arXiv subjects

Hae-Young Kee

Publications and source records attributed to Hae-Young Kee.

At least 127 records · Page 7Linked to original sources

arXiv2014

Engineering spin-orbital magnetic insulator by tailoring superlattices

Novel interplay of spin-orbit coupling and electron correlations in complex Ir oxides recently emerged as a new paradigm for correlated electron physics. Because of a large spin-orbit coupling of ~0.5 eV, which is comparable to the transfer energy t and the crystal field splitting $Δ$ and Coulomb U, a variety of ground states including magnetic insulator, band insulator, semimetal and metal, shows up in a narrow materials phase space. Utilizing such subtle competition of the ground states, we successfully tailor a spin-orbital magnetic insulator out of a semimetal SrIrO$_3$ by controlling dimensionality using superlattice of [(SrIrO$_3$)$_m$, SrTiO$_3$] and show that a magnetic ordering triggers the transition to magnetic insulator. Those results can be described well by a first-principles calculation. This study is an important step towards the design and the realization of topological phases in complex Ir oxides with very strong spin-orbit coupling.

cond-mat.str-el↗

arXiv2013

Cooper-pair based photon entanglement without isolated emitters

We propose a novel approach for efficient generation of entangled photons, based on Cooper-pair luminescence in semiconductors, which does not require isolated emitters such as single atoms or quantum dots. We show that in bulk materials, electron-spin entanglement in Cooper pairs should not be expected to be translated into pure entangled photons despite the selection rules, due to mixing introduced by light-hole heavy-hole degeneracy. Semiconductor quantum wells, however, remove this degeneracy, allowing efficient photon entanglement generation in simple electrically-driven structures, taking advantage of the superconducting macroscopic coherence. The second-order decay of two-electron states in Cooper-pair luminescence leaves no which-path information, resulting in perfect coherence between two pathways and hence in principle, perfect entanglement. We calculate the purity of the entangled-photon state and find that it increases for larger light-hole heavy-hole energy splitting and for lower temperatures. These results provide new insights into light-matter interaction in solids and enable realization of novel quantum photonics based on matter condensates.

cond-mat.mes-hall↗

arXiv2013

Releasing half-quantum vortices via the coupling of spin polarization, charge- and spin-current

Motivated by recent experiments observing half-integer flux quanta in mesoscopic loops of superconducting Sr2RuO4, a theory is developed based on a "d-soliton", a topological defect of spin-triplet superconductors involving a change of the spin configuration. A phenomenological Ginzburg-Landau free energy is given which allows to describe such a soliton yielding a half-integer flux quantum inside a loop, assuming a chiral p-wave superconducting phase. The d-soliton couples to spin polarization perpendicular to the loop axis, which allows to reduce the energy of the d-soliton, fitting well to the experimental findings. The origin of this coupling lies in the combination of spin and charge current within the d-soliton generating a spin polarization analogous to a spin Hall effect. The effect of Fermi liquid corrections is discussed, in particular, in view of the spatial extension of the d-soliton and the energetics in comparison with a standard integer flux state.

cond-mat.supr-con↗

arXiv2013

Symmetry breaking via Kondo hybridization: Chiral nematic metal in Pr2Ir2O7

In frustrated magnets when magnetic ordering is suppressed down to low temperature, the formation of a quantum spin liquid becomes a possibility. How such a spin liquid manifests in the presence of conduction electrons is a question with potentially rich physical consequences, particularly when both the localized spins and conduction electrons reside on frustrated lattices. We propose a novel mechanism for symmetry breaking in systems where conduction electrons hybridize with a quantum spin liquid through Kondo couplings. We apply this to the pyrochlore iridate Pr2Ir2O7, which exhibits an anomalous Hall effect without clear indications of magnetic order. We show that Kondo hybridization between the localized Pr pseudo-spins and Ir conduction electrons breaks some of the spatial symmetries, in addition to time-reversal regardless of the form ofthe coupling. These broken symmetries result in an anomalous Hall conductivity and induce small magnetic, quadrupolar and charge orderings. Further experimental signatures are proposed.

cond-mat.str-el↗

arXiv2013

Theory of magnetism and metal-insulator transition in layered perovskite iridates

