arXiv subjects

K. -W. Lee

Publications and source records attributed to K. -W. Lee.

At least 37 records · Page 2Linked to original sources

arXiv2015

Large orbital moment and spin-orbit enabled Mott transition in the Ising Fe honeycomb lattice BaFe2(PO4)2

BaFe2(PO4)2 is an unusual Ising insulating ferromagnet based on the Fe$^{2+}$ spin $S$ = 2 ion, the susceptibility of which suggests a large orbital component to the Fe local moment. We apply density functional theory based methods to obtain a microscopic picture of the competing interactions and the critical role of spin-orbit coupling (SOC) in this honeycomb lattice system. The low-temperature ferromagnetic phase displays a half-semimetallic Dirac point pinning the Fermi level and preventing gap opening before consideration of SOC, presenting a case in which correlation effects modeled by a repulsive Hubbard $U$ fail to open a gap. Simultaneous inclusion of both correlation and SOC drives a large orbital moment in excess of 0.7 $μ_B$ (essentially $L$ = 1) for spin aligned along the $\hat{c}$ axis, with a gap comparable with the inferred experimental value. The large orbital moment accounts for the large Ising anisotropy, in spite of the small magnitude of the SOC strength on the 3$d$ (Fe) ion. Ultimately, the Mott-Hubbard gap is enabled by degeneracy lifting by SOC and the large Fe moments, rather than by standard Hubbard interactions alone. We suggest that competing orbital occupations are responsible for the structural transitions involved in the observed re-entrant rhombohedral-triclinic-rhombohedral sequence.

cond-mat.str-el↗

arXiv2014

Unquenched $e_g^1$ orbital moment in the Mott insulating antiferromagnet KOsO4

Applying the correlated electronic structure method based on density functional theory plus the Hubbard $U$ interaction, we have investigated the tetragonal scheelite structure Mott insulator KOsO$_4$, whose $e_g^1$ configuration should be affected only slightly by spin-orbit couping (SOC). The method reproduces the observed antiferromagnetic Mott insulating state, populating the Os $d_{z^2}$ majority orbital. The quarter-filled $e_g$ manifold is characterized by a symmetry breaking due to the tetragonal structure, and the Os ion shows a crystal field splitting $Δ_{cf}$ = 1.7 eV from the $t_{2g}$ complex, which is relatively small considering the high formal oxidation state Os$^{7+}$. The small magnetocrystalline anisotropy before including correlation (i.e., in the metallic state) is increased by more than an order of magnitude in the Mott-insulating state, a result of a strong interplay between large SOC and a strong correlation. In contrast to conventional wisdom that the $e_g$ complex will not support orbital magnetism, we find that for the easy axis [100] direction the substantial Os orbital moment $M_L\approx-0.2 μ_B$ compensates half of the Os spin moment $M_S$ = 0.4$μ_B$. The origin of the orbital moment is analyzed and understood in terms of additional spin-orbital lowering of symmetry, and beyond that due to structural distortion, for magnetization along [100]. Further interpretation is assisted by analysis of the spin density and the Wannier function with SOC included.

cond-mat.str-el↗

arXiv2014

Strain and Spin-Orbit Coupling Induced Orbital-Ordering in Mott Insulator BaCrO3

Using ab initio calculations, we have investigated an insulating tetragonally distorted perovskite BaCrO$_3$ with a formal $3d^2$ configuration, the volume of which is apparently substantially enhanced by a strain due to SrTiO$_3$ substrate. Inclusion of both correlation and spin-orbit coupling (SOC) effects leads to a metal-insulator transition and in-plane zigzag orbital-ordering (OO) of alternating singly filled $d_{xz}+id_{yz}$ and $d_{xz}-id_{yz}$ orbitals, which results in a large orbital moment $M_L$ ~ -0.78 $μ_B$ antialigned to the spin moment $M_S$ ~ $2|M_L|$ in Cr ions. Remarkably, this ordering also induces a considerable $M_L$ for apical oxygens. Our findings show metal-insulator and OO transitions, driven by an interplay among strain, correlation, and SOC, which is uncommon in 3d systems.

