arXiv subjects

Hae-Young Kee

Publications and source records attributed to Hae-Young Kee.

At least 145 records · Page 8Linked to original sources

arXiv2010

Role of electronic nematicity in the interplay between s- and d-wave broken-symmetry states

To understand the role of electronic nematic order in the interplay between s- and d-wave particle-particle or particle-hole condensate states, relations between various s- and d-wave order parameters are studied. We find that the nematic operator transforms two independent six-dimensional vectors. The d-wave superconducting, d-density wave, and antiferromagnetic orders are organized into one vector, and the s-wave superconducting, charge density wave, and spin-triplet d-density wave orders into the other vector. Each vector acts as a superspin and transforms under the action of SO(6) where charge, spin, eta- and pi-pairing, spin-triplet nematic operators satisfy the SO(6) Lie algebra. Electronic nematic order is not a part of the SO(6) group. It commutes with all 15 generators. Our findings imply that nematic order does not affect the competition among the order parameters within the same superspin, while it strongly interferes the interplay between two order parameters that belong to different superspins. For example, nematicity allows a linear coupling between d- and s-wave superconducting order parameters which modifies the superconducting transition temperature. A generalized Ginzburg-Landau theory and further physical implications are discussed.

cond-mat.str-el↗

arXiv2009

Nematicity as a route to a magnetic field-induced spin density wave order; application to the high temperature cuprates

The electronic nematic order characterized by broken rotational symmetry has been suggested to play an important role in the phase diagram of the high temperature cuprates. We study the interplay between the electronic nematic order and a spin density wave order in the presence of a magnetic field. We show that a cooperation of the nematicity and the magnetic field induces a finite coupling between the spin density wave and spin-triplet staggered flux orders. As a consequence of such a coupling, the magnon gap decreases as the magnetic field increases, and it eventually condenses beyond a critical magnetic field leading to a field-induced spin density wave order. Both commensurate and incommensurate orders are studied, and the experimental implications of our findings are discussed.

cond-mat.str-el↗

arXiv2009

Collective modes and emergent SO(6) symmetry in the iron pnictides

We show the existence of an emergent SO(6) symmetry in the low energy description of the iron pnictides. This approximate symmetry provides a unifying framework for the occurrence of spin density wave (SDW) and superconductivity (SC) in these materials. We use this symmetry to make several predictions for future experiments, including the topology of the phase diagram and the presence of various resonant modes in neutron scattering experiments in both the SC and SDW phases. We also predict the existence of a new "Orbital Density Wave" state, which competes with both SDW and SC orders.

cond-mat.supr-con↗

arXiv2008

Quantum oscillations of ortho-II YBa_2Cu_3O_6.5

Motivated by quantum oscillations observed in highly ordered ortho-II YBa_2Cu_3O_6.5, we study the Fermi surface topology of d-density wave (ddw) or antiferromagnetic (AF) ordering in the presence of the ortho-II potential. We find that the electron pocket is unaffected by the presence of the ortho-II potential. This further strengthens the proposal that quantum oscillations observed in ortho-II-free YBa_2Cu_4O_8 arise from an electron pocket. On the other hand, the hole pocket topology is sensitive to the ortho-II potential. We show there exist generically three distinct quantum oscillations associated with one electron-like and two hole-like Fermi pockets. We compare our results to the quantum oscillations observed in experiments. We discuss possible ways to distinguish between ddw and antiferromagnetic orders in a potential single-layer ortho-II material.

cond-mat.supr-con↗

arXiv2008

Origin and consequence of an unpinned helical magnet: application to partial order in MnSi under pressure

We study a classical ferromagnetic Heisenberg model in the presence of Dzyaloshinskii-Moriya interactions on the corner-shared triangle lattice formed by the Mn sites of MnSi. We show that a sizable spin helicity can be obtained only when the microscopic Moriya vectors lie parallel to the Mn-Mn bonds. Further, such vectors are shown to produce an unpinned helical order characterized by a particular ordering wavevector magnitude but unpinned direction, dubbed partial order, at physically realizable temperatures. A consequence of such an unpinned helical ordering is that the neutron scattering intensity is sharply peaked at this wavevector magnitude. The surface formed by connecting these wavevectors is a sphere, around which the neutron scattering weight is spread. We further show that the observed neutron scattering intensity can be anisotropic along this surface and that this anisotropy is dependent on the experimentalist's choice of lattice Bragg peak through a geometric factor. A neutron scattering measurement near the Bragg point ($\frac{2π}{a}$,$\frac{2π}{a}$,0) naturally leads to a highest intensity along the (1,1,0) direction consistent with the observed anisotropy in MnSi [Pfleiderer {\it{et al.}} Nature {\bf{427}} 227 (2004)]. A possible mechanism for pinning the helical order, and a way to distinguish an ordered and a partially ordered state in the context of neutron scattering are discussed.