We investigate the metal-insulator transition in the layered Ruddelsden Popper series of strontium iridates Srn+1IrnO3n+1. Tight-binding models of t2g orbitals for n = 1, 2, and infinity are constructed, and changes in band dispersion due to dimensionality and spin-orbit coupling are presented. Identifying the states near the Fermi level to be predominantly Jeff = 1/2, we use an effective Hubbard model to study the effect of correlations. Transitions from a metallic state to various magnetically ordered states at different critical interactions are obtained. A canted antiferromagnetic insulator is found for Sr2IrO4, a c-axis collinear antiferromagnetic insulator for Sr3Ir2O7, and non-coplanar canted antiferromagnetic insulator via magnetic metal for SrIrO3. We derive the strong-coupling spin-model and compare the magnetic ordering patterns obtained in the weak and strong coupling limits. We find that they are identical, indicating that magnetic ordering is not sufficient to justify Mott physics in this series of iridates.

cond-mat.str-el↗

arXiv2013

Transport signatures of spatially modulated electronic nematic phases

Electronic nematic phases are broadly characterized by spontaneously broken rotational symmetry. Although they have been widely recognized in the context of high temperature cuprates, bilayer ruthenates, and iron-based superconductors, the focus so far has been exclusively on the uniform nematic phase. Recently, however, it was proposed that on a square lattice a nematic instability in the d-wave charge channel could lead to a spatially modulated nematic state, where the modulation vector q is determined by the relative location of the Fermi level to the van Hove singularity. Interestingly, this finite-q nematic phase has also been identified as an additional leading instability that is as strong as the superconducting instability near the onset of spin density wave order. Here we study the electrical conductivity tensor in the modulated nematic phase for a general modulation vector. Our results can be used to identify modulated nematic phases in correlated materials.

cond-mat.str-el↗

arXiv2012

Microscopic theory of magnetism in Sr3Ir2O7

An intriguing idea of spin-orbit Mott insulator has been proposed to explain magnetic insulating behavior in various iridates. This scenario relies on the strength of the spin-orbit coupling being comparable to electronic correlations, and it is not a priori obvious whether this picture is valid for all iridates. In particular, Sr3Ir2O7 exhibits a small charge gap and magnetic moment compared to Sr2IrO4, questioning the validity of such hypothesis. To understand the microscopic mechanism for magnetism in Sr3Ir2O7, we construct a tight binding model taking into account the full t2g- orbitals, the staggered rotation of the local octahedra, and the bilayer structure. The bands near the Fermi level are mainly characterized by the total angular momentum Jeff = 1/2, except below the Γ point, supporting a reasonably strong spin-orbit coupling picture. A first order transition to a collinear antiferromagnet via multi-orbital Hubbard interactions is found within the mean field approximation. The magnetic moment jump at the transition is consistently smaller than Sr2IrO4, originated from the underlying band structure of a barely band insulator. Given the small charge gap and moment observed in Sr3Ir2O7, the system is close to a magnetic transition. A comparison to a spin-model is presented and connection to the Mott insulator is also discussed.

cond-mat.str-el↗

arXiv2012

Interplay between spin-orbit coupling and Hubbard interaction in SrIrO3 and related Pbnm perovskites

There has been a rapidly growing interest on the interplay between spin-orbit coupling (SOC) and Hubbard interaction U in correlated materials. A current consensus is that the stronger the SOC, the smaller is the critical interaction Uc required for a spin-orbit Mott insulator, because the atomic SOC splits a band into different total angular momentum bands narrowing the effective bandwidth. It was further claimed that at large enough SOC, the stronger the SOC, the weaker the Uc because in general the effective SOC is enhanced with increasing electron-electron interaction strength. Contrary to this expectation, we find that, in orthorhombic perovskite oxides (Pbnm), the stronger the SOC, the bigger the Uc. This is originated from a line of Dirac node in Jeff=1/2 bands near the Fermi level inherited from a combination of the lattice structure and a large SOC. Due to this protected line of nodes, there are small hole and electron pockets in SrIrO3, and such a small density of states makes Hubbard interaction less efficient in building a magnetic insulator. The full phase diagram in U vs. SOC is obtained, where non-magnetic semimetal, magnetic metal, and magnetic insulator are found. Magnetic ordering patterns beyond Uc are also presented. We further discuss implications of our finding in relation to other perovskites such as SrRhO3 and SrRuO3.

