arXiv subjects

Bumsoo Kyung

Publications and source records attributed to Bumsoo Kyung.

18 recordsLinked to original sources

arXiv2011

Optical and DC conductivity of the two-dimensional Hubbard model in the pseudogap regime and across the antiferromagnetic quantum critical point, including vertex corrections

The conductivity of the two-dimensional Hubbard model is particularly relevant for high-temperature superconductors. Vertex corrections are expected to be important because of strongly momentum dependent self-energies. We use the Two-Particle Self-Consistent approach that satisfies crucial constraints such as the Mermin-Wagner theorem, the Pauli principle and sum rules in order to reach non-perturbative regimes. This approach is reliable from weak to intermediate coupling. A functional derivative approach ensures that vertex corrections are included in a way that satisfies the f sum-rule. The two types of vertex corrections that we find are the antiferromagnetic analogs of the Maki-Thompson and Aslamasov-Larkin contributions of superconducting fluctuations to the conductivity but, contrary to the latter, they include non-perturbative effects. The resulting analytical expressions must be evaluated numerically. The calculations are impossible unless a number of advanced numerical algorithms are used. A maximum entropy approach is specially developed for analytical continuation of our results. The numerical results are for nearest neighbor hoppings. In the pseudogap regime induced by two-dimensional antiferromagnetic fluctuations, the effect of vertex corrections is dramatic. Without vertex corrections the resistivity increases as we enter the pseudogap regime. Adding vertex corrections leads to a drop in resistivity, as observed in some high temperature superconductors. At high temperature, the resistivity saturates at the Ioffe-Regel limit. At the quantum critical point and beyond, the resistivity displays both linear and quadratic temperature dependence and there is a correlation between the linear term and the superconducting transition temperature. A hump is observed in the mid-infrared range of the optical conductivity in the presence of antiferromagnetic fluctuations.

cond-mat.str-el↗

arXiv2008

First order Mott transition at zero temperature in two dimensions: Variational plaquette study

The nature of the metal-insulator Mott transition at zero temperature has been discussed for a number of years. Whether it occurs through a quantum critical point or through a first order transition is expected to profoundly influence the nature of the finite temperature phase diagram. In this paper, we study the zero temperature Mott transition in the two-dimensional Hubbard model on the square lattice with the variational cluster approximation. This takes into account the influence of antiferromagnetic short-range correlations. By contrast to single-site dynamical mean-field theory, the transition turns out to be first order even at zero temperature.

cond-mat.str-el↗

arXiv2007

Electronic Properties of the Hubbard Model on a Frustrated Triangular Lattice

We study novel electronic properties of the Hubbard model on a triangular lattice using the cellular dynamical mean-field theory. The interplay of strong geometric frustration and electron correlations causes a Mott transition at the Hubbard interaction $U/t=10.5$ and an unusual suppression of low energy spin excitations. Doping of a triangular Mott insulator leads to a quasiparticle peak (no pseudogap) at the Fermi surface and to an unexpected increase of low energy spin excitations, in stark contrast to the unfrustrated square lattice case. The present results give much insight into strongly frustrated electronic systems. A few predictions are made.

cond-mat.str-el↗

arXiv2002

Phenomenological description of competing antiferromagnetism and d-wave superconductivity in high $T_{c}$ cuprates

