3,330 research outputs found
Antiferromagnetic and structural transitions in the superoxide KO2 from first principles: A 2p-electron system with spin-orbital-lattice coupling
KO2 exhibits concomitant antiferromagnetic (AFM) and structural transitions,
both of which originate from the open-shell 2p electrons of O
molecules. The structural transition is accompanied by the coherent tilting of
O molecular axes. The interplay among the spin-orbital-lattice
degrees of freedom in KO2 is investigated by employing the first-principles
electronic structure theory and the kinetic-exchange interaction scheme. We
have shown that the insulating nature of the high symmetry phase of KO2 at high
temperature (T) arises from the combined effect of the spin-orbit coupling and
the strong Coulomb correlation of O 2p electrons. In contrast, for the low
symmetry phase of KO2 at low T with the tilted O molecular axes, the
band gap and the orbital ordering are driven by the combined effects of the
crystal-field and the strong Coulomb correlation. We have verified that the
emergence of the O 2p ferro-orbital ordering is essential to achieve the
observed AFM structure for KO2
Correlated Electronic Structures and the Phase Diagram of Hydrocarbon-based Superconductors
We have investigated correlated electronic structures and the phase diagram of electron-doped hydrocarbon molecular solids, based on the dynamical mean-field theory. We have found that the ground state of hydrocarbon-based superconductors such as electron-doped picene and coronene is a multi-band Fermi liquid, while that of non-superconducting electron-doped pentacene is a single-band Fermi liquid in the proximity of the metal-insulator transition. The size of the molecular orbital energy level splitting plays a key role in producing the superconductivity of electron-doped hydrocarbon solids. The multi-band nature of hydrocarbon solids would boost the superconductivity through the enhanced density of states at the Fermi level.X11910sciescopu
Observation of a kink during the formation of the Kondo resonance band in a heavy-fermion system
We have shown that the kink behavior in the spectral function of a heavy fermion can appear during the formation of the Kondo resonance (KR) band and the hybridization gap. We have investigated the heavy fermion compound CeCoGe2, using a combined approach of the density functional theory and the dynamical mean field theory. Low temperature T spectral functions show dispersive KR states, similarly to the recent experimental observation. During the evolution from the non-f conduction band state at high T to the dispersive KR band state at low T, which have topologically different band shapes, we have found the existence of kinks in the non-f spectral function near the Fermi level E-F. The observation of kink is clearly in correspondence with the multiple temperature scales of the formation of the KR band.X1186sciescopu
Correlated normal state fermiology and topological superconductivity in UTe2
UTe2 is a promising candidate for spin-triplet superconductors, in which a
paramagnetic normal state becomes superconducting due to spin fluctuations. The
subsequent discovery of various unusual superconducting properties has promoted
the use of UTe2 as an exciting playground to study unconventional
superconductivity, but fathoming the normal state fermiology and its influence
on the superconductivity still requires further investigation. Here, we
theoretically show that electron correlation induces a dramatic change in the
normal state fermiology with an emergent correlated Fermi surface (FS) driven
by Kondo resonance at low temperatures. This emergent correlated FS can account
for various unconventional superconducting properties in a unified way. In
particular, the geometry of the correlated FS can naturally host topological
superconductivity in the presence of odd-parity pairings, which become the
leading instability due to strong ferromagnetic spin fluctuations. Moreover,
two pairs of odd-parity channels appear as accidentally degenerate solutions,
which can naturally explain the multicomponent superconductivity with broken
time-reversal symmetry. Interestingly, the resulting time-reversal breaking
superconducting state is a Weyl superconductor in which Weyl points migrate
along the correlated FS as the relative magnitude of nearly degenerate pairing
solutions varies. We believe that the correlated normal state fermiology we
discovered provides a unified platform to describe the unconventional
superconductivity in UTe2.Comment: 13 pages, 4 figures and 1 table in the main text, and 10 figures and
1 table in the Supplementary Informatio
Temperature-dependent Fermi surface evolution in heavy fermion CeIrIn5
In Cerium-based heavy electron materials, the 4f electron's magnetic moments
bind to the itinerant quasiparticles to form composite heavy quasiparticles at
low temperature. The volume of the Fermi surfacein the Brillouin zone
incorporates the moments to produce a "large FS" due to the Luttinger theorem.
When the 4f electrons are localized free moments, a "small FS" is induced since
it contains only broad bands of conduction spd electrons. We have addressed
theoretically the evolution of the heavy fermion FS as a function of
temperature, using a first principles dynamical mean-field theory (DMFT)
approach combined with density functional theory (DFT+DMFT). We focus on the
archetypical heavy electrons in CeIrIn5, which is believed to be near a quantum
critical point. Upon cooling, both the quantum oscillation frequencies and
cyclotron masses show logarithmic scaling behavior (~ ln(T_0/T)) with different
characteristic temperatures T_0 = 130 and 50 K, respectively. The resistivity
coherence peak observed at T ~ 50 K is the result of the competition between
the binding of incoherent 4f electrons to the spd conduction electrons at Fermi
level and the formation of coherent 4f electrons.Comment: 5 pages main article,3 figures for the main article, 2 page
Supplementary information, 2 figures for the Supplementary information.
Supplementary movie 1 and 2 are provided on the
webpage(http://www-ph.postech.ac.kr/~win/supple.html
Potential of IGCC slag as an alkali activated material
Integrated gasification combined cycle (IGCC) is a next generation energy production technology that converts coal into syngas with enhanced power generation efficiency and environmental performance. IGCC produces coal gasification slag as the solid by-product. Recycling of IGCC slag is still in the early stages, but the recycling process has been around the cement and concrete industry. We calculated the reactive Si/Al ratio of IGCC slag which is generated from a pilot plant in South Korea, and evaluated the potential of it as an alkali-activated material. Samples which were activated with the combined activator of sodium silicate solution and caustic soda had an average compressive strength of 4.5 MPa, showing swelling on the top free surface. Expansion of the alkali-activated slag was possibly caused by free CaO and MgO in the slag. While the samples that were activated with the combined activator of sodium aluminate and caustic soda had an average compressive strength of 10 MPa. Hydroxy sodalite and C3AH6 were found to be the new crystalline phases. IGCC slag can be used as an alkali-activated material, but the strength performance should be improved with proper mix design approach which can alleviate the expansion issue at the same time.
Acknowledgement
This study was supported by Korea Western Power Co., Ltd. in South Korea
Topological acoustic triple point
Acoustic phonon in a crystalline solid is a well-known and ubiquitous example
of elementary excitation with a triple degeneracy in the band structure.
Because of the Nambu-Goldstone theorem, this triple degeneracy is always
present in the phonon band structure. Here, we show that the triple degeneracy
of acoustic phonons can be characterized by a topological charge
that is a property of three-band systems with symmetry, where
and are the inversion and the time-reversal
symmetries, respectively. We therefore call triple points with nontrivial
the topological acoustic triple point (TATP). The topological
charge can equivalently be characterized by the skyrmion number
of the longitudinal mode, or by the Euler number of the transverse modes, and
this strongly constrains the nodal structure around the TATP. The TATP can also
be symmetry-protected at high-symmetry momenta in the band structure of phonons
and spinless electrons by the and the groups. The nontrivial
wavefunction texture around the TATP can induce anomalous thermal transport in
phononic systems and orbital Hall effect in electronic systems. Our theory
demonstrates that the gapless points associated with the Nambu-Goldstone
theorem are an avenue for discovering new classes of degeneracy points with
distinct topological characteristics.Comment: 7+15 pages, 5+6 figure
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