arXiv subjects

Wei-Cheng Lee

Publications and source records attributed to Wei-Cheng Lee.

At least 55 records · Page 3Linked to original sources

arXiv2013

Elementary Excitations due to Orbital Degrees of Freedom in Iron Based Superconductors

One central issue under intense debate in the study of the iron based superconductors is the origin of the structural phase transition that changes the crystal lattice symmetry from tetragonal to orthorhombic. This structural phase transition, occurring universally in almost every family of the iron-based superconductors, breaks the lattice $C_4$ rotational symmetry and results in an anisotropy in a number of physical properties. Due to the unique topology of the Fermi surface, both orbital- and spin-based scenarios have been proposed as the driving force. In this review, we focus on theories from the orbital-based scenario and discuss several related experiments. It is pointed out that although both scenarios lead to the same macroscopic phases and are not distinguishable in bulk measurements of the thermodynamic properties, the elementary excitations could be fundamentally different, and provide us with the possibility to resolve this long-standing debate between orbital- and spin-based theories.

cond-mat.supr-con↗

arXiv2013

Non-Fermi Liquid behavior at the Orbital Ordering Quantum Critical Point in the Two-Orbital Model

The critical behavior of a two-orbital model with degenerate $d_{xz}$ and $d_{yz}$ orbitals is investigated by multidimensional bosonization. We find that the corresponding bosonic theory has an overdamped collective mode with dynamical exponent $z=3$, which appears to be a general feature of a two-orbital model and becomes the dominant fluctuation in the vicinity of the orbital-ordering quantum critical point. Since the very existence of this $z=3$ overdamped collective mode induces non-Fermi liquid behavior near the quantum critical point, we conclude that a two-orbital model generally has a sizable area in the phase diagram showing non-Fermi liquid behavior. Furthermore, we show that the bosonic theory resembles the continuous model near the d-wave Pomeranchuk instability, suggesting that orbital order in a two-orbital model is identical to nematic order in a continuous model. Our results can be applied to systems with degenerate $d_{xz}$ and $d_{yz}$ orbitals such as iron-based superconductors and bilayer strontium ruthenates Sr$_3$Ru$_2$O$_7$.

cond-mat.str-el↗

arXiv2012

Non-Fermi Liquid due to Orbital Fluctuations in Iron Pnictide Superconductors

We study the influence of quantum fluctuations on the electron self energy in the normal state of iron-pnictide superconductors using a five orbital tight binding model with generalized Hubbard on-site interactions. Within a one-loop treatment, we find that an overdamped collective mode develops at low frequency in channels associated with quasi-1D $d_{xz}$ and $d_{yz}$ bands. When the critical point for the $C_4$ symmetry broken phase (structural phase transition) is approached, the overdamped collective modes soften, and acquire increased spectral weight, resulting in non-Fermi liquid behavior at the Fermi surface characterized by the frequency dependence of the imaginary part of electron self energy of the form $ω^λ$, $0<λ<1$. We argue that this non-Fermi liquid behavior is responsible for the recently observed zero-bias enhancement in the tunneling signal in quantum point contact spectroscopy. A key experimental test of this proposal is the absence of the non-Fermi liquid behaviour in the hole-doped materials. Our result suggests that quantum criticality plays an important role in understanding the normal state properties of iron-pnictide superconductors.

cond-mat.str-el↗

arXiv2012

Impact of Dynamic Orbital Correlations on Magnetic Excitations in the Normal State of Iron-Based Superconductors

We show here that orbital degrees of freedom produce a distinct signature in the magnetic excitation spectrum of iron-based superconductors above the magnetic ordering temperature. Because $d_{xz}$ and $d_{yz}$ orbitals are strongly connected with the Fermi surface topology, the nature of magnetic excitations can be modified significantly due to the presence of either static or fluctuating orbital correlations. Within a five-orbital itinerant model, we show that static orbital order generally leads to an enhancement of commensurate magnetic excitations even when the original Fermi surface lacks nesting at $(π,0)$ or $(0,π)$. When long-range orbital order is absent, Gaussian fluctuations beyond the standard random-phase approximation (RPA) capture the effects of fluctuating orbital correlations on the magnetic excitations. We find that commensurate magnetic excitations can also be enhanced if the orbital correlations are strong. Our results offer a natural explanation for the incommensurate-to-commensurate transformation observed in a recent neutron scattering measurement (Z. Xu, et. al., arXiv:1201.4404), and we propose that this unusual transformation is an important signature to distinguish orbital from spin physics in the normal state of the pnictides. Implications for the magnetic and superconducting states are discussed.