In this paper the phase diagram of high $T_{c}$ cuprates is {\it qualitatively} studied in the context of competing orders: antiferromagnetism, d-wave superconductivity and $d$-density wave. {\it Local} correlation functions are estimated from a mean-field solution of the $t-J$ Hamiltonian. With decreasing doping the superconducting mean-field $T^{MF}_{c}$ and order parameter $d$ begin to decrease below some characteristic doping $x_{c} \simeq 0.2$ where short-range antiferromagnetic correlations begin to develop. {\it Dynamical} properties that involve the energy spectrum, such as the normal state pseudogap, are calculated from effective interactions that are consistent with the above-mentioned local correlation functions. The total excitation gap $Δ_{tg}$ (in the superconducting state) and the normal state pseudogap $Δ_{pg}$ are in good agreement with experimental results. Properties of the condensate are estimated using an effective pairing interaction $V_{eff}$ which takes into account (pair breaking) antiferromagnetic correlations. These condensate properties include condensation energy U(0), coherence gap $Δ_{cg}$ and critical field $H_{c2}$. The calculated coherence gap closely follows the doping dependence of $T_{c}$ or $d$, and is approximately given as $Δ_{cg} \sim Δ_{tg}-Δ_{pg}$ within our numerical uncertainties. The systematic decrease of superfluidity ($d$, U(0), $Δ_{cg}$, $H_{c2}$), and systematic increase of $Δ_{pg}$ and $Δ_{tg}$ with decreasing doping below $x_{c}$ have their natural explanation in our approach. The overall description is however qualitative since it does not appear possible to obtain results that are in quantitative agreement with experiment for all physical quantities.

cond-mat.str-el↗

arXiv2001

Induced local spin-singlet amplitude and pseudogap in high $T_{c}$ cuprates

In this paper we show that local spin-singlet amplitude with d-wave symmetry, $<|Δ_{d}(0)|^{2}>$, can be induced by short-range spin correlations even in the absence of pairing interactions. Fluctuation theory is formulated to make connection between pseudogap temperature $T^{*}$, pseudogap size $Δ_{pg}$ and $<|Δ_{d}(0)|^{2}>$. In the present scenario for the pseudogap, the normal state pseudogap is caused by the induced local spin-singlet amplitude due to short-range spin correlations, which compete in the low energy sector with superconducting correlations to make $T_{c}$ go to zero near half-filling. Calculated $T^{*}$ falls from a high value onto the $T_{c}$ line and closely follows mean-field Néel temperature $T_{N}^{MF}$. The calculated $Δ_{pg}$ is in good agreement with experimental results. We propose an experiment in which the present scenario can be critically tested.

cond-mat.str-el↗

arXiv2001

Short-range spin correlations and pseudogap in underdoped cuprates

In this paper we show that local spin-singlet amplitude with d-wave symmetry can be induced by short-range spin correlations even in the absence of pairing interactions. In the present scenario for the pseudogap, the normal state pseudogap is caused by the induced local spin-singlet amplitude due to short-range spin correlations, which compete in the low energy sector with superconducting correlations to make $T_{c}$ go to zero near half-filling.

cond-mat.str-el↗

arXiv2001

Short-range spin correlations and induced local spin-singlet amplitude in the Hubbard model

In this paper, from the microscopic Hubbard Hamiltonian we extract the local spin-singlet amplitude due to short-range spin correlations, and quantify its strength near half-filling. As a first application of the present approach, we study a problem of the energy dispersion and its d-wave modulation in the insulating cuprates, Sr$_{2}$CuO$_{2}$Cl$_{2}$ and Ca$_{2}$CuO$_{2}$Cl$_{2}$. Without any adjustable parameters, most puzzling issues are naturally and quantitatively explained within the present approach.

cond-mat.str-el↗

arXiv2000

Spectral properties and pseudogaps in a model with d-wave pairing symmetry

A model with d-wave pairing symmetry is studied by employing a non-perturbative sum rule approach. At low temperature the magnitude of a normal state pseudogap shows strong $\vec{k}$ or angle dependence well fitted by $\cos 2ϕ$ form. With increasing temperature, the pseudogap closes at some critical angle $ϕ_{c}$ and beyond this angle a single quasiparticle-like peak appears. The resulting Fermi surface is strongly temperature dependent. Both in the spectral function and the density of states, the pseudogap disappears in a manner that the spectral weight fills in the pseudogap instead of closing it with increasing temperature. All these features are qualitatively consistent with ARPES for underdoped cuprates.

