arXiv subjects

Wei-Cheng Lee

Publications and source records attributed to Wei-Cheng Lee.

At least 37 records · Page 2Linked to original sources

arXiv2020

The Breakdown of Mott Physics at VO$_2$ Surfaces

Transition metal oxides such as vanadium dioxide (VO$_2$), niobium dioxide (NbO$_2$), and titanium sesquioxide (Ti$_2$O$_3$) are known to undergo a temperature-dependent metal-insulator transition (MIT) in conjunction with a structural transition within their bulk. However, it is not typically discussed how breaking crystal symmetry via surface termination affects the complicated MIT physics. Using synchrotron-based x-ray spectroscopy, low energy electron diffraction (LEED), low energy electron microscopy (LEEM), transmission electron microscopy (TEM), and several other experimental techniques, we show that suppression of the bulk structural transition is a common feature at VO$_2$ surfaces. Our density functional theory (DFT) calculations further suggest that this is due to inherent reconstructions necessary to stabilize the surface, which deviate the electronic structure away from the bulk d$^1$ configuration. Our findings have broader ramifications not only for the characterization of other "Mott-like" MITs, but also for any potential device applications of such materials.

cond-mat.str-el↗

arXiv2020

Unraveling the role of V-V dimer on the vibrational properties of VO$_2$ by first-principles simulations and Raman spectroscopic analysis

We investigate the vibrational properties of VO2, particularly the low temperature M1 phase by first-principles calculations using the density functional theory as well as Raman spectroscopy. We perform the structural optimization using SCAN meta-GGA functional and obtain the optimized crystal structures for metallic rutile and insulating M1 phases satisfying all expected features of the experimentally derived structures. Based on the harmonic approximation around the optimized structures at zero temperature, we calculate the phonon properties and compare our results with experiments. We show that our calculated phonon density of states is in excellent agreement with the previous neutron scattering experiment. Moreover, we reproduce the phonon softening in the rutile phase as well as the phonon stiffening in the M1 phase. By comparing with the Raman experiments, we find that the Raman-active vibration modes of the M1 phase is strongly correlated with the V-V dimer distance of the crystal structure. Our combined theoretical and experimental framework demonstrates that Raman spectroscopy could serve as a reliable way to detect the subtle change of V-V dimer in the strained VO$_2$.

cond-mat.mtrl-sci↗

arXiv2020

Correlation induced emergent charge order in metallic vanadium dioxide

Recent progress in growth and characterization of thin-film VO$_2$ has shown its electronic properties can be significantly modulated by epitaxial matching. To throw new light on the concept of `Mott engineering', we develop a symmetry-consistent approach to treat structural distortions and electronic correlations in epitaxial VO$_2$ films under strain, and compare our design with direct experimental probes. We find strong evidence for the emergence of correlation-driven charge order deep in the metallic phase, and our results indicate that exotic phases of VO$_2$ can be controlled with epitaxial stabilization.

cond-mat.mtrl-sci↗

arXiv2019

Real-Space Visualization of Quantum Phase Transition by Network Topology

We demonstrate that with appropriate quantum correlation function, a real-space network model can be constructed to study the phase transitions in quantum systems. For the three-dimensional bosonic system, the single-particle density matrix is adopted to construct the adjacency matrix. We show that the Bose-Einstein condensate transition can be interpreted as the transition into a small-world network, which is accurately captured by the small-world coefficient. For the one-dimensional disordered system, using the electron diffusion operator to build the adjacency matrix, we find that the Anderson localized states create many weakly-linked subgraphs, which significantly reduces the clustering coefficient and lengthens the shortest path. We show that the crossover from delocalized to localized regimes as a function of the disorder strength can be identified as the loss of global connection, which is revealed by the small-world coefficient as well as other independent measures like the robustness, the efficiency, and the algebraic connectivity. Our results suggest that the quantum phase transitions can be visualized in real space and characterized by the network analysis with suitable choices of quantum correlation functions.