cond-mat.str-el↗

arXiv2014

Electronic Structures, Magnetism, and Phonon Spectra in the Metallic Cubic Perovskite BaOsO3

Using ab initio calculations, we have investigated a cubic perovskite BaOsO3 and a few related compounds that have been synthesized recently and formally have a metallic d^4 configuration. In BaOsO3, which shows obvious 3-dimensional fermiology, a nonmagnetism is induced by a large spin-orbit coupling (SOC), which is precisely equal to an exchange splitting ~0.4 eV of the $t_{2g}$ manifold. However, the inclusion of on-site Coulomb repulsion as small as $U^c$~1.2 eV, only 1/3 of the $t_{2g}$ bandwidth, leads to the emergence of a spin-ordered moment, indicating that this system is on the verge of magnetism. In contrast to BaOsO3, our calculations suggest that the ground state of an orthorhombic CaOsO3 is a magnetically ordered state due to the reduction of the strength of SOC (about a half of that of BaOsO3) driven by the structure distortion, although the magnetization energy is only a few tenths of meV. Furthermore, in the cubic BaOsO3 and BaRuO3, our full-phonon calculations show several unstable modes, requiring further research.

cond-mat.str-el↗

arXiv2013

Effects of Magnetovolume and Spin-orbit Coupling in the Ferromagnetic Cubic Perovskite BaRuO3

BaRuO3 having five different crystal structures has been synthesized by varying the pressure while sintering. Contrary to the other phases being nonmagnetic, the cubic perovskite phase synthesized recently shows an itinerant ferromagnetic character. We investigated this ferromagnetic BaRuO3 using first principles calculations. A few van Hove singularities appear around the Fermi energy, causing unusually high magnetovolume effects of $ΔM/Δa$ ~ 4.3 $μ_B$/Åas well as a Stoner instability [IN(0) ~ 1.2]. At the optimized lattice parameter a, the magnetic moment M is 1.01 $μ_B$ in the local spin density approximation. When spin-orbit coupling is included, the topologies of some Fermi surfaces are altered, and the net moment is reduced by 10% to a value very close to the experimentally observed value of ~ 0.8 $μ_B$. Our results indicate that this ferromagnetism is induced by the Stoner instability, but the combined effects of the p-d hybridization, the magnetovolume, and the spin-orbit coupling determine the net moment. In addition, we briefly discuss the results of the tight-binding Wannier function technique.

cond-mat.mtrl-sci↗

arXiv2013

Electronic Structures and Phonon Spectra in Boronitride Superconductors LaMBN (M= Ni, Pt)

We have investigated electronic structures and phonon spectra of newly discovered isostructural superconductors LaNiBN (T_c = 4.1 K) and LaPtBN (T_c = 6.7 K). We have found that their electronic structures are substantially three-dimensional, leading to metallicity both in NiB (PtB) and the intervening LaN layers. Our ab initio phonon calculations show that almost all phonon modes contribute to the electron-phonon coupling (EPC) mechanism, reflecting that both layers are involved in the superconductivity. For LaNiBN, we obtain an EPC strength of λ= 0.52 and a logarithmically averaged characteristic phonon frequency of ω_{log} = 376 K, leading to T_c = 3.9 K. Compared with the Ni B_{1g} mode in LaNiBN, the Pt B_{1g} mode in LaPtBN is reduced by ~70%, leading to a slightly enhanced λ= 0.56 and an ~20 % reduced ω_{log}. The estimated T_c is 5.4 K for LaPtBN, in good agreement with the experiment. We do not find any indication of magnetic instability for either LaNiBN or LaPtBN, which implies that both systems are EPC mediated superconductors. Further, we have found an interesting trend of monotonic increase of T_c with respect to the boron height in the NiB (PtB) layer of both borocarbide and boronitride superconductors, which suggests a possible way to enhance T_c in these systems.