cond-mat.str-el↗

arXiv2000

Conjectures for the microscopic theory of high temperature superconductivity

Based on experimental results and our previous theoretical work, a microscopic theory of high temperature superconductivity is conjectured. In this conjecture, superconducting and antiferromagnetic long-range orders are driven by interlayer coupling. Strictly in two dimensions, the microscopic Hubbard model has an (resonating valence bond) insulator-to-metal transition at $x=x_{c}$ near optimal doping for zero temperature, leading to a quantum critical point, and one of the crossover lines is given by the pseudogap temperature $T^{*}$. We argue that various singular and non-Fermi liquid properties observed near optimal doping are due to the presence of this quantum critical point. In our conjecture, the crossover line $T^{*}$ also practically divides the superconducting region into two, depending on the doping level with respect to $x_{c}$. For $x \leq x_{c}$ the superconducting state has significant antiferromagnetic correlations, while for $x > x_{c}$ it has virtually no antiferromagnetic correlations, thus justifying the conventional BCS theory based on the noninteracting electrons. Inelastic neutron scattering resonance and systematically reduced superfluid density in the superconducting state below $x_{c}$ have their natural explanations in the present scenario. The present approach supports interlayer pair tunneling model where the superconducting condensation energy comes from the lowering of the c-axis kinetic energy in the superconducting state. Comparison of the present scenario with some of the leading theories based on the Hubbard and $t-J$ models is given. The generic features of both hole-doped and electron-doped cuprates as well as heavy-fermion superconductors may be understood in the {\em unified} framework within the present picture.

cond-mat.str-el↗

arXiv2000

New interpretation of slave boson mean-field theory of the $t-J$ model: short-range antiferromagnetic and d-wave pairing correlations

The $t-J$ Hamiltonian is studied in a mean-field approximation by taking into account antiferromagnetic and d-wave pairing correlations. Considering the presence of antiferromagnetic fluctuations, the weaknesses of a mean-field approximation and the limitation of the $t-J$ model near half-filling, we give a new interpretation to the slave boson mean-field theory of the $t-J$ model. We argue that due to phase coherence-breaking antiferromagnetic fluctuations and quantum fluctuations, superconducting long-range order does not appear strictly in two dimensions. $T_{c}$ resulting from interlayer pairing hopping can lead to a universal relation, when $T_{c}$ is scaled by $T^{max}_{c}$. Systematic reduction of superfluid density and increase of $(Δ_{d})_{max}/K_{B}T_{c}$ ratio below and near optimal doping have their natural explanation in our picture. A crossover temperature $T^{0}$ found in some of magnetic experiments such as NMR is also easily understood in the present framework.

cond-mat.str-el↗

arXiv2000

Mean-field study of the interplay between antiferromagnetism and d-wave superconductivity

The interplay between antiferromagnetism and d-wave superconductivity is studied in a mean-field approximation for a generic microscopic Hamiltonian with short-range repulsion and near-neighbor attraction. In the presence of competing microscopic interactions, the phase boundaries of antiferromagnetic and superconducting states are significantly modified in some region of the doping-temperature plane. The transition between superconductivity and antiferromagnetism occurs through a phase where both order parameters coexist with a third, dynamically generated, spin-triplet amplitude. This dynamical generation of a new order parameter is not restricted to a system with antiferromagnetism and d-wave superconductivity, but is a generic feature for fermionic systems. The dynamically generated spin-triplet order parameter is found to be robust to variations in the mean-field Hamiltonian.

cond-mat.str-el↗

arXiv1999

Pseudogap and photoemission spectra in the attractive Hubbard model

Angle-resolved photoemission spectra are calculated microscopically for the two-dimensional attractive Hubbard model. A system of self-consistent T-matrix equations are solved numerically in the real-time domain. The single-particle spectral function has a two-peak structure resulting from the presense of bound states. The spectral function is suppressed at the chemical potential, leading to a pseudogap-like behavior. At high temperatures and densities the pseudogap diminishes and finally disappears; these findings are similar to experimental observations for the cuprates.

