The properties of an interacting electron system depend on the
electron correlations and the effective dimensionality. For example, Coulomb
repulsion between electrons may inhibit, or completely ...block, conduction by
intersite electron hopping, thereby determining whether a material is a metal
or an insulator. Furthermore, correlation effects increase as
the number of effective dimensions decreases; in three-dimensional systems, the
low-energy electronic states behave as quasiparticles, whereas in
one-dimensional systems, even weak interactions break the quasiparticles into
collective excitations. Dimensionality is particularly important
for exotic low-dimensional materials where one- or two-dimensional building
blocks are loosely connected into a three-dimensional whole. Here we examine
two such layered metallic systems with angle-resolved photoemission
spectroscopy and electronic transport measurements, and we find a crossover in
the number of effective dimensions-from two to three-with
decreasing temperature. This is apparent from the observation that, in the
direction perpendicular to the layers, the materials have an insulating
character at high temperatures but become metal-like at low temperatures,
whereas transport within the layers remains metallic over the whole temperature
range. We propose that this change in effective dimensionality correlates with
the presence of coherent quasiparticles within the layers.
The effects of structural supermodulation with the period λ ≈ 26 Å along the b axis of Bi2Sr2CaCu2O8+δ have been observed in photoemission studies from the early days as the presence of "diffraction ...replicas" of the intrinsic electronic structure. Although predicted to affect the electronic structure of the Cu-O plane, the influence of supermodulation potential on Cu-O electrons has never been observed in photoemission. In the present study, we clearly see the effects on the Bi2Sr2CaCu2O8+δ electronic structure; we observe a hybridization of the intrinsic bands with the supermodulation replica bands in the form of avoided crossings and a corresponding reconstruction of the Fermi surface. We estimate the hybridization gap, 2Δh ∼ 25 meV, in the slightly underdoped samples. The hybridization weakens with doping and the anticrossing can no longer be resolved in strongly overdoped samples. In contrast, the "shadow" replica, shifted by ( π , π ) , is found not to hybridize with the original bands within our detection limits.
We report a new, cleavable, strong topological metal, Zr sub(2) Te sub(2) P, which has the same tetradymite-type crystal structure as the topological insulator Bi sub(2) Te sub(2) Se. Instead of ...being a semiconductor, however, Zr sub(2) Te sub(2) P is metallic with a pseudogap between 0.2 and 0.7 eV above the Fermi energy (E sub(F)). Inside this pseudogap, two Dirac dispersions are predicted: one is a surface-originated Dirac cone protected by time-reversal symmetry (TRS), while the other is a bulk-originated and slightly gapped Dirac cone with a largely linear dispersion over a 2 eV energy range. A third surface TRS-protected Dirac cone is predicted, and observed using angle-resolved photoemission spectroscopy, making Zr sub(2) Te sub(2) P the first system, to our knowledge, to realize TRS-protected Dirac cones at M points. The high anisotropy of this Dirac cone is similar to the one in the hypothetical Dirac semimetal BiO sub(2). We propose that if E sub(F) can be tuned into the pseudogap where the Dirac dispersions exist, it may be possible to observe ultrahigh carrier mobility and large magnetoresistance in this material.
We report the temperature-dependent optical conductivity and angle-resolved photoemission spectroscopy (ARPES) studies of the multiband iron-based superconductor Sr0.67Na0.33Fe2As2. Measurements were ...made in the high-temperature tetragonal paramagnetic phase, below the structural and magnetic transitions at TN≃125 K in the orthorhombic spin-density-wave (SDW)-like phase and Tr≃42 K in the reentrant tetragonal double-Q magnetic phase where both charge and SDW order exist, and below the superconducting transition at Tc≃10 K. The free-carrier component in the optical conductivity is described by two Drude contributions: one strong and broad and the other weak and narrow. The broad Drude component decreases dramatically below TN and Tr, with much of its strength being transferred to a bound excitation in the midinfrared, while the narrow Drude component shows no anomalies at either of the transitions, actually increasing in strength at low temperature while narrowing dramatically. The behavior of an infrared-active mode suggests zone folding below Tr. Below Tc the dramatic decrease in the low-frequency optical conductivity signals the formation of a superconducting energy gap. ARPES reveals holelike bands at the center of the Brillouin zone (BZ), with both electron- and holelike bands at the corners. Below TN, the hole pockets at the center of the BZ decrease in size, consistent with the behavior of the broad Drude component; however, below Tr the electronlike bands shift and split, giving rise to a low-energy excitation in the optical conductivity at ≃20 meV. The C2 and C4 magnetic states, with resulting spin-density-wave and charge-SDW order, respectively, lead to a significant reconstruction of the Fermi surface that has profound implications for the transport originating from the electron and hole pockets but appears to have relatively little impact on the superconductivity in this material.