cond-mat.supr-con↗

arXiv2013

Structural and Correlation Effects in the Itinerant Insulating Antiferromagnetic Perovskite NaOsO3

The orthorhombic perovskite NaOsO3 undergoes a continuous metal-insulator transition (MIT), accompanied by antiferromagnetic (AFM) order at T_N=410 K, suggested to be an example of the rare Slater (itinerant) MIT. We study this system using ab initio and related methods, focusing on the origin and nature of magnetic ordering and the MIT. The rotation and tilting of OsO6 octahedra in the GdFeO3 structure result in moderate narrowing the band width of the t_{2g} manifold, but sufficient to induce flattening of bands and AFM order within the local spin density approximation (LSDA), where it remains metallic but with a deep pseudogap. Including on-site Coulomb repulsion U, at U_c ~2 eV a MIT occurs only in the AFM state. Effects of spin-orbit coupling (SOC) on the band structure seem minor as expected for a half-filled $t_{2g}^{3}$ shell, but SOC doubles the critical value U_c necessary to open a gap and also leads to large magnetocrystalline energy differences in spite of normal orbital moments no greater than 0.1$μ_B$. Our results are consistent with a Slater MIT driven by magnetic order, induced by a combination of structurally-induced band narrowing and moderate Coulomb repulsion, with SOC necessary for a full picture. Strong p-d hybridization reduces the moment, and when bootstrapped by the reduced Hund's rule coupling (proportional to the moment) gives a calculated moment of ~1 $μ_B$, consistent with the observed moment and only a third of the formal $d^3$ value. We raise and discuss one important question: since this AFM ordering is at q=0 (in the 20 atom cell) where nesting is a moot issue, what is the microscopic driving force for ordering and the accompanying MIT?

cond-mat.str-el↗

arXiv2012

Effects of metallic spacer in layered superconducting Sr2(Mg$_y$Ti$_{1-y}$)O3FeAs

The highly two-dimensional superconducting system Sr2(Mg$_y$Ti$_{1-y}$)O3FeAs, recently synthesized in the range of 0.2 < y < 0.5, shows an Mg concentration-dependent $T_c$. Reducing the Mg concentration from y=0.5 leads to a sudden increase in $T_c$, with a maximum $T_c$ ~40 K at y=0.2. Using first principles calculations, the unsynthesized stoichiometric y=0 and the substoichiometric y=0.5 compounds have been investigated. For the 50% Mg-doped phase (y=0.5), Sr2(Mg$_y$Ti$_{1-y}$)O3 layers are completely insulating spacers between FeAs layers, leading to the fermiology such as that found for other Fe pnictides. At y=0, representing a phase with metallic Sr2TiO3 layers, the $Γ$-centered Fe-derived Fermi surfaces (FSs) considerably shrink or disappear. Instead, three $Γ$-centered Ti FSs appear, and in particular two of them have similar size, like in MgB2. Interestingly, FSs have very low Fermi velocity in large fractions: the lowest being 0.6$\times10^6$ cm/s. Furthermore, our fixed spin moment calculations suggest the possibility of magnetic ordering, with magnetic Ti and nearly nonmagnetic Fe ions. These results indicate a crucial role of Sr2(Mg$_y$Ti$_{1-y}$)O3 layers in this superconductivity.

cond-mat.supr-con↗

arXiv2012

Evaluation of Half-metallic Antiferromagnetism in ${\cal A}_2$CrFeO$_6$ ({$\cal A$}=La, Sr