cond-mat.str-el↗

arXiv1998

Evolution of Hole and Spin Dynamics in High Temperature Superconductors within the Small Hole Density Limit of the t-J Model

The evolution of hole and spin dynamics in high temperature superconductors is studied within the self-consistent noncrossing approximation of the t-J model in the small hole density limit. As the doping concentration is increased, long-range electron correlations disappear rapidly and the quasiparticle energy band becomes considerably narrow. At a small hole density long-range antiferromagnetic order is destroyed leading to the inadequacy of spin wave basis approximation near small wave vectors. Spin excitations near the antiferromagnetic zone boundary are strongly renormalized and damped but they are still well described within spin wave basis approximation.

cond-mat↗

arXiv1998

Precursors of Antiferromagnetic and Hubbard Bands in the 2D Hubbard Model

We formulate a theory to the 2D Hubbard model in a framework free of finite size effect and numerical analytical continuation, yet containing the essential features of the 2D Hubbard model, i.e., the correct atomic limit for large frequencies and 2D spin fluctuations. As temperature is decreased for a 2D half-filled band, 2D critical fluctuations give rise to a strong local maximum in the scattering rates at the chemical potential leading to a split peak in the spectral function. As U is increased, four peaks associated with antiferromagnetic and Hubbard bands begin to develop in small and intermediate frequency regimes.

cond-mat↗

arXiv1998

Destruction of Strong Critical Spin Fluctuations by Doping in the 2D Hubbard Model

We present the doping dependence of the spectral functions, density of states and low frequency behavior of the self-energy for the 2D Hubbard model on the basis of our recently developed theory for the Hubbard model. Strong 2D critical spin fluctuations dominating near half-filling are completely destroyed by 13 and 20 percent doping concentrations for U=4 and 8, respectively. Below these concentrations the imaginary part of the self-energy vanishes quadratically in frequency near the Fermi energy, a characteristic feature for the Fermi liquid.

cond-mat↗

arXiv1998

Band Kondo Effect in the Doped 2D Hubbard Model

We present strong numerical evidence for the band Kondo effect in the doped 2D Hubbard model by showing the systematic change of the density of states, imaginary part of the self-energy, effective magnetic moment and quasiparticle residue upon decreasing the temperature. Quadratically vanishing (in frequency) scattering rates near the Fermi energy and a linearly vanishing (in temperature) effective magnetic moment at low temperatures strongly support the screening of magnetic moments from singly occupied electrons by doped holes in the 2D Hubbard model.

cond-mat↗

arXiv1998

Density-Induced Breaking of Pairs in the Attractive Hubbard Model

A conserving T-matrix approximation is applied to the two-dimensional attractive Hubbard model in the low-density regime. A set of self-consistent equations is solved in the real-frequency domain to avoid the analytic continuation procedure. By tuning the chemical potential the particle density was varied in the limits 0.01 < n < 0.18. For the value of the attractive potential U=8t the binding energy of pairs monotonically decreases with increasing n, from its zero-density limit 2.3t and vanishes at a critical density n=0.19. A pairing-induced pseudogap in the single-particle density of states is found at low densities and temperatures.

cond-mat↗

arXiv1998

Formation of a Heavy-Fermion State in the 2D Periodic Anderson Model

We study the formation of a heavy-fermion state in the 2D periodic Anerson model. For U=2, the density of states, imaginary part of the self-energy and effective magnetic moment all indicate the Kondo screening of local f electrons, leading to a coherent heavy-fermion state. For U=3 and 4, the dominance of RKKY interaction over Kondo screening at low temperatures indicates a magnetic instability at zero temperature. A partial screening of magnetic moments, however, still gives rise to a relatively sharp peak at the Fermi energy in the density of states.

cond-mat↗