cond-mat.stat-mech↗

arXiv2019

Cooperative Effects of Strain and Electron Correlation in Epitaxial VO2 and NbO2

We investigate the electronic structure of the epitaxial VO$_2$ films in the rutile phase using the density functional theory combined with the slave spin method (DFT+SS). In DFT-SS, the multiorbital Hubbard interactions are added to a DFT-fit tight-binding model, and we employ the slave-spin method to treat the electron correlation. We find that while stretching the system along the rutile $c$-axis results in a band structure favoring an anisotropic orbital fillings, the electron correlation favors an equal electron filling among $t_{2g}$ orbitals. These two distinct effects cooperatively induce interesting orbital-dependent redistributions of the electron occupations and the spectral weights, which pushes the strained VO$_2$ toward an orbital selective Mott transition (OSMT). The simulated single-particle spectral functions are directly compared to V L-edge resonant X-ray photoemission spectroscopy of epitaxial 10 nm VO$_2$/TiO$_2$ (001) and (100) strain orientations. Excellent agreement is observed between the simulations and experimental data regarding the strain-induced evolution of the lower Hubbard band. Simulations of rutile NbO$_2$ under similar strain conditions as VO$_2$ are performed, and we predict that OSMT will not occur in rutile NbO$_2$. Our results indicates that the electron correlation in VO$_2$ is important and can be modulated even in the rutile phase before the Peierls instability sets in.

cond-mat.str-el↗

arXiv2018

Emergent charge order near the doping-induced Mott-insulating quantum phase transition in Sr$_{3}$Ru$_{2}$O$_{7}$

We investigate the double layered Sr$_{3}$(Ru$_{1-x}$Mn$_{x}$)$_{2}$O$_{7}$ and its doping-induced quantum phase transition (QPT) from a metal to an antiferromagnetic (AFM) Mott insulator. Using spectroscopic imaging with the scanning tunneling microscope (STM), we visualize the evolution of the electronic states in real- and momentum-space. We find a partial-gap in the tunneling density of states at the Fermi energy (E$_{F}$) that develops with doping to form a weak Mott insulating (Δ ~ 100meV) state. Near the QPT, we discover a spatial electronic reorganization into a commensurate checkerboard charge order. These findings share some resemblance to the well-established universal charge order in the pseudogap phase of cuprates. Our experiments therefore demonstrate the ubiquity of the incipient charge order that emanates from doped Mott insulators.

cond-mat.str-el↗

arXiv2018

Importance of orbital fluctuations for the magnetic dynamics in heavy-fermion compound SmB$_6$

The emergent dynamical processes associated with magnetic excitations in heavy-fermion SmB$_6$ are investigated. By imposing multiorbital interactions on a first-principles model, we find the interplay between spin and orbital fluctuations in the $f$ manifold is highly sensitive to local correlations. The magnetic phase diagram constructed at zero temperature reveals quantum critical features with the existence of several competing phases. Within the random phase approximation, we perform a comprehensive study of the spin-spin correlation function, and our results agree with neutron scattering experiments. Spectral weight analysis shows the low energy spin excitations are selectively accompanied by orbital fluctuations, indicating a non-trivial entanglement between the spin and orbital degree of freedom driven by relativistic couplings.

cond-mat.str-el↗

arXiv2017

Revisit of Antiferromagnetism in Hubbard Model by A Cluster Slave-Spin Method

The cluster slave-spin method is introduced to systematically investigate the solutions of the Hubbard model including the symmetry-broken phases. In this method, the electron operator is factorized into a fermioninc spinon describing the physical spin and a slave-spin describing the charge fluctuations. Following the $U(1)$ formalism derived by Yu and Si [Phys. Rev. B 86, 085104 (2012)], it is shown that the self-consistent equations to explore various symmetry-broken density wave states can be constructed in general with a cluster of multiple slave-spin sites. We employ this method to study the antiferromagnetic (AFM) state in the single band Hubbard model with the two and four-site clusters of slave spins. While the Hubbard gap, the charge gap due to the doubly-occupied states, scales with the Hubbard interaction $U$ as expected, the AFM gap $Δ$, the gap in the spinon dispersion in the AFM state, exhibits a crossover from the weak to strong-coupling behaviors as $U$ increases. Our cluster slave-spin method reproduces not only the traditional mean-field behavior of $Δ\sim U$ in the weak-coupling limit, but also the behavior of $Δ\sim t^2/U$ predicted by the superexchange mechanism in the strong coupling limit. In addition, the holon-doublon correlator as functions of $U$ and doping $x$ is also computed, which exhibits a strong tendency toward the holon-doublon binding in the strong coupling regime. We further show that the quasiparticle weight obtained by the cluster slave-spin method is in a good agreement with the generalized Gutzwiller approximation in both AFM and paramagnetic states, and the results can be improved beyond the generalized Gutzwiller approximation as the cluster is enlarged from a single site to 4 sites. Our results demonstrate that the cluster slave-spin method can be a powerful tool to systematically investigate the strongly correlated system.