cond-mat.str-el↗

arXiv2008

Theory of spin nematic to spin-Peierls quantum phase transition in ultracold spin-1 atoms in optical lattices

We present a theory of the anisotropy tuned quantum phase transition between spin nematic and spin-Peierls phases in S=1 systems with significant bi-quadratic exchange interactions. Based on quantum Monte Carlo studies on finite size systems, [K. Harada et al., J. Phys. Soc. Jpn. 76, 013703 (2007)] it has been proposed that this phase transition is second order with new deconfined fractional excitations that are absent in either of the two phases [T. Grover and T. Senthil, Phys. Rev. Lett. 98, 247202 (2007)]. The possibility of a weak first order transition, however, cannot be ruled out. To elucidate the nature of the transition, we construct a large-N SO(3N) model for this phase transition and find in the $N\to\infty$ limit that the transition is generically of first order. Furthermore, we find a critical point in the 1D limit, where two transition lines, separating spin nematic, ferromagnetic, and spin-Peierls phases, meet. Our study indicates that the spin nematic phase is absent in 1D, while its correlation length diverges at the critical point. Predictions for $^{23}$Na atoms trapped in an optical lattice, where this quantum phase transition naturally arises, are discussed.

cond-mat.str-el↗

arXiv2008

Meta-nematic transitions in a bilayer system: Application to the bilayer ruthenate

It was suggested that the two consecutive metamagnetic transitions and the large residual resistivity discovered in Sr$_3$Ru$_2$O$_7$ can be understood via the nematic order and its domains in a single layer system. However, a recently reported anisotropy between two longitudinal resistivities induced by tilting the magnetic field away from the c-axis cannot be explained within the single layer nematic picture. To fill the gap in our understanding within the nematic order scenario, we investigate the effects of bilayer coupling and in-plane magnetic field on the electronic nematic phases in a bilayer system. We propose that the in-plane magnetic field in the bilayer system modifies the energetics of the domain formation, since it breaks the degeneracy of two different nematic orientations. Thus the system reveals a pure nematic phase with a resistivity anisotropy in the presence of an in-plane magnetic field. In addition to the nematic phase, the bilayer coupling opens a novel route to a hidden nematic phase that preserves the x-y symmetry of the Fermi surfaces.

cond-mat.str-el↗

arXiv2008

Intimate relations between electronic nematic, d-density wave and d-wave superconducting states

This paper consists of two important theoretical observations on the interplay between l = 2 condensates; d-density wave (ddw), electronic nematic and d-wave superconducting states. (1) There is SO(4) invariance at a transition between the nematic and d-wave superconducting states. The nematic and d-wave pairing operators can be rotated into each other by pseudospin SU(2) generators, which are s-wave pairing and electron density operators. The difference between the current work and the previous O(4) symmetry at a transition between the ddw and d-wave superconducting states (Nayak 2000 Phys. Rev. B 62 R6135) is presented. (2) The nematic and ddw operators transform into each other under a unitary transformation. Thus, when a Hamiltonian is invariant under such a transformation, the two states are exactly degenerate. The competition between the nematic and ddw states in the presence of a degeneracy breaking term is discussed.

cond-mat.str-el↗

arXiv2008

Fate of partial order on the trillium and distorted windmill lattices

The classical Heisenberg model on the trillium and distorted windmill lattices exhibits a degenerate ground state within large-$N$ theory, where the degenerate wavevectors form a surface and line, in 3-dimensional space, respectively. We name such states partially ordered to represent the existence of long-range order along the direction normal to these degenerate manifolds. We investigate the effects of thermal fluctuations using Monte Carlo (MC) methods, and find a first order transition to a magnetically ordered state for both cases. We further show that the ordering on the distorted windmill lattice is due to order by disorder, while the ground state of the trillium lattice is unique. Despite these different routes to the realization of low temperature ordered phases, the static structure factors obtained by large-$N$ theory and MC simulations for each lattice show quantitative agreement in the cooperative paramagnetic regime at finite temperatures. This suggests that a remnant of the characteristic angle-dependent spin correlations of partial order remains above the transition temperatures for both lattices. The possible relevance of these results to $β$-Mn, CeIrSi, and MnSi is discussed.