The nearly well-ordered double perovskite La$_2$CrFeO$_6$ has been synthesized recently. Contrary to previous theoretical predictions, but in agreement with experimental observations, our first principle calculations indicate an insulating ferrimagnet La$_2$CrFeO$_6$ with antialigned S=3/2 Cr$^{3+}$ and S=5/2 Fe$^{3+}$ ions,using the local spin density approximation (LSDA), a correlated band theory LDA+U, and a semilocal functional modified Becke-Johnson method. Additionally, we investigated the double perovskite Sr$_2$CrFeO$_6$, which is as yet unsynthesized. In LSDA calculations, this system shows formally tetravalent Cr and Fe ions both having antialigned $S$=1 moments, but is a simple metal. Once applying on-site Coulomb repulsion U on both Cr and Fe ions, this system becomes half-metallic and the moment of Fe is substantially reduced, resulting in zero net moment. These results are consistent with our fixed spin moment studies. Our results suggest a precisely compensated half-metallic Sr$_2$CrFeO$_6$.

cond-mat.mtrl-sci↗

arXiv2011

Half-metallic Ferrimagnetism Driven by Coulomb Enhanced Spin-Orbit Coupling in PdCrO3

Recently, in the seemingly narrow gap insulating NiCrO$_3$ with the trigonally distorted (R-3c) perovskite-like structure, a compensated half-metal (CHM) is predicted, as applying a modest pressure. Using ab initio calculations including both Coulomb correlations and spin-orbit coupling (SOC), we investigate the as-yet-unsynthesized PdCrO$_3$, isostructural and isovalent to NiCrO$_3$. Upon applying the on-site Coulomb repulsion $U$ to both Pd and Cr ions, the Cr spin moment is precisely compensated with the antialigned spin moments of Pd and oxygens. Coincidentally only one spin channel remains metallic due to the twice larger width of the Pd 4d bands than the Ni 3d bands in NiCrO$_3$, indicating CHM in ambient pressure. Inclusion of SOC as well as correlation effects (LDA+U+SOC) produces a SOC constant enhanced twice over the value of LDA+SOC, leading to unusually large orbital moment of --0.25 $μ_B$ on Pd. However, the half-metallicity still survives, so that a transition of CHM to a half-metallic ferrimagnet occurs due to Coulomb enhanced SOC. On the other hand, an isovalent, but presumed cubic double perovskite La$_2$PdCrO$_6$ is expected to be a half-metal ferromagnet with tiny orbital moments.

cond-mat.str-el↗

arXiv2011

Compensated Half-metallicity in the Trigonally Distorted Perovskite-type NiCrO$_3$

Using first principles calculations, we investigate the electronic and magnetic properties of the trigonally distorted (R-3c) perovskite-derived NiCrO$_3$. Within the local spin density approximation (LSDA), our calculations show that this system is an exactly compensated half-metal (CHM). The local spin moments of Cr 2.04, and antialigned Ni -1.41 and three oxygens -0.63 (in the units of $μ_B$), indicate high spin S=3/2 Cr$^{3+}$ and S=3/2 (NiO$_3$)$^{3-}$ units. Considering reasonable values of the on-site Coulomb repulsion U on both Ni and Cr ions with LDA+U approach, this system becomes an insulator (as reported by Chamberland and Cloud) having a narrow gap in the spin-up channel, whereas the other channel has a large gap of ~3 eV. Although inclusion of U seemingly leads to the transition Ni$^{2+} --> high spin S=3/2 Ni$^{3+}$, consistent with the experimentally observed effective moment, the zero net moment remains unchanged due to either reduction of oxygen local moments or enhancement of Cr local moment. Compression of volume by 10% leads to CHM even when correlation effects are included.These results suggest the possibility of a CHM state in NiCrO$_3$ and provide another route to search for CHM, which is a property sought by many.