cond-mat.str-el↗

arXiv2016

Resonant Plasmon-Axion Excitations Induced by Charge Density Wave Order in Weyl Semimetal

We investigate the charge excitations of a Weyl semimetal in the axionic charge density wave (axionic CDW) state. While it has been shown that the topological response (anomalous Hall conductivity) is protected against the CDW state, we find that the long wavelength plasmon excitation is radically influenced by the dynamics of the CDW order parameter. In the normal state, we show that an undamped collective mode should exist at q~Q_(CDW) if there is an attractive interaction favoring the formation of the CDW state. The undamped nature of this collective mode is attributed to a gap-like feature in the particle-hole continuum at q~Q_(CDW) due to the chirality of the Weyl nodes, which is not seen in other materials with CDW instability. In the CDW state, the long wavelength plasmon excitations become more dispersive due to the additional interband scattering not allowed in the normal state. Moreover, because the translational symmetry is spontaneously broken, Umklapp scattering, the process conserving the total momentum only up to nQ_(CDW) with n an integer and Q_(CDW) the ordering wave vector, emerges in the CDW state. We find that the plasmon excitation couples to the phonon mode of the CDW order via the Umklapp scattering, leading to two branches of resonant collective modes observable in the density-density correlation function at q~0 and q~Q_(CDW). Based on our analysis, we propose that measuring these resonant plasmon-axion excitations around q~0 and q~Q_(CDW) by the momentum-resolved electron energy loss spectroscopy (M-EELS) could serve as a reliable way to detect the axionic CDW state in Weyl semimetals.

cond-mat.str-el↗

arXiv2016

Recent Progress of Point Contact Spectroscopy as a Probe of Correlated Electron States

We review recent progress in point contact spectroscopy (PCS) to extract spectroscopic information out of correlated electron materials, with the emphasis on non-superconducting states. PCS has been used to detect bosonic excitations in normal metals, where signatures (e.g. phonons) are usually less than 1$\%$ of the measured conductance. In the superconducting state, point contact Andreev reflection (PCAR) has been widely used to study properties of the superconducting gap in various superconductors. In the last decade, there have been more and more experimental results suggesting that the point contact conductance could reveal new features associated with the unusual single electron dynamics in non-superconducting states, shedding a new light on exploring the nature of the competing phases in correlated materials. We will summarize the theories for point contact spectroscopy developed from different approaches and highlight these conceptual differences distinguishing point contact spectroscopy from tunneling-based probes. Moreover, we will show how the Schwinger-Kadanoff-Baym-Keldysh (SKBK) formalism together with the appropriate modeling of the nano-scale point contacts randomly distributed across the junction leads to the conclusion that the point contact conductance is proportional to the {\it effective density of states}, a physical quantity that can be computed if the electron self energy is known. The experimental data on iron based superconductors and heavy fermion compounds will be analyzed in this framework. These recent developments have extended the applicability of point contact spectroscopy to correlated materials, which will help us achieve a deeper understanding of the single electron dynamics in strongly correlated systems.

cond-mat.str-el↗

arXiv2016

Crossover From Strong to Weak Pairing States in $t-J-U$ Model Studied by A Slave Spin Method

We investigate the superconductivity in the $t-J-U$ model within a slave-spin formalism. We show that the BCS mean-field theory implemented with the slave spin formalism naturally predicts two distinct gaps which are the pairing gap of the spinons $Δ_f$ and the Cooper pairing gap of the electrons $Δ_{SC} = ZΔ_f$, where $Z$ is the quasiparticle weight. Furthermore, we find that the nature of the superconducting state depends crucially on the interaction providing the pairing mechanism. For spin interactions, the bandwidth of the spinon hopping term is renormalized by $Z$ but the its pairing term is not. As a result, if $U$ exceeds the critical value for the Mott insulating state at half-filling, $Z$ develops a strong doping dependence, leading to a doping-driven crossover from strong to weak pairing states. In the strong pairing state, while $Δ_f$ is enhanced as $x\to 0$, $Δ_{SC}\sim x$ due to the renormalization of $Z$. In the weak pairing state, $Z$ does not change with $x$ significantly. Therefore, $Δ_{SC}$ is mainly controlled by $Δ_f$, and both of them go to zero at larger doping. The crossover from strong to weak pairing states is well captured by the slave spin formalism within reasonable range of parameters just at the mean-field level, indicating the slave spin formalism is a powerful tool to study correlated materials. For charge interactions, we find that the bandwidth of the spinon hopping term and its pairing term are renormalized by $Z$ and $Z^2$ respectively. Consequently, both $Δ_f$ and $Δ_{SC}$ are suppressed at small $x$ in large U, and no crossover will occur. The implication of our results for the superconducting states in correlated materials will be discussed.

