141 research outputs found
Strong One-Dimensional Characteristics of Hole-Carriers in ReS2 and ReSe2.
Each plane of layered ReS2 and ReSe2 materials has 1D chain structure, from which intriguing properties such as 1D character of the exciton states and linearly polarized photoluminescence originate. However, systematic studies on the 1D character of charge carriers have not been done yet. Here, we report on systematic and comparative studies on the energy-momentum dispersion relationships of layered transition metal dichalcogenides ReS2 and ReSe2 by angle resolved photoemission. We found that the valence band maximum or the minimum energy for holes is located at the high symmetric Z-point for both materials. However, the out-of-plane ([Formula: see text]) dispersion for ReSe2 (20 meV) is found to be much smaller than that of ReS2 (150 meV). We observe that the effective mass of the hole carriers along the direction perpendicular to the chain is about 4 times larger than that along the chain direction for both ReS2 and ReSe2. Remarkably, the experimentally measured hole effective mass is about twice heavier than that from first principles calculation for ReS2 although the in-plane anisotropy values from the experiment and calculations are comparable. These observation indicate that bulk ReS2 and ReSe2 are unique semiconducting transition metal dichalcogenides having strong one-dimensional characters
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Polarization control at the microscopic and electronic structure observatory
The new Microscopic and Electronic Structure Observatory (MAESTRO) at the Advanced Light Source (ALS) in Berkeley provides X-rays of variable polarization, produced by an elliptically polarized undulator (EPU), for angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM) experiments. The interpretation of photoemission data, in particular of dichroism effects in ARPES, requires the precise knowledge of the exact polarization state. Numerical simulations show that the first harmonics of the EPU at MAESTRO provides soft X-rays of almost 100% on axis polarization. However, the higher harmonics as well as the downstream optical elements of the beamline, have a considerable impact on the polarization of the light delivered to the experimental end-station. Employing a simple reflective polarimeter, the polarization is characterized for variable EPU and beamline settings and the overall degree of polarization in the MAESTRO end-stations is estimated to be on the order of 83%
Superconductivity below 20 K in heavily electron-doped surface layer of FeSe bulk crystal
A superconducting transition temperature (T-c) as high as 100 K was recently discovered in one monolayer FeSe grown on SrTiO3. The discovery ignited efforts to identify the mechanism for the markedly enhanced T-c from its bulk value of 8 K. There are two main views about the origin of the T-c enhancement: interfacial effects and/or excess electrons with strong electron correlation. Here, we report the observation of superconductivity below 20 K in surface electron-doped bulk FeSe. The doped surface layer possesses all the key spectroscopic aspects of the monolayer FeSe on SrTiO3. Without interfacial effects, the surface layer state has a moderate T-c of 20 K with a smaller gap opening of 4.2 meV. Our results show that excess electrons with strong correlation cannot induce the maximum T-c, which in turn reveals the need for interfacial effects to achieve the highest T-c in one monolayer FeSe on SrTiO3.1116Ysciescopu
Nearly-free-electron system of monolayer Na on the surface of single-crystal HfSe2
The electronic structure of a single Na monolayer on the surface of single-crystal HfSe2 is investigate dusing angle-resolved photoemission spectroscopy. We find that this system exhibits analmost perfect “nearly-free-electron” behavior with an extracted effective mass of ∼1me, in contrast to heavier masses found previously for alkali metal monolayers on other substrates. Our density functional-theory calculations indicate that this is due to the large lattice constant, causing both exchange and correlation interactions to be suppressed, and to the weak hybridization between the overlayer and the substrate. This is therefore an ideal model system for understanding the properties of two-dimensional materials.PostprintPeer reviewe
Fermi surface instability at the hidden-order transition of URu2Si2
Solids with strong electron correlations generally develop exotic phases of
electron matter at low temperatures. Among such systems, the heavy-fermion
semi-metal URu2Si2 presents an enigmatic transition at To = 17.5 K to a `hidden
