arXiv subjects

Ki-Seok Kim

Publications and source records attributed to Ki-Seok Kim.

At least 127 records · Page 7Linked to original sources

arXiv2007

Heavy-fermion spin liquid in the strong hybridization limit of the finite-U Anderson lattice model

Studying the finite-U Anderson lattice model in the strong hybridization limit, we find a heavy-fermion spin liquid phase, where both conduction and localized fermions are strongly hybridized to form heavy fermions but this heavy-fermion phase corresponds to a symmetric Mott insulating state owing to the presence of charge gap, resulting from large Hubbard-U interactions in localized fermions. We show that this heavy-fermion spin liquid phase differs from the "fractionalized" Fermi liquid state, where the latter corresponds to a metallic state with a small Fermi surface of conduction electrons while localized fermions decouple from conduction electrons to form a spin liquid state. We discuss the stability of this anomalous spin liquid phase against antiferromagnetic ordering and gauge fluctuations, in particular, instanton effects associated with confinement of slave particles. Furthermore, we propose a variational wave function to check its existence from the microscopic model.

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arXiv2007

Competition between superconductivity and charge density waves

We derive an effective field theory for the competition between superconductivity (SC) and charge density waves (CDWs) by employing the SO(3) pseudospin representation of the SC and CDW order parameters. One important feature in the effective nonlinear $σ$ model is the emergence of Berry phase even at half filling, originating from the competition between SC and CDWs, i.e., the pseudospin symmetry. A well known conflict between the previous studies of Oshikawa\cite{Oshikawa} and D. H. Lee et al.\cite{DHLee} is resolved by the appearance of Berry phase. The Berry phase contribution allows a deconfined quantum critical point of fractionalized charge excitations with $e$ instead of $2e$ in the SC-CDW quantum transition at half filling. Furthermore, we investigate the stability of the deconfined quantum criticality against quenched randomness by performing a renormalization group analysis of an effective vortex action. We argue that although randomness results in a weak disorder fixed point differing from the original deconfined quantum critical point, deconfinement of the fractionalized charge excitations still survives at the disorder fixed point owing to a nonzero fixed point value of a vortex charge.

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arXiv2006

How to control pairing fluctuations: SU(2) slave-rotor gauge theory of the Hubbard model

We study how to incorporate Mott physics in the BCS-type superconductor, motivated from the fact that high $T_c$ superconductivity results from a Mott insulator via hole doping. The U(1) slave-rotor representation was proposed to take local density fluctuations into account non-perturbatively, describing the Mott-Hubbard transition at half filling. Since this decomposition cannot control local pairing fluctuations, the U(1) slave-rotor representation does not give a satisfactory treatment for charge fluctuations. Extending the U(1) slave-rotor representation, we introduce an SU(2) slave-rotor representation to allow not only local density fluctuations but also local pairing excitations. We find an SU(2) slave-rotor gauge theory of the Hubbard model in terms of two kinds of collective boson excitations associated with density and pairing fluctuations that interact with gapless fermion excitations via SU(2) gauge fluctuations. An interesting observation in this effective description is that phase fluctuations of fermion pairs arise as SU(2) gauge fluctuations. Thus, fermion-pairing excitations can be controlled by dynamics of collective bosons in the SU(2) slave-rotor gauge theory. Performing the standard saddle-point analysis based on the SU(2) slave-rotor action, we find ......

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arXiv2006

Antiferromagnetic metal to heavy-fermion metal quantum phase transition in the Kondo lattice model: A strong coupling approach

We study the quantum phase transition from an antiferromagnetic metal to a heavy fermion metal in the Kondo lattice model. Based on the strong coupling approach we {\it first} diagonalize the Kondo coupling term. Since this strong coupling approach makes the resulting Kondo term {\it relevant}, the Kondo hybridization persists even in the antiferromagnetic metal, indicating that fluctuations of Kondo singlets are not critical in the phase transition. We find that the quantum transition in our strong coupling approach results from {\it softening of antiferromagnetic spin fluctuations of localized spins}, driven by the Kondo interaction. Thus, the volume change of Fermi surface becomes continuous across the transition. ......