cond-mat.str-el↗

arXiv2016

Structural and magnetic field effects on spin fluctuations in Sr$_{3}$Ru$_2$O$_7$

We investigate the evolution of magnetic excitations in Sr$_3$Ru$_2$O$_7$ using a three band tight binding model that takes into account the influence of Mn and Ti dopant ions. The effect of dopant ions on the Sr$_3$Ru$_2$O$_7$ band structure has been included by taking into account the dopant induced suppression of the oxygen octahedral rotation in the tight binding band structure and changes in electron occupation. We find that the low energy spin fluctuations are dominated by three wave vectors around Q=$((0,0),(π/2,π/2))$, and $(π,0)$ which compete with each other. As the octahedral rotation is suppressed with increasing doping, the three wave vectors evolve differently. In particular, the undoped compound has dominant wavevectors at Q=$((0,0),(π/2,π/2))$, but doping Sr$_3$Ru$_2$O$_7$ leads to a significant enhancement in the spin susceptibility at the Q=$(π,0)$ wavevector bringing the system closer to a magnetic instability. All the features calculated from our model are in agreement with neutron scattering experiments. We have also studied the effect of a c-axis Zeeman field on the low energy spin fluctuations. We find that an increasing magnetic field suppresses the AFM fluctuations and leads to stronger competition between the AFM and FM spin fluctuations. The magnetic field dependence observed in our calculations therefore supports the scenario that the observed nematic phase in the metamagnetic region in Sr$_3$Ru$_2$O$_7$ is intimately related to the presence of a competing ferromagnetic instability.

cond-mat.str-el↗

arXiv2016

Tuning a Strain-Induced Orbital Selective Mott Transition in Epitaxial VO$_2$

We present evidence of strain-induced modulation of electron correlation effects and increased orbital anisotropy in the rutile phase of epitaxial VO$_2$/TiO$_2$ films from hard x-ray photoelectron spectroscopy and soft V L-edge x-ray absorption spectroscopy, respectively. By using the U(1) slave spin formalism, we further argue that the observed anisotropic correlation effects can be understood by a model of orbital selective Mott transition at a filling that is non-integer, but close to the half-filling. Because the overlaps of wave functions between $d$ orbitals are modified by the strain, orbitally-dependent renormalizations of the bandwidths and the crystal fields occur with the application of strain. These renormalizations generally result in different occupation numbers in different orbitals. We find that if the system has a non-integer filling number near the half-filling such as for VO$_2$, certain orbitals could reach an occupation number closer to half-filling under the strain, resulting in a strong reduction in the quasiparticle weight $Z_α$ of that orbital. Moreover, an orbital selective Mott transition, defined as the case with $Z_α = 0$ in some, but not all orbitals, could be accessed by epitaxial strain-engineering of correlated electron systems.

cond-mat.str-el↗

arXiv2015

Characterizing Featureless Mott Insulating State by Quasiparticle Interferences - A Dynamical Mean Field Theory Prospect

The quasiparticle interferences (QPIs) of a Mott insulator are investigated using the $T$-matrix formalism implemented with the dynamical mean-field theory ($T$-DMFT). In Mott insulating state, because DMFT predicts a singularity in the real part of the electron self energy $\Re Σ(ω) \sim η/ω$ at low frequency, where $η$ can be considered as the 'order parameter' for Mott insulating state, QPIs are completely washed out at the small bias voltage. However, a further analysis shows that $\Re Σ(ω)$ in fact serves as an energy-dependent chemical potential shift. As a result, the effective bias voltage seen by the system is $eV' = eV - \Re Σ(eV)$. Due to the singular behavior of $\Re Σ(ω)$, a critical bias voltage $eV_c = \sqrtη$ satisfying $eV'=0$ exists if and only if in Mott insulating state. Consequently, the same QPI patterns produced by the non-interacting Fermi surfaces appears at this critical bias voltage $eV_c$ in Mott insulating state. We propose that this reentry of non-interacting QPI patterns at $eV_c$ could serve as an experimental signature of Mott insulating state, and the 'order parameter' can be experimentally measured as $η\sim (eV_c)^2$.