cond-mat.mtrl-sci↗

arXiv2010

Sr$_2$VO$_3$FeAs: A Nanolayered Bimetallic Iron Pnictide Superconductor

One of the unifying concepts in the iron-pnictide superconductors, both for the mechanism of magnetic ordering and of unconventional order parameter character, has been the electron and hole Fermi surfaces that are approximately nested. Using the density functional methods that have predicted Fermi surfaces correctly in SrFe$_2$P$_2$, we find that the recently reported superconducting Sr$_2$VO$_3$FeAs, with $T_c$=37 K and no apparent competition between magnetism and superconductivity, possesses different Fermi surface geometry and character than previous classes of iron pnictides. The intervening layer (a V bilayer) gives rise to bands that cross the Fermi level. Coupling to the FeAs layer is small except for interaction along the zone boundary, however that coupling degrades the Fermi surface nesting. Sr$_2$VO$_3$FeAs, with its alternating layers of open shell atoms, deserves further close study that should help to understand the origin of the properties of iron pnictide compounds.

cond-mat.supr-con↗

arXiv2009

Orbital-ordering driven structural distortion in metallic SrCrO3

In contrast to the previous reports that the divalent perovskite SrCrO$_3$ was believed to be cubic structure and nonmagnetic metal, recent measurements suggest coexistence of majority tetragonally distorted weak antiferromagnetic phase and minority nonmagnetic cubic phase. Within the local (spin) density approximation (L(S)DA) our calculations confirm that a slightly tetragonally distorted phase indeed is energetically favored. Using the correlated band theory method (LDA+ Hubbard U) as seems to be justified by the unusual behavior observed in SrCrO$_3$, above the critical value $U_c$=4 eV only the distorted phase undergoes an orbital-ordering transition, resulting in $t_{2g}^2 --> d_{xy}^1$($d_{xz}d_{yz}$)$^1$ corresponding to the filling of the $d_{xy}$ orbital but leaving the other two degenerate. The Fermi surfaces of the cubic phase are simple with nesting features, although the nesting wavevectors do not correlate with known data. This is not uncommon in perovskites; the strongly directional d-d bonding often leads to boxlike Fermi surfaces, and either the nesting is not strong enough, or the matrix elements are not large enough, to promote instabilities. Fixed spin moment calculations indicate the cubic structure is just beyond a ferromagnetic Stoner instability (IN(0)~1.1) in L(S)DA, and that the energy is unusually weakly dependent on the moment out to 1.5$μ_B$/Cr (varying only by 11 meV/Cr), reflecting low energy long-wavelength magnetic fluctuations. We observe that this system shows strong magneto-phonon coupling (change in Cr local moment is ~7.3 $μ_B$/Å) for breathing phonon modes.

cond-mat.str-el↗

arXiv2008

Magnetism Driven by Anion Vacancies in Superconducting $α$--FeSe$_{1-x}$

To study the microscopic electronic and magnetic interactions in the substoichiometric iron chalcogenide FeSe$_{1-x}$ which is observed to superconduct at x~1/8 up to $T_c$=27 K, we use first principles methods to study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a ferrimagnetic cluster of eight Fe atoms, which for the ordered x=1/8 alloy leads to half metallic conduction. Similar magnetic clusters are obtained for FeTe$_{1-x}$ and for BaFe$_2$As$_2$ with an As vacancy, although neither of these are half metallic. Based on fixed spin density results, we suggest the low energy excitations in FeSe$_{1-x}$ are antiparamagnon-like with short correlation length.