cond-mat.str-el↗

arXiv2008

Fermi pockets and quantum oscillations of the Hall coefficient in high temperature superconductors

Recent quantum oscillation measurements in high temperature superconductors in high magnetic fields and low temperatures have ushered in a new era. These experiments explore the normal state from which superconductivity arises and provide evidence of a reconstructed Fermi surface consisting of electron and hole pockets in a regime in which such a possibility was previously considered to be remote. More specifically, the Hall coefficient has been found to oscillate according to the Onsager quantization condition, involving only fundamental constants and the areas of the pockets, but with a sign that is negative. Here we explain the observations with the theory that the alleged normal state exhibits a hidden order, the $d$-density wave, which breaks symmetries signifying time reversal, translation by a lattice spacing, and a rotation by an angle $π/2$, while the product of any two symmetry operations is preserved. The success of our analysis underscores the importance of spontaneous breaking of symmetries, Fermi surface reconstruction, and conventional quasiparticles. We primarily focus on the version of the order that is commensurate with the underlying crystalline lattice, but also touch upon the consequences if the order were to incommensurate. It is shown that while commensurate order results in two independent oscillation frequencies as a function of the inverse of the applied magnetic field, incommensurate order leads to three independent frequencies. The oscillation amplitudes, however, are determined by the mobilities of the charge carriers comprising the Fermi pockets.

cond-mat.supr-con↗

arXiv2007

Topological spin liquid on the hyper-kagome lattice of Na_4Ir_3O_8

Recent experiments on the "hyper-kagome" lattice system Na$_4$Ir$_3$O$_8$ have demonstrated that it is a rare example of a three dimensional spin-1/2 frustrated antiferromagnet. We investigate the role of quantum fluctuations as the primary mechanism lifting the macroscopic degeneracy inherited by classical spins on this lattice. In the semi-classical limit we predict, based on large-N calculations, that an unusual $\vec q=0$ coplaner magnetically ordered ground state is stabilized with no local "weather vane" modes. This phase melts in the quantum limit and a gapped topological Z$_2$ spin liquid phase emerges. In the vicinity of this quantum phase transition, we study the dynamic spin structure factor and comment on the relevance of our results for future neutron scattering experiments.

cond-mat.str-el↗

arXiv2007

Half quantum vortices and Majorana fermions in triplet superconductors

Half quantum vortices (HQVs) in superfluid $^3He-A$ have been speculated since 1976. Two years ago, Yamashita et al reported a very unusual NMR satellite in the rotating superfluid $^3He-A$ in a parallel plate geometry. Recently this satellite has been interpreted in terms of HQVs. Also, the strong flux pinnings in the triplet superconductors $UPt_3, Sr_2RuO_4$ and $U_{1-x}Th_xBe_{13}$ have been discovered by Mota and her colleagues which are interpreted in terms of HQVs. We shall first review the bound state spectra around a single vortex in unconventional superconductors, which provides the necessary background. In particular, we find the zero modes of the Majorana fermions in some of triplet superconductors $UPt_3, Sr_2RuO_4$ and $CePt_3Sn$. Armed with these results we analyze the microscopic aspects of HQVs in $UPt_3$ and $Sr_2RuO_4$. We compute the quasiparticle density of states and the local magnetic field around HQVs, which should be accessible to scanning tunneling microscopy (STM), micromagnetometry and neutron scattering below 300 mK.

cond-mat.supr-con↗

arXiv2007

Classical antiferromagnet on a hyperkagome lattice

Motivated by recent experiments on Na_4Ir_3O_8 [Y. Okamoto, M. Nohara, H. Aruga-Katori, and H. Takagi, arXiv:0705.2821 (unpublished)], we study the classical antiferromagnet on a frustrated three-dimensional lattice obtained by selectively removing one of four sites in each tetrahedron of the pyrochlore lattice. This ``hyperkagome'' lattice consists of corner-sharing triangles. We present the results of large-N mean field theory and Monte Carlo computations on O(N) classical spin models. It is found that the classical ground states are highly degenerate. Nonetheless a nematic order emerges at low temperatures in the Heisenberg model (N=3) via ``order by disorder'', representing the dominance of coplanar spin configurations. Implications for ongoing experiments are discussed.