cond-mat.str-el↗

arXiv2012

Hidden and antiferromagnetic order as a rank-5 superspin in URu2Si2

We propose a candidate for the hidden order in URu2Si2: a rank-5 E type spin density wave between Uranium 5f crystal field doublets breaking time reversal and lattice tetragonal symmetry in a manner consistent with recent torque measurements [R. Okazaki et al, Science 331, 439 (2011)]. We argue that coupling of this order parameter to magnetic probes can be hidden by crystal field effects, while still having significant effects on transport, thermodynamics and magnetic susceptibilities. In a simple tight-binding model for the heavy quasiparticles, we show the connection between the hidden order and antiferromagnetic phases arises since they form different components of this single rank-5 pseudo-spin vector. Using a phenomenological theory, we show the experimental pressure-temperature phase diagram can be qualitatively reproduced by tuning terms which break pseudo-spin rotational symmetry. As a test of our proposal, we predict the presence of small magnetic moments in the basal plane oriented in the [110] direction ordered at the wave-vector (0,0,1).

cond-mat.str-el↗

arXiv2012

Unveiling a nematic quantum critical point in multi-orbital systems

Electronic nematicity, proposed to exist in a number of transition metal materials, can have different microscopic origins. In particular, the anisotropic resistivity and meta-magnetic jumps observed in Sr3Ru2O7 are consistent with an earlier proposal that the isotropic-nematic transition is generically first order and accompanied by meta-magnetism when tuned by a magnetic field. However, additional striking experimental features such as a non-Fermi liquid resistivity and critical thermodynamic behavior imply the presence of an unidentified quantum critical point (QCP). Here we show that orbital degrees of freedom play an essential role in revealing a nematic QCP, even though it is overshadowed by a nearby meta-nematic transition at low temperature. We further present a finite temperature phase diagram including the entropy landscape and discuss our findings in light of the phenomena observed in Sr3Ru2O7.

cond-mat.str-el↗

arXiv2012

Identifying spin-triplet pairing in spin-orbit coupled multi-band superconductors

We investigate the combined effect of Hund's and spin-orbit (SO) coupling on superconductivity in multi-orbital systems. Hund's interaction leads to orbital-singlet spin-triplet superconductivity, where the Cooper pair wave function is antisymmetric under the exchange of two orbitals. We identify three d-vectors describing even-parity orbital-singlet spin-triplet pairings among t2g-orbitals, and find that the three d-vectors are mutually orthogonal to each other. SO coupling further assists pair formation, pins the orientation of the d-vector triad, and induces spin-singlet pairings with a relative phase difference of π/2. In the band basis the pseudospin d-vectors are aligned along the z-axis and correspond to momentum-dependent inter- and intra-band pairings. We discuss quasiparticle dispersion, magnetic response, collective modes, and experimental consequences in light of the superconductor Sr2RuO4.

cond-mat.supr-con↗

arXiv2012

Semimetal and Topological Insulator in Perovskite Iridates

The two-dimensional layered perovskite Sr2IrO4 was proposed to be a spin-orbit Mott insulator, where the effect of Hubbard interaction is amplified on a narrow J_{eff} = 1/2 band due to strong spin-orbit coupling. On the other hand, the three-dimensional orthorhombic perovskite (Pbnm) SrIrO3 remains metallic. To understand the physical origin of the metallic state and possible transitions to insulating phases, we construct a tight-binding model for SrIrO3. The band structure possesses a line node made of J_{eff} = 1/2 bands below the Fermi level. As a consequence, instability towards magnetic ordering is suppressed and the system remains metallic. This line node, originating from the underlying crystal structure, turns into a pair of three-dimensional nodal points on the introduction of a staggered potential or spin-orbit coupling strength between alternating layers. Increasing this potential beyond a critical strength induces a transition to a strong topological insulator, followed by another transition to a normal band insulator. We propose that materials constructed with alternating Ir- and Rh-oxide layers along the (001) direction, such as Sr2IrRhO6, are candidates for a strong topological insulator.

cond-mat.str-el↗

arXiv2011

Hidden spin liquid in an antiferromagnet: Applications to FeCrAs

The recently studied material FeCrAs exhibits a surprising combination of experimental signatures, with metallic, Fermi liquid like specific heat but resistivity showing strong non-metallic character. The Cr sublattice posseses local magnetic moments, in the form of stacked (distorted) Kagome lattices. Despite the high degree of magnetic frustration, anti-ferromagnetic order develops below ~125K suggesting the non-magnetic Fe sublattice may play a role in stabilizing the ordering. From the material properties we propose a microscopic Hamiltonian for the low energy degrees of freedom, including the non-magnetic Fe sublattice, and study its properties using slave-rotor mean field theory. Using this approach we find a spin liquid phase on the Fe sublattice, which survives even in the presence of the magnetic Cr sublattice. Finally, we suggest that the features of FeCrAs can be qualitatively explained by critical fluctuations in the non-magnetic sublattice Fe due to proximity to a metal-insulator transition.