cond-mat.str-el↗

arXiv2012

Phase soliton and pairing symmetry of a two-band superconductor: Role of the proximity effect

We suggest a mechanism which promotes the existence of a phase soliton -- topological defect formed in the relative phase of superconducting gaps of a two-band superconductor with s+- type of pairing. This mechanism exploits the proximity effect with a conventional s-wave superconductor which favors the alignment of the phases of the two-band superconductor which, in the case of s+- pairing, are pi-shifted in the absence of proximity. In the case of a strong proximity such effect can be used to reduce soliton's energy below the energy of a soliton-free state thus making the soliton thermodynamically stable. Based on this observation we consider an experimental setup, applicable both for stable and metastable solitons, which can be used to distinguish between s+- and s++ types of pairing in the iron-based multiband superconductors.

cond-mat.supr-con↗

arXiv2011

Vacancy Driven Orbital and Magnetic Order in (K,Tl,Cs)$_y$Fe$_{2-x}$Se$_2$

We investigate the effects of the $\sqrt{5}\times\sqrt{5}$ Fe vacancy ordering on the orbital and magnetic order in (K,Tl,Cs)$_y$Fe$_{2-x}$Se$_2$ using a three-orbital ($t_{2g}$) tight-binding Hamiltonian with generalized Hubbard interactions. We find that vacancy order enhances electron correlations, resulting in the onset of a block antiferromagnetic phase with large moments at smaller interaction strengths. In addition, vacancy ordering modulates the kinetic energy differently for the three $t_{2g}$ orbitals. This results in a breaking of the degeneracy between the $d_{xz}$ and $d_{yz}$ orbitals on each Fe site, and the onset of orbital order. Consequently, we obtain a novel inverse relation between orbital polarization and the magnetic moment. We predict that a transition from high-spin to low-spin states accompanied by a crossover from orbitally-disordered to orbitally-ordered states will be driven by doping the parent compound with electrons, which can be verified by neutron scattering and soft X-ray measurements.

cond-mat.str-el↗

arXiv2011

Spectral Weight Transfer in Multi-Orbital Mott Systems

We develop here a general formalism for multi-orbital Mott systems which can be used to understand dynamical and static spectral weight transfer. We find that the spectral weight transferred from the high energy scales is greatly increased as a result of the multi-orbital structure. As a consequence certainly dynamically generated symmetries obtain at lower values of doping than in the single-band Hubbard model. For example, in the atomic limit, the particle-hole symmetric condition in the lower band shifts from the one-band result of $x=1/3$ to $x=1/(2n_o+1)$, where $n_o$ is the number of orbitals with an unpaired spin. Transport properties computed from effective low-energy theories which forbid double occupancy of bare electrons, such as the multi-orbital t-J generalization, should all be be sensitive to this particle-hole symmetric condition. Away from the atomic limit, the dynamical contributions increase the transferred spectral weight. Consequently, any phenomena which are sensitive to an emergent particle-hole symmetry should obtain at $x<(1/(2n_o+1)$.

cond-mat.str-el↗

arXiv2011

Comment on "High-Spin Polaron in Lightly Doped CuO$_2$ Planes"

In a recent Letter (Phys. Rev. Lett. 106, 036401 (2011)), Lau, et al. investigated the single hole problem in an effective model containing Cu $d$ and O $2p$ orbitals by exact diagonalization on clusters composed of 32 CuO$_2$ unit cells. With full quantum fluctuations due to the antiferromagnetic (AFM) backgroud, they found that spin-polaron solutions with spin 3/2 become lowest energy solutions in certain region of Brillouin zone (BZ), which they claimed no similiar solutions were obtained in other models or approximations before. In this Comment, we would like to point out that such high-spin polaron solutions have been found in our previous Letter (Phys. Rev. Lett. 91, 057001, (2003)) and many of features described by Lau et al. have in fact been seen in our Letter.