order' state whose order parameter remains unknown after 23 years of intense
research. Various experiments point to the reconstruction and partial gapping
of the Fermi surface when the hidden-order establishes. However, up to now, the
question of how this transition affects the electronic spectrum at the Fermi
surface has not been directly addressed by a spectroscopic probe. Here we show,
using angle-resolved photoemission spectroscopy, that a band of heavy
quasi-particles drops below the Fermi level upon the transition to the
hidden-order state. Our data provide the first direct evidence of a large
reorganization of the electronic structure across the Fermi surface of URu2Si2
occurring during this transition, and unveil a new kind of Fermi-surface
instability in correlated electron systemsComment: 15 pages, 5 figure
Angle-resolved and resonant photoemission spectroscopy study of the Fermi surface reconstruction in the charge density wave systems CeTe2 and PrTe2
The electronic structures of a charge density wave (CDW) system RTe2 (R = Ce, Pr) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES) and the first-principles band structure method. The R 4f hybridization peak (4f(n)c(m-1)) in the R 4f PES spectrum is located deeper in PrTe2 than in CeTe2 and R 4f spectral intensity near EF is much weaker in PrTe2 than in CeTe2, implying the importance of the hybridization between Ce 4f and Te(1) 5p electrons. For both CeTe2 and PrTe2, the metallic states crossing the Fermi level (EF) are observed below the CDW transition temperature, indicating the existence of the partially ungapped Fermi surfaces (FSs). The zigzag features having the fourfold rotational symmetry are observed near the X point in the FS of CeTe2, but not in the FS of PrTe2. The tight-binding model calculations show that the zigzag FS features in CeTe2 can be described as the CDW-induced FS reconstruction due to the 4 x 4 CDW supercell structure. The effect of the linear dichroism is observed in ARPES, suggesting that the E-F-crossing states have mainly the in-plane orbital character. The photon-energy maps for the near-E-F states exhibit the straight vertical dispersions for both CeTe2 and PrTe2, demonstrating the dominant two-dimensional character in RTe2 (R = Ce, Pr).open1122sciescopu
Spin-orbit density wave induced hidden topological order in URu2Si2
The conventional order parameters in quantum matters are often characterized
by 'spontaneous' broken symmetries. However, sometimes the broken symmetries
may blend with the invariant symmetries to lead to mysterious emergent phases.
The heavy fermion metal URu2Si2 is one such example, where the order parameter
responsible for a second-order phase transition at Th = 17.5 K has remained a
long-standing mystery. Here we propose via ab-initio calculation and effective
model that a novel spin-orbit density wave in the f-states is responsible for
the hidden-order phase in URu2Si2. The staggered spin-orbit order 'spontaneous'
breaks rotational, and translational symmetries while time-reversal symmetry
remains intact. Thus it is immune to pressure, but can be destroyed by magnetic
field even at T = 0 K, that means at a quantum critical point. We compute
topological index of the order parameter to show that the hidden order is
topologically invariant. Finally, some verifiable predictions are presented.Comment: (v2) Substantially modified from v1, more calculation and comparison
with experiments are include
Electronic Structure of YbB 6 : Is it a Topological Insulator or Not?
To finally resolve the controversial issue of whether or not the electronic structure of YbB6 is nontrivially topological, we have made a combined study using angle-resolved photoemission spectroscopy (ARPES) of the nonpolar (110) surface and density functional theory (DFT). The flat-band conditions of the (110) ARPES avoid the strong band bending effects of the polar (001) surface and definitively show that YbB6 has a topologically trivial B 2p-Yb 5d semiconductor band gap of similar to 0.3 eV. Accurate determination of the low energy band topology in DFT requires the use of a modified Becke-Johnson exchange potential incorporating spin-orbit coupling and an on-site Yb 4f Coulomb interaction U as large as 7 eV. The DFT result, confirmed by a more precise GW band calculation, is similar to that of a small gap non-Kondo nontopological semiconductor. Additionally, the pressure-dependent electronic structure of YbB6 is investigated theoretically and found to transform into a p-d overlap semimetal with small Yb mixed valency.open11119sciescopu
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