Previous Band Calculations Research Articles

Intrinsic Anomalous Thermal Hall Effect in the Unconventional Superconductor UTe2

We investigate the intrinsic anomalous thermal Hall effect as an effective probe of the order parameters of non-unitary pairing states of the putative spin-triplet superconductor UTe$_2$. We consider all symmetry-allowed non-unitary states under magnetic fields comprehensively, and calculate the anomalous Hall conductivity arising from time-reversal symmetry-breaking Weyl point nodes of the gap functions, assuming three types of Fermi surfaces; ellipsoid-shaped, cylinder-shaped, and ring-shaped ones, which are predicted by previous band calculations. We find the nonzero anomalous thermal Hall conductivity in the certain ratio of the two irreducible representations. The results may be utilized for future exploration of pairing states of UTe$_2$.

Effects of unique band structure of h-BN probed by photocurrent excitation spectroscopy

By employing a photocurrent excitation spectroscopy measurement, a direct bandgap of ∼6.46 eV has been resolved for the first time in thick B-10 enriched h-BN films. Together with previous band calculations, an unconventional energy diagram has been constructed to capture the unique features of h-BN: h-BN has a minimum direct bandgap of ∼6.5 eV and a bandgap of ∼6.1 eV which is indirect with the conduction band minimum (CBM) at M-point and valence band maximum (VBM) at K-point in the Brillouin zone, and the energy levels of the donor and acceptor impurities are measured relative to CBM and VBM, respectively.

Effects of thermal and spin fluctuations on the band structure of purple bronze Li2Mo12O34

The band structures of ordered and thermally disordered Li$_2$Mo$_{12}$O$_{34}$ are calculated by use of ab-initio DFT-LMTO method. The unusual, very 1-dimensional band dispersion obtained in previous band calculations is confirmed for the ordered structure, and the overall band structure agrees reasonably with existing photoemission data. Dispersion and bandstructure perpendicular to the main dispersive direction is obtained. A temperature dependent band broadening is calculated from configurations with thermal disorder of the atomic positions within the unit cell. This leads a band broadening of the two bands at the Fermi energy which can become comparable to their energy separation. The bands are particularly sensitive to in-plane movements of Mo sites far from the Li-sites, where the density-of-states is highest. The latter fact makes the effect of Li vacancies on the two bands relatively small. Spin-polarized band results for the ordered structure show a surprisingly large exchange enhancement on the high DOS Mo sites. Consequences for spin fluctuations associated with a cell doubling along the conducting direction are discussed.

X-Ray Photoelectron Spectroscopy and Electronic Structures of Scheelite- and Wolframite-Type Tungstate Crystals

X-ray photoelectron spectroscopy (XPS) of metal tungstates, MWO 4 , has been carried out to investigate the electronic structures of scheelites (M=Pb, Ca, and Ba) and wolframites (M=Cd and Zn). Relativistic molecular orbital calculations of these tungstates have also been performed. The theoretical results are discussed in comparison with the previous band calculations, and are shown to be in satisfactory agreement with the XPS spectra. The valence and conduction bands are mainly composed of the O 2 p and W 5 d states, respectively, in all cases. It is stressed that the metal states contribute, to some extent, to the top of the valence band and/or the bottom of the conduction band in PbWO 4 , CdWO 4 , and ZnWO 4 , but such contributions are negligible in CaWO 4 and BaWO 4 . The similarity and difference of the fundamental optical spectra among the tungstates are explained on the basis of the present results.

Singlet pairing versus triplet pairing in a cobaltate superconductor NaxCoO2·yH2O

Abstract Here, we propose that a close competition between spin singlet and spin-triplet pairings may be taking place in Na x CoO 2 · y H 2 O, which we model by an (extended) Hubbard Hamiltonian that takes into account the pocket-like Fermi surfaces and the van Hove singularity found in previous band calculation results. We show that spin-triplet f-wave pairing dominates when off-site repulsions are not too strong, while for larger values of off-site repulsions, spin-singlet i-wave pairing may take place.

Shubnikov–de Haas oscillations in the two-dimensional organic conductorτ−(EDO−S,S−DMEDT−TTF)2(AuBr2)1+y(y∼0.75)

The quasi-two-dimensional organic τ-type conductor, τ-(EDO-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1 + y (y ∼0.75), shows metallic temperature dependence of the resistivity down to T m i n ∼50 K, below which resistance upturn is observed. There has been no evidence of the phase transition through T m i n . In this work, in pursuit of direct evidence for metal at low temperature by transport measurement, we show an observation of the Shubnikov-de Haas (SdH) oscillations for this salt. The observed frequencies of the oscillations are inconsistent with the previous band calculation. Further, it is remarkable that the amplitude of the oscillation was so giant (‖Δρ/ρ 0 ‖∼30%), and the oscillation above 20 T corresponds to a N = 2 Landau state, i.e., next to a quantum limit. We also observed the SdH oscillations in other τ-type conductor, τ-(P-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1 + y (y∼0.75), which almost reproduced the previous report by Storr et al. [Phys. Rev. B 64, 045107 (2001)].