cond-mat.stat-mech↗

arXiv2007

Half quantum vortex in superfluid $^3$He-A phase in parallel plate geometry

The half quantum vortex(HQV) in condensate has been studied, since it was predicted by Salomaa and Volovik in superfluid $^3$He-A phase. However, an experimental evidence for its existence has not been reported so far. Motivated by a recent experimental report by Yamashita et al\cite{yamashita}, we study the HQVs in superfluid $^3$He confined between two parallel plates with a gap D $\sim$ 10 $μ$m in the presence of a magnetic field H $\sim$ 26 mT perpendicular to the parallel plates. We find that the bound HQVs are more stable than the singular vortices and free pairs of HQVs, when the rotation perpendicular to the parallel plates is below the critical speed, $Ω_c \sim$ 2 rad/s. The bound pair of HQVs accompanies the tilting of ${\hat d}$-vector out of the plane, which leads to an additional absorption in NMR spectra. Our study appears to describe the temperature and rotation dependence of the observed satellite NMR signal, which supports the existence of the HQVs in $^3$He.

cond-mat.supr-con↗

arXiv2007

A Direct Probe of Electronic Nematic Order; Symmetry Information in Scanning Tunneling Microscope Images

An electronic nematic state spontaneously breaks a point-group symmetry of an underlying lattice. As a result, the nematic-isotropic transition accompanies a Fermi surface distortion. However, the anisotropic nature of the nematic state at a macroscopic scale can be easily wiped out when domains of different orientations of nematic order exist. We suggest that a spatial pattern of local density of states (LDOS) in the presence of a non-magnetic impurity can be a direct probe of the nematic order. We study various patterns of LDOS across the quantum phase transition between the isotropic and nematic phases. Especially the Fourier transformed local density of states (FT-LDOS), which can be deduced from scanning tunneling microscope images, represent a transparent symmetry of an electronic structure. The application of our results to the bilayer ruthenate, Sr$_3$Ru$_2$O$_7$ is also discussed.

cond-mat.str-el↗

arXiv2007

Theory of non-Fermi liquid near a diagonal electronic nematic state on a square lattice

We study effects of Fermi surface fluctuations on a single-particle life time near the diagonal electronic nematic phase on a two-dimensional square lattice. It has been shown that there exists a quantum critical point (QCP) between the diagonal nematic and isotropic phases. We study the longitudinal fluctuations of the order parameter near the critical point, where the singular forward scattering leads to a non-Fermi liquid behavior over the whole Fermi surface except along the k_x- and k_y-directions. We will also discuss the temperature and chemical potential dependence of the single-particle decay rate.

cond-mat.str-el↗

arXiv2006

Geometric frustration inherent to the trillium lattice, a sublattice of the B20 structure

We study the classical Heisenberg model on a recently identified three dimensional corner-shared equilateral triangular lattice, a magnetic sublattice to a large class of systems with the symmetry group P2$_1$3. Since the degree of geometric frustration of the nearest neighbor antiferromagnetic model on this lattice lies on the border between the pyrochlore (not ordered) and hexagonal (ordered) lattices, it is non-trivial to predict its ground state. Using a classical rotor model, we find an ordered ground state with wavevector $(\frac{2π}{3a_0},0,0)$ featuring 120$^o$ rotated spins on each triangle. However, a mean field approximation on this lattice fails to find an ordered ground state, finding instead a non-trivially degenerate ground state. As the mean field approach is known to agree with Monte Carlo on the pyrochlore lattice, the reasons for this discrepancy are discussed. We also discuss the possible relevance of our results to MnSi.

cond-mat.str-el↗

arXiv2006

A microscopic model for spiral ordering along (110) on the MnSi lattice

We study an extended Heisenberg model on the MnSi lattice. In the cubic B20 crystal structure of MnSi, Mn atoms form lattices of of corner-shared equilateral triangles. We find an ubiquitous spiral ordering along (110) for J1 <0, and J2=J3 >0, where J1, J2, and J3 are 1st, 2nd and 3rd nearest neighbor Heisenberg interactions, respectively. While the ordering direction of (110) is reasonably robust to the presence of the Dzyaloshinskii-Moriya interaction, it can be shifted to the (111) direction with the introduction of a magnetic anisotropy term for small J2/|J1|. We discuss the possible relevance of these results to the partially ordered state recently reported in MnSi.

cond-mat.str-el↗