cond-mat.str-el↗

arXiv2011

Frustrated Cooper pairing and the $f$-wave supersolidity

Geometric frustration in quantum magnetism refers to that magnetic interactions on different bonds cannot be simultaneously minimized. The usual Cooper pairing systems favor the uniform distribution of the pairing phase among lattice sites without frustration. In contrast, we propose "frustrated Cooper pairing" in non-bipartite lattices which leads to frustrated supersolid states with non-uniform distributions of the Cooper pair phase and density. This exotic pairing state naturally occurs in the $p$-orbital band in optical lattices with ultra-cold spinless fermions. In the triangular lattice, it exhibits an unconventional supersolid state with the $f$-wave symmetry.

cond-mat.supr-con↗

arXiv2010

$F$-wave pairing of cold atoms in optical lattices

The tremendous development of cold atom physics has opened up a whole new opportunity to study novel states of matter which are not easily accessible in solid state systems. Here we propose to realize the $f$-wave pairing superfluidity of spinless fermions in the $p_{x,y}$-orbital bands of the two dimensional honeycomb optical lattices. The non-trivial orbital band structure rather than strong correlation effects gives rise to the unconventional pairing with the nodal lines of the $f$-wave symmetry. With a confining harmonic trap, zero energy Andreev bound states appear around the circular boundary with a six-fold symmetry. The experimental realization and detection of this novel pairing state are feasible.

cond-mat.quant-gas↗

arXiv2010

Microscopic Theory of the Thermodynamic Properties of Sr$_3$Ru$_2$O$_7$

The thermodynamic properties of the bilayer Sr$_3$Ru$_2$O$_7$ at very low temperatures are investigated by a realistic tight-binding model with the on-site interactions treated at the mean-field level. Due to the strong spin-orbit coupling, the band structure undergoes a significant change in Fermi surface topology as the external magnetic field is applied, invalidating the rigid band picture in which the Zeeman energy only causes chemical potential shifts. In addition, since Sr$_3$Ru$_2$O$_7$ is a $t_{2g}$ active system with unquenched orbital moments, the orbital Zeeman energy is not negligible and plays an important role in the phase diagram on the magnetic field orientation. We find that both the total density of states at the Fermi energy and the entropy exhibit a sudden increase near the critical magnetic field for the nematic phase, echoing the experimental findings. Our results suggest that extra cares are necessary to isolate the contributions due to the quantum criticality from the band structure singularity in this particular material. The effects of quantum critical fluctuations are briefly discussed.

cond-mat.str-el↗

arXiv2010

Quasiparticle Interference in the Unconventional Metamagnetic Compound Sr$_3$Ru$_2$O$_7$

Quasiparticle interference (QPI) in spectroscopic imaging scanning tunneling microscopy provides a powerful method to detect orbital band structures and orbital ordering patterns in transition metal oxides. We use the $T$-matrix formalism to calculate the QPI spectra for the unconventional metamagnetic system of Sr$_3$Ru$_2$O$_7$ with a $t_{2g}$-orbital band structure. A detailed tight-binding model is constructed accounting for features such as spin-orbit coupling, bilayer splitting, and the staggered rotation of the RuO octahedra. The band parameters are chosen by fitting the calculated Fermi surfaces with those measured in the angular-resolved photo-emission spectroscopy experiment. The calculated quasiparticle interference at zero magnetic field exhibits a hollow square-like feature arising from the nesting of the quasi-1d $d_{xz}$ and $d_{yz}$ orbital bands, in agreement with recent measurements by J. Lee {\it et al.} (Nature Physics {\bf 5}, 800 (2009)). Rotational symmetry breaking in the nematic metamagnetic state also manifests in the quasi-particle interference spectra.

cond-mat.str-el↗

arXiv2010

Quasiparticle Interference on the Surface of the Topological Insulator Bi$_2$Te$_3$