cond-mat.str-el↗

arXiv2015

Superconductivity-Induced Changes in Density-Density Correlation Function Enabled by Umklapp Processes

Motivated by the mid-infrared scenario for high-temperature superconductivity proposed by Leggett, the effects of Umklapp processes on the density-density correlation function in the presence of long-range Coulomb interaction have been investigated on a microscopic model. We show that because Umklapp processes enable scatterings that conserve total momentum only up to $n\vec{K}$, where $n$ is an integer and $\vec{K}$ is the reciprocal wave vector, significant amounts of spectral weight in the plasmonic excitations at long wavelength are transferred into lower frequency around mid-infrared regime. We further find that regardless of the gap symmetry, superconductivity generally suppresses the Umklapp scatterings due to the formation of the electron pairing. This suppression is unique for the superconductivity due to the interplay between electron pairing and the odd parity of the matrix elements associated with Umklapp channels, which usually does not occur in other known competing orders. Specific predictions for the experimental signatures in optical conductivity and electron energy loss spectroscopy (EELS) will be discussed.

cond-mat.supr-con↗

arXiv2014

Theory of Point Contact Spectroscopy in Correlated Materials

We develop a microscopic theory for the point-contact conductance between a metalic electrode and a strongly correlated material using the non-equilibrium Schwinger-Kadanoff-Baym-Keldysh formalism. We explicitly show that in the classical limit, contact size shorter than the scattering length of the system, the microscopic model can be reduced to an effective model with transfer matrix elements that conserve in-plane momentum. We find that the conductance $dI/dV$ is proportional to the {\it effective density of states}, that is, the integrated single-particle spectral function $A(ω=eV)$ over the whole Brillouin zone. From this conclusion, we are able to establish the conditions under which a non-Fermi liquid metal exhibits a zero-bias peak in the conductance. This finding is discussed in the context of recent point-contact spectroscopy on the iron pnictides and chalcogenides which has exhibited a zero-bias conductance peak.

cond-mat.str-el↗

arXiv2014

High-Energy Damping by Particle-Hole Excitations in the Spin-Wave Spectrum of Iron-Based Superconductors

Using a degenerate double-exchange model, we investigate the spin excitation spectra of iron pnictides. The model consists of local spin moments on each Fe site, as well as itinerant electrons from the degenerate $d_{xz}$ and $d_{yz}$ orbitals. The local moments interact with each other through antiferromagnetic $J_{1}$-$J_{2}$ Heisenberg interactions, and they couple to the itinerant electrons through a ferromagnetic Hund coupling. We employ the fermionic spinon representation for the local moments and perform a generalized random-phase approximation calculation on both spinons and itinerant electrons. We find that in the $\left(π,0\right)$ magnetically-ordered state, the spin-wave excitation at $\left(π,π\right)$ is pushed to a higher energy due to the presence of itinerant electrons, which is consistent with a previous study using the Holstein-Primakoff transformation. In the paramagnetic state, the particle-hole continuum keeps the collective spin excitation near $\left(π,π\right)$ at a higher energy even without any $C_{4}$ symmetry breaking. The implications for recent high temperature neutron scattering measurements will be discussed.

cond-mat.str-el↗

arXiv2013

Orbital Resonance Mode in Superconducting Iron Pnictides

We show that the fluctuations associated with ferro orbital order in the $d_{xz}$ and $d_{yz}$ orbitals can develop a sharp resonance mode in the superconducting state with a nodeless gap on the Fermi surface. This orbital resonance mode appears below the particle-hole continuum and is analogous to the magnetic resonance mode found in various unconventional superconductors. If the pairing symmetry is $s_{\pm}$, a dynamical coupling between the orbital ordering and the d-wave subdominant pairing channels is present by symmetry. Therefore the nature of the resonance mode depends on the relative strengths of the fluctuations in these two channels, which could vary significantly for different families of the iron based superconductors. The application of our theory to a recent observation of a new $δ$-function-like peak in the B$_{1g}$ Raman spectrum of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is discussed.

cond-mat.supr-con↗