Appropriate consideration of the electron correlation is essential to reproduce the intriguing metal-insulator transition accompanying the Peierls-type structural transition in VO
2
. In the density ...functional theory-based approach, this depends on the choice of the exchange-correlation functional. Here, using a newly developed strongly constrained and appropriately norm (SCAN) functional, we investigate the lattice and electronic properties of the metallic rutile phase of VO
2
(
R
-VO
2
) from the first-principles calculations. We also explored the role of the Coulomb correlation
U
. By adding
U
, we found that the phonon instability properly describes the Peierls-type distortions. The orbital-decomposed density of states presents the orbital selective behavior with the SCAN+
U
, which is susceptible to the one-dimensional Peierls distortion. Our results suggest that even with the SCAN functional, the explicit inclusion of the Coulomb interaction is necessary to describe the structural transition of VO
2
.
Cuprates have been at the center of long debate regarding their superconducting mechanism; therefore, predicting the critical temperatures of cuprates remains elusive. Herein, using machine learning ...and first-principles calculations, we predict the maximum superconducting transition temperature (T c,max) of hole-doped cuprates and suggest the functional form for T c,max with the root-mean-square-error of 3.705 K and R 2 of 0.969. We have found that the Bader charge of apical oxygen, the bond strength between apical atoms, and the number of superconducting layers are essential to estimate T c,max. Furthermore, we predict the T c,max of hypothetical cuprates generated by replacing apical cations with other elements. Among the hypothetical structures, the cuprates with Ga show the highest predicted T c,max values, which are 71, 117, and 131 K for one, two, and three CuO2 layers, respectively. These findings suggest that machine learning could guide the design of new high-T c superconductors in the future.
Abstract
Superconductivity in the vicinity of a competing electronic order often manifests itself with a superconducting dome, centered at a presumed quantum critical point in the phase diagram. This ...common feature, found in many unconventional superconductors, has supported a prevalent scenario in which fluctuations or partial melting of a parent order are essential for inducing or enhancing superconductivity. Here we present a contrary example, found in IrTe
2
nanoflakes of which the superconducting dome is identified well inside the parent stripe charge ordering phase in the thickness-dependent phase diagram. The coexisting stripe charge order in IrTe
2
nanoflakes significantly increases the out-of-plane coherence length and the coupling strength of superconductivity, in contrast to the doped bulk IrTe
2
. These findings clarify that the inherent instabilities of the parent stripe phase are sufficient to induce superconductivity in IrTe
2
without its complete or partial melting. Our study highlights the thickness control as an effective means to unveil intrinsic phase diagrams of correlated van der Waals materials.
Abstract
Monolayers of two-dimensional van der Waals materials exhibit novel electronic phases distinct from their bulk due to the symmetry breaking and reduced screening in the absence of the ...interlayer coupling. In this work, we combine angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy to demonstrate the emergence of a unique insulating 2 × 1 dimer ground state in monolayer 1
T
-IrTe
2
that has a large band gap in contrast to the metallic bilayer-to-bulk forms of this material. First-principles calculations reveal that phonon and charge instabilities as well as local bond formation collectively enhance and stabilize a charge-ordered ground state. Our findings provide important insights into the subtle balance of interactions having similar energy scales that occurs in the absence of strong interlayer coupling, which offers new opportunities to engineer the properties of 2D monolayers.
The intriguing coexistence of the charge density wave (CDW) and superconductivity in SrPt2As2 and LaPt2Si2 has been investigated based on the ab initio density functional theory band structure and ...phonon calculations. We have found that the CDW instabilities for both cases arise from the q-dependent electron-phonon coupling with quasi-nesting feature of the Fermi surface. The band structure obtained by the band-unfolding technique reveals the sizable q-dependent electron-phonon coupling responsible for the CDW instability. The local split distortions of Pt atoms in the As-Pt-As layers play an essential role in driving the five-fold supercell CDW instability as well as the phonon softening instability in SrPt2As2. By contrast, the CDW and phonon softening instabilities in LaPt2Si2 occur without split distortions of Pt atoms. The phonon calculations suggest that the CDW and the superconductivity coexist in X-Pt-X layers (X = As or Si) for both cases.
In this study, we investigate the effectiveness of machine learning (ML) models in constructing empirical formulas for the superconducting transition temperature (
T
c
) by comparing ML-derived ...equations with McMillan’s equation. We utilized artificially generated data with a size of 10,000 from McMillan’s equation and employed the parametric brute force searching (BFS) algorithm to search for model equations varying model complexity and dataset size. The BFS models with features of the Debye temperature and electron–phonon coupling exhibit the RMSE of 0.830 K and
R
2
of 0.976 even with a small dataset size of 100. The ML-derived formula is also close to McMillan’s equation showing a linear relationship between the Debye temperature and
T
c
, as well as a cubic relationship between electron–phonon coupling and
T
c
. Furthermore, we analyzed feature contributions using non-parametric random forest (RF) regression and found the strong relevance of electron–phonon coupling on
T
c
. Our results demonstrate the importance of feature selection and model complexity in effectively predicting
T
c
rather than simply adding more data.
The study of the high critical temperature (Tc) of hydrogen compounds under high pressure has resulted in a considerable focus on Bardeen-Cooper-Schrieffer superconductors. Nb has the highest Tc ...among the elemental metals at ambient pressure, so reviewing Nb films again is worthwhile. In this study, we investigated the factors that determine the Tc of Nb films by strain introduction and carrier doping. We deposited Nb films of various thicknesses onto Si substrates and evaluated the Tc variation with thickness. In-plane compressive strain in the (110) plane due to residual stress reduced the Tc. First-principles calculations showed that adjusting the density of states at the Fermi level is key for both strain-induced suppression and doping-induced enhancement of the Nb Tc. The application of hydrostatic pressure compensated for the intrinsic strain of the film and increased its Tc, which could also be enhanced by increasing the hole concentration with an electric double-layer transistor. A liquid electrolyte should be used as a pressure medium for applying hydrostatic pressure to increase the Tc of correlated materials, where this increase results from changes in material structure and carrier concentration.
P2-type sodium cathode materials generally exhibit a P2–O2 phase transition upon deintercalation at high voltage, which is detrimental to their cycling performance. Herein, using first-principles ...calculations, we investigate the structural stability and phase transition of P2-Na0MnO2 upon substitution of Li for Mn as a model of a high-voltage phase. The phonon of P2-Na0MnO2 shows an imaginary phonon frequency, indicating instability, which is consistent with the experimental P2–O2 transformation. On the contrary, the phonon of P2-Na0Li0.25Mn0.75O2 shows dynamic stability. We demonstrate that the substitution of the Li ion induces the redistribution of charge from the out-of-plane to in-plane orbitals along with a reduced charge of oxygen. Furthermore, we consider the various Li doping compositions and suggest that the density of the next-nearest-neighbor Li-ion pairs also plays an important role in stabilizing the P2 phase. On the basis of our findings, we propose a minimum of ∼20% Li doping to stabilize P2-NaLi x Mn1–x O2 at high voltage.
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