Giant Shubnikov-de Haas oscillation and the new metallic state in the organic τ-type conductors

Two-dimensional organic τ-type conductors show metallic temperature dependence of resistivity down to T min =30–50 K, below which a resistivity upturn is observed. A large negative magnetoresistance appears below T min suddenly in temperature. Below about 4 K, magnetoresistance in any direction is extremely hysteretic in field-, temperature- or field-angle-sweep, and ρ ⊥( H//plane) shows switching between two- and four-fold depending on magnetic and thermal history, indicating magnetic domain generation. Static susceptibility shows slight ferromagnetic component. NMR is in favor of anti-ferromagnetism below about 15 K. These two results imply that the system is a weak ferromagnetic one. Preliminary optical reflectance and specific heat studies implied metallic state at low temperature, but no clear-cut proof has been shown whether or not the ground state is metallic. This report presents giant Shubnikov de Haas (SdH) oscillation in both τ-(EDO- S, S-DMEDT-TTF) 2(AuBr 2) 1+ y , and τ-(P- S, S-DMEDT-TTF) 2(AuBr 2) 1+ y , ( y∼0.75), which is a clear evidence for the metallic ground state. However, the observed Fermi surfaces (FS) are 0.65 and 6.1% of the first Brillouin zone for τ-(EDO- S, S-DMEDT-TTF) 2(AuBr 2) 1+ y , and 2.3 and 6.9% for τ-(P- S, S-DMEDT-TTF) 2(AuBr 2) 1+ y , ( y∼0.75), which suggest extremely small FS pockets, and were not expected by the previous band calculation. Further, it is surprising that the Dingle temperature was around 1 K, indicating unexpectedly clean, while residual resistivity ratio is about 1. We note that the fundamental oscillation of 0.65% is already n=2 state, which is already next to the Landau quantum limit.

Experimental surface-state band structure of the Ba-inducedSi(111)3×1surface

The surface-state band structure of the Ba-induced single-domain $\mathrm{Si}(111)3\ifmmode\times\else\texttimes\fi{}1$ surface was studied in detail via angle-resolved photoelectron spectroscopy using synchrotron radiation. Applying different photon energies and experimental geometries, we obtained the experimental surface-state band structure, including information of the symmetry of the surface states. Our results indicate that the surface electronic state is undoubtedly semiconducting, in contrast to the prediction of previous band calculations. However, the dispersion feature of the three nonmetallic surface states is in almost perfect agreement with that calculated for the honeycomb-chained channel model. The inconsistency between our results and the theoretical calculation can be explained by the fact that the coverage of alkali-earth metal is not the same as that of alkali metal. Rather, the coverage of alkali-earth metal on the $3\ifmmode\times\else\texttimes\fi{}1$ surface is half that of alkali metal, i.e., 1/6 monolayer.

The mechanism of the photo-induced magnetic transition in Co–Fe cyanide with ab initio calculations

Following our previous band structure calculation for stoichiometric crystal, ab initio cluster calculations with partially ligand substitution were carried out to clarify the mechanism of the heat-induced charge transfer observed in a Co–Fe cyanide complex exhibiting a photo-induced magnetic transition system. The existence of iron vacancies is found to be another reason for the stabilization of the cobalt high-spin (HS) state as compared with the low-spin (LS) state rather than the crystal structure expansion evidenced by the previous band calculation. Doped electrons are localized on the nearest Co sites of vacancies inducing charge transfer from Fe to Co around them. We suggest that this plays a role of domain nucleation in the heat/photo-induced LS–HS transition.

Ab initio study on the electronic structure and magnetism of MnAs, MnSb, and MnBi

We report the results of first-principles calculations on the electronic structure in ferromagnetic and non-magnetic hexagonal MnV (V = As, Sb, Bi). The calculations are based on the local-spin-density approximation (LSDA) of the density-functional theory (DFT) as well as the atomic sphere approximation (ASA) in the linear muffin-tin orbitals (LMTO) method. For the non-spin-polarized case, the calculated bands in these compounds exhibit p-d mixing in the vicnity of Fermi energy and the Mn 3d bands dominate the antibonding parts of p–d hybride. The spin-polarization in ferromagnetic states are mainly due to the splitting of anti-bonding bands from p–d mixing. The calculated spin moments in these compounds agree fairly well with experimental values and refine previous band calculations. In the spin-polarized band structure, the Mn 3d electrons are found to exhibit week dispersions.