cond-mat.str-el↗

arXiv2011

Dimensional crossover of a frustrated distorted kagome Heisenberg Model: Application to FeCrAs

Motivated by recent experimental work on FeCrAs, we study the magnetic properties of classical spin models on the hexagonal Fe$_2$P/ZrNiAl structure (space group $P{\bar{6}}2m$). When both transition metal/rare earth sites carry magnetic moments, one has alternating distorted kagome and triangular lattice layers. Each point of the triangular lattice lies at the centre of a distorted hexagon and actually corresponds to three isolated magnetic atoms: a "trimer". We show that a simple model consisting of antiferromagnetically correlated Heisenberg spins on the frustrated distorted kagome lattice coupling either ferromagnetically or antiferromagnetically to Heisenberg spins at the average position of a trimer, leads to a rich phase diagram, including the possible selection of a $(1/3,1/3,0)$ magnetic order as may have been seen in FeCrAs.

cond-mat.str-el↗

arXiv2010

Emergence of superconductivity, valence bond order and Mott insulators in Pd[(dmit)2] based organic salts

The $\EtMeP$ and $\EtMeSb$ nearly triangular organic salts are distinguished from most other Pd[(dmit)$_2$] based salts, as they display valence bond and no long range order, respectively. Under pressure, a superconducting phase is revealed in EtMe$_3$P near the boundary of valence bond order. We use slave-rotor theory with an enlarged unit cell to study competition between uniform and broken translational symmetry states, offering a theoretical framework capturing the superconducting, valence bond order, spin liquid, and metallic phases on an isotropic triangular lattice. Our finite temperature phase diagram manifests a remarkable resemblance to the phase diagram of the EtMe$_3$P salt, where the re-entrant transitions of the type insulator-metal-insulator can be explained by an entropy difference between metal and the U(1) spin liquid. We find that the superconducting pairing symmetry is $d \pm id$, and predict different temperature dependences of the specific heat between the spin liquid and metal.

cond-mat.str-el↗

arXiv2010

Searching for topological density wave insulators in multi-orbital square lattice systems

We study topological properties of density wave states with broken translational symmetry in two-dimensional multi-orbital systems with a particular focus on t$_{2g}$ orbitals in square lattice. Due to distinct symmetry properties of d-orbitals, a nodal charge or spin density wave state with Dirac points protected by lattice symmetries can be achieved. When an additional order parameter with opposite reflection symmetry is introduced to a nodal density wave state, the system can be fully gapped leading to a band insulator. Among those, topological density wave (TDW) insulators can be realized, when an effective staggered on-site potential generates a gap to a pair of Dirac points connected by the inversion symmetry which have the same topological winding numbers. We also present a mean-field phase diagram for various density wave states, and discuss experimental implications of our results.

cond-mat.str-el↗

arXiv2010

Microscopic Route to Nematicity in Sr3Ru2O7

An anisotropic metallic phase dubbed electronic nematic phase bounded by two consecutive metamagnetic transitions has been reported in the bilayer ruthenate Sr3Ru2O7. It has also been shown that the nematic and the accompanying metamagnetic transitions are driven by an effective momentum-dependent quadrupole-type interaction. Here, we study the microscopic origin of such an effective interaction. To elucidate the mechanism behind the spontaneous Fermi surface distortion associated with the nematic, we identify a simple tight binding model based on t2g orbitals, spin-orbit coupling and the rotation of RuO6 octahedra as starting point, consistent with the Fermi surface obtained from recent angle-resolved photoemission data. Within an extended Hubbard model the nematic state, characterized by an anisotropy between the bands near $(\pm π,0)$ and $(0,\pm π)$, then strongly competes with ferromagnetic order but pre-empts it via a finite nearest neighbor interaction. We discuss experimental means to confirm our proposal.

cond-mat.str-el↗

arXiv2010

Magnetic breakdown in ortho-II high-temperature cuprates

The recent confirmation of closed Fermi surface in the high temperature cuprates via de Haas-van Alphen and Shubnikov-de haas oscillations calls for a theoretical investigation of the origin of such oscillations. We study de Haas-van Alphen oscillations and magnetic breakdown in the context of ortho-II high temperature cuprates to understand the origin of the multiple frequencies. We find that the magnetic breakdown is highly sensitive not only to the ortho-II potential, but also to the Fermi surface topology, and is thus useful to distinguish between various theoretical proposals related to quantum oscillations.

cond-mat.supr-con↗