cond-mat.supr-con↗

arXiv2007

Half Semimetallic Antiferromagnetism in the Sr$_2$CrTO$_6$ System, T=Os, Ru

Double perovskite Sr$_2$CrOsO$_6$ is (or is very close to) a realization of a spin-asymmetric semimetallic compensated ferrimagnet, according to first principles calculations. This type of near-half metallic antiferromagnet is an unusual occurrence, and more so in this compound because the zero gap is accidental rather than being symmetry determined. The large spin-orbit coupling (SOC) of osmium upsets the spin balance (no net spin moment without SOC): it reduces the Os spin moment by 0.27 $μ_B$ and induces an Os orbital moment of 0.17 $μ_B$ in the opposite direction. The effects combine (with small oxygen contributions) to give a net total moment of 0.54 $μ_B$ per cell in \scoo, reflecting a large impact of SOC in this compound. This value is in moderately good agreement with the measured saturation moment of 0.75 $μ_B$. The value of the net moment on the Os ion obtained from neutron diffraction (0.73 $μ_B$ at low temperature) differs from the calculated value (1.14 $μ_B$). Rather surprisingly, in isovalent Sr$_2$CrRuO$_6$ the smaller SOC-induced spin changes and orbital moments (mostly on Ru) almost exactly cancel. This makes Sr$_2$CrRuO$_6$ a "half (semi)metallic antiferromagnet" (practically vanishing net total moment) even when SOC is included, with the metallic channel being a small-band-overlap semimetal. Fixed spin moment (FSM) calculations are presented for each compound, illustrating how they provide different information than in the case of a nonmagnetic material. These FSM results indicate that the Cr moment is an order of magnitude stiffer against longitudinal fluctuations than is the Os moment.

cond-mat.mtrl-sci↗

arXiv2007

Orbital-quenching-induced magnetism in Ba_2NaOsO_6

The double perovskite \bnoo with heptavalent Os ($d^1$) is observed to remain in the ideal cubic structure ({\it i.e.} without orbital ordering) despite single occupation of the $t_{2g}$ orbitals, even in the ferromagnetically ordered phase below 6.8 K. Analysis based on the {\it ab initio} dispersion expressed in terms of an Os $t_{2g}$-based Wannier function picture, spin-orbit coupling, Hund's coupling, and strong Coulomb repulsion shows that the magnetic OsO$_6$ cluster is near a moment-less condition due to spin and orbital compensation. Quenching (hybridization) then drives the emergence of the small moment. This compensation, unprecedented in transition metals, arises in a unified picture that accounts for the observed Mott insulating behavior.

cond-mat.str-el↗

arXiv2007

Correlation Effects in the Triangular Lattice Single-band System Li_xNbO_2

Superconductivity in hole-doped Li_xNbO_2 has been reported with T_c ~ 5 K in the range 0.45 < x < 0.8. The electronic structure is based on a two-dimensional triangular Nb lattice. The strong trigonal crystal field results in a single Nb d(z^2) band isolated within a wide gap, leading to a single-band triangular lattice system. The isolated, partially filled band has a width W=1.5 eV with dominant second neighbor hopping. To identify possible correlation effects, we apply DMFT(QMC) using on-site Coulomb repulsion U=0-3 eV, and check selected results using determinant QMC. For U as small as 1 eV, the single particle spectrum displays a robust lower Hubbard band, suggesting the importance of correlation effects in Li_xNbO_2 even for U < W. At half-filling (x=0), a Mott transition occurs at U_c ~ 1.5 eV. Coupling between the O A_g phonon displacement and correlation effects is assessed.

cond-mat.supr-con↗

arXiv2007

Chemical Differences between K and Na in Alkali Cobaltates

K$_x$CoO$_2$ shares many similarities with Na$_x$CoO$_2$, as well as some important differences (no hydration-induced superconductivity has been reported). At $T_{c2}$=20 K, K$_{0.5}$CoO$_2$ becomes an insulator with a tiny optical gap as happens in Na$_{0.5}$CoO$_2$ at 52 K. This similarity, with a known common structure, enables direct comparisons to be made. Using the K-zigzag structure recently reported and the local density approximation, we compare and contrast these cobaltates at x=0.5. Although the electronic structures are quite similar as expected, substantial differences are observed near the Fermi level. These differences are found to be attributable mostly to the chemical, rather than structural difference: although Na is normally considered to be fully ion, K has somewhat more highly ionic character than does Na in these cobaltates.

cond-mat.str-el↗