Two-dimensional band mapping of 2H-TaSe2 using a display-type photoelectron spectrometer

Abstract We have observed the two-dimensional photoelectron angular distribution (PEAD) patterns of the valence band of the 2 H -TaSe 2 using a display type photoelectron spectrometer. The PEAD patterns which correspond to the two-dimensional band map show the various changes with the change of the binding energies, reflecting the complex band structure of 2 H -TaSe 2 . The intensity suppression area of the pattern at the Ta 5d band indicates that the half-filled Ta 5d band consists mainly of d z 2 -like orbitals as predicted by previous band calculation.

Symmetry Analysis of the Fermi Surface States of Sr2RuO4by Display-Type Photoelectron Spectroscopy

The Fermi surface of the noncuprate superconductor Sr 2 RuO 4 has been studied in detail by two-dimensional angle resolved photoelectron spectroscopy (2D-ARPES) using a display-type electron spectrometer and a linearly polarized synchrotron radiation. The existence of the large Fermi surfaces agrees with previous band calculations and conventional ARPES measurements. From the comparison of the observed photoelectron intensity distribution of the Fermi surface with that of the simulation for atomic orbitals, the symmetry of the atomic orbitals composing the Fermi surfaces is revealed to be the d e ; d x y , d y z , d z x -like in contrast to the d x 2 - y 2 -like orbital in the cuprate high- T c super conductor. The possibility of the existence of the extended van-Hove singularity just below the Fermi level, which was suggested by recent conventional ARPES measurements, is also discussed.

X-ray diffraction studies of expanded fluid mercury using synchrotron radiation

Energy-dispersive x-ray diffraction measurements using synchrotron radiation at SPring-8 for expanded fluid Hg were carried out over the wide density range from the liquid to the dense-vapour region including the metal-non-metal (M-NM) transition region. The density ranges from 13.6 to . We developed a high-pressure vessel and a sapphire cell for the x-ray diffraction measurements under high temperature and pressure up to and 1765 bar. We obtained the structure factor S(k) and the pair distribution functions g(r). The density variations of the interatomic distance and coordination number obtained were discussed in relation to the M-NM transition in fluid Hg. It was found that the volume expansion of liquid Hg in the metallic region is not accompanied by a uniform increase of , but is mainly caused by a decrease of . When the transition region is crossed, the rate of decrease of becomes small and starts to elongate. We discussed some previous band calculations for expanded fluid Hg and concluded that it is essential for explaining the M-NM transition in liquid Hg to take the fluctuation of into account. When the density decreases down to the dense-vapour region, the first maxima of g(r) are located at around 3.3-3.4 Å, which is close to the interatomic distance of Hg dimers.

First-principles study of the ortho-KC60polymer

We present results of first-principles band calculations as well as structural optimization of the orthorhombic ${\mathrm{KC}}_{60}$ polymer. Our band dispersion is significantly anisotropic, and different from previous band calculations. The character of the conduction band is explained in terms of linear combinations of the molecular orbitals of ${I}_{h} {\mathrm{C}}_{60}$ and a perturbation coming from the polymerization.

Structural studies of expanded fluid mercury up to the liquid-vapor critical region

The energy-dispersive x-ray-diffraction measurements for expanded fluid Hg were carried out in the density region from 13.55 to 6.6 ${\mathrm{g}\mathrm{}\mathrm{cm}}^{\ensuremath{-}3}$ including the metal-nonmetal $(M\ensuremath{-}\mathrm{NM})$ transition region. We developed a new type of high-pressure vessel and a new sample cell made of single-crystal sapphire for the x-ray diffraction measurements under high temperature and pressure up to 1530 \ifmmode^\circ\else\textdegree\fi{}C and 1980 bars using the energy-dispersive method. We obtained the interference functions and the pair-distribution functions, the termination error of which was corrected using the method by Kaplow et al. The obtained local structure, such as interatomic distance ${r}_{1}$ and coordination number ${N}_{1},$ was discussed in relation to the M-NM transition in liquid Hg. It was found that the volume expansion of liquid Hg in the metallic region is not accompanied by a uniform increase of ${r}_{1},$ but mainly caused by a decrease of ${N}_{1}.$ When the transition region is crossed, the rate of decrease of ${N}_{1}$ becomes small and ${r}_{1}$ starts to elongate. We discussed some previous band calculations for expanded fluid Hg using the obtained ${N}_{1}$ values and concluded that it is essential for explaining the M-NM transition in liquid Hg to take the fluctuation of ${N}_{1}$ into account. Such a microscopic inhomogeneity may be much more enhanced in the lower-density region of the present experiments. The minimum of $S(k)$ found in the small-k region below 1 ${\AA{}}^{\ensuremath{-}1}$ at $\ensuremath{\rho}<~8.5{\mathrm{g}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ indicates a large enhancement of the density fluctuation and the $g(r)$ data strongly suggest that small clusters exist near the critical range.

