy and momentum spaces from Auger recombination in a model semiconductor, GaSb. The Auger rate is modulated by a coherent phonon mode at 2 THz, confirming phonon participation in momentum conservation. The commonly assumed Auger rate constant is found not to be a constant, but rather decreases by 4 orders of magnitude as hot electrons cool down by ~90 meV. These findings provide quantitative guidance in understanding Auger recombination and in designing materials for efficient optoelectronics. [ABSTRACT FROM AUTHOR]

We performed the angle dependent magnetoresistance (MR), Hall effect measurements, the temperature dependent magneto-thermoelectric power (TEP) S(T) measurements, and the first-principles calculations to study the electronic properties of orthorhombic phase Mo Te2 (Td - Mo Te2), which was proposed to be electronically two dimensional (2D). There are some interesting findings about Td - Mo Te2. (1) A scaling approach ɛθ = (sin²θ + γ-2 cos²θ)1/2 is applied, where θ is the magnetic field angle with respect to the c axis of the crystal and γ is the mass anisotropy. Unexpectedly, the electronically three-dimensional (3D) character with γ as low as 1.9 is observed in Td - Mo Te2. (2) The possible Lifshitz transition and the following electronic structure change can be verified around T ~ 150 K and T ~ 60 K, which is supported by the evidence of the slope changing of the temperature dependence of TEP, the carrier density extracted from Hall resistivity, and the onset temperature of γ obtained from the MR measurements. The extremely large MR effect in Td - Mo Te2 could originate from the combination of the electron-hole compensation and a particular orbital texture on the electron pocket, which is supported by the calculations of electronic structure. Our results may provide a general scaling relation for the anisotropic MR and help to recognize the origins of the MR effect in other systems, such as the Weyl semimetals and the Dirac ones. [ABSTRACT FROM AUTHOR]

The transition-metal dichalcogenide 1T-TaS2 exhibits a rich set of charge-density-wave (CDW) orders. Recent investigations suggested that using light or an electric field can manipulate the commensurate CDW (CCDW) ground state. Such manipulations are considered to be determined by charge-carrier doping. Here we use first-principles calculations to simulate the carrier-doping effect on the CCDW in 1T-TaS2. We investigate the charge-doping effects on the electronic structures and phonon instabilities of the 1T structure, and we analyze the doping-induced energy and distortion ratio variations in the CCDW structure. We found that both in bulk and monolayer 1T-TaS2, the CCDW is stable upon electron doping, while hole doping can significantly suppress the CCDW, implying different mechanisms of such reported manipulations. Light or positive perpendicular electric-field-induced hole doping increases the energy of the CCDW, so that the system transforms to a nearly commensurate CDW or a similar metastable state. On the other hand, even though the CCDW distortion is more stable upon in-plane electric-field-induced electron injection, some accompanied effects can drive the system to cross over the energy barrier from the CCDW to a nearly commensurate CDW or a similar metastable state. We also estimate that hole doping can introduce potential superconductivity with a Tc of 6-7 K. Controllable switching of different states such as a CCDW/Mott insulating state, a metallic state, and even a superconducting state can be realized in 1T-TaS2. As a result, this material may have very promising applications in future electronic devices. [ABSTRACT FROM AUTHOR]