The quasiparticle interference of the spectroscopic imaging scanning tunneling microscopy has been investigated for the surface states of the large gap topological insulator Bi$_2$Te$_3$ through the T-matrix formalism. Both the scalar potential scattering and the spin-orbit scattering on the warped hexagonal isoenergy contour are considered. While backscatterings are forbidden by time-reversal symmetry, other scatterings are allowed and exhibit strong dependence on the spin configurations of the eigenfunctions at k points over the isoenergy contour. The characteristic scattering wavevectors found in our analysis agree well with recent experiment results.

cond-mat.mes-hall↗

arXiv2009

Spectroscopic Imaging Scanning Tunneling Microscopy as a Probe of Orbital Structures and Ordering

Unlike charge and spin, the orbital degree of freedom of electrons in transition metal oxides is difficult to detect. We present the theoretical study of a new detection method in metallic orbitally active systems by analyzing the quasiparticle scattering interference (QPI) pattern of the spectroscopic imaging scanning tunneling spectroscopy, which is sensitive to orbital structures and orbital ordering. The QPIs for the $d_{xz}$ and $d_{yz}$-orbital bands in the $t_{2g}$-orbital systems show a characteristic stripe-like feature as a consequence of their quasi-one-dimensional nature, which is robust against orbital hybridization. With the occurrence of orbital ordering proposed in Sr$_3$Ru$_2$O$_7$ and iron-pnictides, the stripe-like QPI patterns exhibit nematic distortion breaking the $C_4$-symmetry.

cond-mat.str-el↗

arXiv2009

Renormalized Landau Levels and Particle-Hole Symmetry in Graphene

In this proceedings paper we report on a calculation of graphene's Landau levels in a magnetic field. Our calculations are based on a self-consistent Hartree-Fock approximation for graphene's massless-Dirac continuum model. We find that because of graphene's chiral band structure interactions not only shift Landau-level energies, as in a non-relativistic electron gas, but also alter Landau level wavefunctions. We comment on the subtle continuum model regularization procedure necessary to correctly maintain the lattice-model's particle hole symmetry properties.

cond-mat.mes-hall↗

arXiv2009

Pairing State with a time-reversal symmetry breaking in FeAs based superconductors

We investigate the competition between the extended $s_{\pm}$-wave and $d_{x^2-y^2}$-wave pairing order parameters in the iron-based superconductors. Because of the frustrating pairing interactions among the electron and the hole fermi pockets, a time reversal symmetry breaking $s+id$ pairing state could be favored. We analyze this pairing state within the Ginzburg-Landau theory, and explore the experimental consequences. In such a state, spatial inhomogeneity induces supercurrent near a non-magnetic impurity and the corners of a square sample. The resonance mode between the $s_{\pm}$ and $d_{x^2-y^2}$-wave order parameters can be detected through the $B_{1g}$-Raman spectroscopy.

cond-mat.supr-con↗

arXiv2009

Quantum Wells in Polar-Nonpolar Oxide Heterojunction Systems

We address the electronic structure of quantum wells in polar-nonpolar oxide heterojunction systems focusing on the case of non-polar BaVO$_3$ wells surrounded by polar LaTiO$_3$ barriers. Our discussion is based on a density functional description using the local spin density approximation with local correlation corrections (LSDA+U). We conclude that a variety of quite different two-dimensional electron systems can occur at interfaces between insulating materials depending on band line-ups and on the geometrical arrangement of polarity discontinuities.

cond-mat.str-el↗

arXiv2009

Berry Phase Coupling and the Cuprate Neutron Scattering Resonance

We examine the influence of coupling between particle-hole and particle-particle spin fluctuations on the inelastic neutron scattering resonance (INSR) in cuprate superconductors in both weak and strong interaction limits. For weak-interactions in the particle-hole channel, we find that the interchannel coupling can eliminate the resonance. For strong interactions which drive the system close to a $\bm{Q}=(π,π)$ magnetic instablity, the resonance frequency always approaches zero but its value is influenced by the interchannel coupling. We comment on constraints imposed on cuprate physics by the INSR phenomenology, and a comparison between the cuprates and the newly-discovered iron pnictide superconductors is discussed.

cond-mat.str-el↗