Fermi surface of Sr 2RuO 4 studied by two-dimensional angle resolved photoelectron spectroscopy

The shape of the Fermi surface of the Sr 2RuO 4, the first noncuprate superconductor, has been directly observed by two-dimensional angle resolved photoelectron spectroscopy (2D-ARPES) technique using a display-type hemispherical mirror analyzer. The gross shape of the Fermi surface agrees with previous band calculations. The photoelectron angular distribution pattern of the Fermi level shows that the Fermi surface is composed of orbitals with d xy, d yz , and d zx -like symmetries which agree well with those predicted by previous band calculations and are in contrast to d x 2− y 2 symmetry of thehigh- T C superconductors.

Magnetic circular dichroism at the K edge of nickel and iron.

The spectra of the K-edge absorption and the magnetic circular dichroism (MCD) for ferromagnetic Ni and Fe are calculated on the basis of a tight-binding approximation including the spin-orbit interaction. The calculated spectra are in good agreement with the experiments and the previous band calculations near the photothreshold. It is shown that the spin-orbit interaction of the 3d states gives rise to a dominant contribution to the MCD while that of the 4p states plays only a minor role. This suggests that the MCD, which probes the orbital moment of p-symmetric (such as 4p) states on the core-hole site, is generated mainly by the 3d orbital moment on the neighboring sites through the p-d hybridization. The effect of the electric-quadrupole transition to the MCD of Ni is found at a close vicinity of the photothreshold. Sum rules for the dipole transition are discussed.

Synthesis and characterization of the pseudo-1-D mixed-valence (Et4-nMenN)Cu2X4 salts: pinned charge density wave systems exhibiting intervalence charge transfer

A series of mixed-valence Cu(I)/Cu(II) linear chain compounds exhibiting pinned charge density waves have been synthesized. The compounds have stoichiometry ACu[sup I]Cu[sup II]X[sub 4] with X = Cl or Br and A = tetraethylammonium (TEA), triethylmethylammonium (TEM), or diethyldimethylammonium (DEM). The crystal structures of the bromide analogues have been determined. The (TEM)Cu[sub 2]Br[sub 4] salt is tetragonal. The (DEM)Cu[sub 2]Br[sub 4] salt is also tetragonal. The (TEA)Cu[sub 2]Br[sub 4] salt is orthorhombic. These structures reveal infinite chains of alternating Cu[sup I]X[sub 4][sup 3[minus]] and Cu[sup II]X[sub 4][sup 2[minus]] tetrahedra (distorted) sharing edges, the chains being effectively isolated by bulky tetraalkylammonium cations. Although the valence states appear to be localized, with a 0.1 [angstrom] difference in the Cu(I)-X and Cu(II)-X bond lengths, several characterization techniques indicate Robin and Day class II behavior. The data also support a Peierls 4k[sub f] distorted charge density wave state for these chains. All salts show an intervalence charge-transfer band in the visible region of the spectrum. Conductivity measurements indicate semiconductive behavior. Temperature-dependent magnetic susceptibility measurements on (TEA)Cu[sub 2]Br[sub 4] reveal weak antiferromagnetic coupling, which is attributed to short Br[center dot][center dot][center dot]Br contacts between Cu[sup II]X[sub 4][sup 2[minus]] species within the chains. The above physicalmore » properties are rationalized in light of previous EHMO band calculations.« less

Electron and optical spectra in the indirect-gap semiconductor CrSi2

Within the density-functional theory, fully relativistic calculations of electron and optical spectra in chromium disilicide have been performed. CrSi2 is found to be an indirect-gap semiconductor; pressure therefore has a subtle effect on the gap. The calculated electronic structure is compared with experimental data and previous band calculations.

Properties of BN thin films deposited by plasma CVD

Boron nitride thin films were deposited from B 2H 6 and N 2 gases using the plasma CVD technique at a low pressure. The formation of cubic boron nitride needs a negative substrate self-bias. The deposited films were characterized by infrared absorption spectroscopy and transmission electron microscopy which showed that the cubic BN consisted of microcrystals with 100–200 Å grain size. Optical reflectance measurements of the films were carried out at room temperature in the photon energy range 5–25 eV. Structure in the imaginary dielectric function spectra was in good agreement with that predicted for crystalline BN by previous energy band calculations.