A comparative study of the gaps measured in two techniques, angle-resolved photoelectron spectroscopy and spectroscopic imaging scanning tunneling microscopy, is presented. In particular the study ...focuses on the more overdoped region of the cuprate phase diagram in the superconducting state. While the total densities of states measured in the two techniques agree very well, it is shown that the peak in the density of states, ΔDOS, is consistently displaced to higher energies relative to the maximal superconducting gap, Δ0, determined in photoemission. The difference between the two gaps is more evident for the less doped samples reflecting increased normalization of bands. This observation will clearly influence the boundaries in the phase diagrams of the cuprates defined by these two techniques.
The ab-plane optical properties of a cleaved single crystal of La2-xBaxCuO4 for x=1/8 (Tc approximately =2.4 K) have been measured over a wide frequency and temperature range. The low-frequency ...conductivity is Drude-like and shows a metallic response with decreasing temperature. However, below approximately =60 K, corresponding to the onset of charge-stripe order, there is a rapid loss of spectral weight below about 40 meV. The behavior is quite different from that typically associated with the pseudogap in the normal state of the cuprates. Instead, the gapping of the normal-state single-particle excitations looks surprisingly similar to that observed in superconducting La2-xSrxCuO4, including the presence of a residual Drude peak with reduced weight.
Ferromagnetic van der Waals (vdW) insulators are of great scientific interest for their promising applications in spintronics. It has been indicated that in the two materials within this class, CrI3 ...and VI3, the magnetic ground state, the band gap, and the Fermi level could be manipulated by varying the layer thickness, strain or doping. To understand how these factors impact the properties, a detailed understanding of the electronic structure would be required. However, the experimental studies of the electronic structure of these materials are still very sparse. Here, we present the detailed electronic structure of CrI3 and VI3 measured by angle-resolved photoemission spectroscopy (ARPES). Our results show a band-gap of the order of 1 eV, sharply contrasting some theoretical predictions such as Dirac half-metallicity and metallic phases, indicating that the intra-atomic interaction parameter (U) and spin-orbit coupling (SOC) were not properly accounted for in the calculations. We also find significant differences in the electronic properties of these two materials, in spite of similarities in their crystal structure. In CrI3, the valence band maximum is dominated by the I 5p, whereas in VI3 it is dominated by the V 3d derived states. Our results represent valuable input for further improvements in the theoretical modeling of these systems.
The self-energy of the photohole in 2H-TaSe2 is measured by angle-resolved photoemission spectroscopy as a function of binding energy and temperature. In the charge-density wave (CDW) state, a ...structure in the self-energy is detected at approximately 65 meV that cannot be explained by electron-phonon scattering. A reduction in the scattering rates below this energy indicates the collapse of a major scattering channel with the formation of the CDW state accompanying the appearance of a bosonic "mode" in the excitation spectrum of the system.
We present an overview of the electronic properties of iron arsenic high temperature superconductors with emphasis on low energy band dispersion, Fermi surface and superconducting gap. ARPES data is ...compared with full-potential linearized plane wave (FLAPW) calculations. We focus on single layer NdFeAsO
0.9F
0.1 (R1111) and two layer Ba
1−
x
K
x
Fe
2As
2 (B122) compounds. We find general similarities between experimental data and calculations in terms of character of Fermi surface pockets, and overall band dispersion. We also find a number of differences in details of the shape and size of the Fermi surfaces as well as the exact energy location of the bands, which indicate that magnetic interaction and ordering significantly affects the electronic properties of these materials. The Fermi surface consists of several hole pockets centered at
Γ and electron pockets located in zone corners. The size and shape of the Fermi surface changes significantly with doping. Emergence of a coherent peak below the critical temperature
T
c and diminished spectral weight at the chemical potential above
T
c closely resembles the spectral characteristics of the cuprates, however the nodeless superconducting gap clearly excludes the possibility of
d-wave order parameter. Instead it points to
s-wave or extended
s-wave symmetry of the order parameter.
► Correlation between stripes, QP properties and superconductivity observed in 214 cuprates. ► At the onset of superconductivity (x∼0.05), the diagonal stripes give away to superconductivity. ► ...Appearance of static parallel stripes at x=1/8 suppresses superconductivity. ► Spectral gap in non-superconducting samples at x=1/8 resembles superconducting gap. ► Parallel stripes seem to enhance pairing, but destroy phase coherence.
Twenty five years after discovery of high-temperature superconductivity (HTSC) in La2−xBaxCuO4 (LBCO), the HTSC continues to pose some of the biggest challenges in materials science. Cuprates are fundamentally different from conventional superconductors in that the metallic conductivity and superconductivity are induced by doping carriers into an antiferromagnetically ordered correlated insulator. In such systems, the normal state is expected to be quite different from a Landau-Fermi liquid – the basis for the conventional BCS theory of superconductivity. The situation is additionally complicated by the fact that cuprates are susceptible to charge/spin ordering tendencies, especially in the low-doping regime. The role of such tendencies on the phenomenon of superconductivity is still not completely clear. Here, we present studies of the electronic structure in cuprates where the superconductivity is strongly suppressed as static spin and charge orders or “stripes” develop near the doping level of x=1/8 and “outside” of the superconducting dome, for x<0.055. We discuss the relationship between the “stripes”, superconductivity, pseudogap and the observed electronic excitations in these materials.
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