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arXiv2006

Quantum phase transition in one dimensional extended Kondo lattice model away from half filling

We study one dimensional {\it extended} Kondo lattice model, described by the $t-J$ Hamiltonian for conduction electrons away from half filling and the Heisenberg Hamiltonian for localized spins at half filling. Following Shankar,\cite{Shankar} we find an effective field theory for this model, where doped holes are represented by massless Dirac fermions (holons) and spin excitations are fractionalized into relativistic bosons (spinons). These holons and spinons interact via U(1) gauge fluctuations. Effects of Berry phase to the localized spins disappear due to the presence of Kondo couplings, causing the spinon excitations gapped. Furthermore, the gauge fluctuations are suppressed by hole doping. As a result, massive spinons are deconfined to arise in the localized spins unless the Kondo hybridization is strong enough. When the Kondo hybridization strength exceeds a certain value, we find that the localized spin chain becomes critical. This indicates that the present one dimensional Kondo lattice model exhibits a phase transition from a spin-gapped phase to a critical state in the localized spin chain, driven by the Kondo interaction.

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arXiv2006

SU(2) slave-rotor theory of the attractive Hubbard model

Extending the U(1) slave-rotor representation\cite{FG_SRR} of the repulsive Hubbard model, we propose an SU(2) slave-rotor decomposition for the attractive Hubbard model, where the SU(2) slave-rotor variables represent order parameter fluctuations associated with superconductivity and charge density wave. This decomposition method allows us to modify the standard Hartree-Fock mean field theory by incorporating order parameter fluctuations on an equal footing. Deriving an effective SU(2) slave-rotor action from the attractive Hubbard model, and analyzing it at the mean field level, we demonstrate a second order phase transition driven by softening of the slave-rotor variables.

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arXiv2006

SU(2) gauge theory of the Hubbard model: Emergence of an anomalous metallic phase near the Mott critical point

We propose one possible mechanism for an anomalous metallic phase appearing frequently in two spatial dimensions, that is, local pairing fluctuations. Introducing a pair-rotor representation to decompose bare electrons into collective pairing excitations and renormalized electrons, we derive an SU(2) gauge theory of the Hubbard model as an extended version of its U(1) gauge theory\cite{Florens,LeeLee} to allow only local density fluctuations. Since our effective SU(2) gauge theory admits two kinds of collective bosons corresponding to pair excitations and density fluctuations respectively, an intermediate phase appears naturally between the spin liquid Mott insulator and Fermi liquid metal of the U(1) gauge theory,\cite{Florens,LeeLee} characterized by softening of density-fluctuation modes as the Fermi liquid, but gapping of pair-excitation modes. We show that this intermediate phase is identified with an anomalous metallic phase because there are no electron-like quasiparticles although it is metallic.

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arXiv2006

Slave rotor theory of antiferromagnetic Hubbard model

The slave-rotor mean-field theory of Florens and Georges is generalized to the antiferromagnetic phase of the Hubbard model. An effective action consisting of a spin rotor and a fermion is derived and the corresponding saddle-point action is analyzed. Zero-temperature phase diagram of the antiferromagnetic Hubbard model is presented. While the magnetic phase persists for all values of the Hubbard interaction U, the single-particle spectral function exhibits a crossover into an incoherent phase when the magnetic moment m (and the corresponding U values) lies within a certain window m_c < m < 1-m_c, indicating a possible deviation from the Hartree-Fock theory.

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arXiv2006

Bandwidth-control vs. doping-control Mott transition in the Hubbard model

We reinvestigate the bandwidth-control and doping-control Mott transitions (BCMT and DCMT) from a spin liquid Mott insulator to a Fermi liquid metal based on the slave-rotor representation of the Hubbard model,\cite{Florens} where the Mott transitions are described by softening of bosonic collective excitations. We find that the nature of the insulating phase away from half filling is different from that of half filling in the respect that a charge density wave coexists with a topological order (spin liquid) away from half filling because the condensation of vortices generically breaks translational symmetry in the presence of "dual magnetic fields" resulting from hole doping while the topological order remains stable owing to gapless excitations near the Fermi surface. Performing a renormalization group analysis, we discuss the role of dissipative gauge fluctuations due to the Fermi surface in both the BCMT and the DCMT.

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arXiv2006

Role of disorder in the Mott-Hubbard transition

We investigate the role of disorder in the Mott-Hubbard transition based on the slave-rotor representation of the Hubbard model, where an electron is decomposed into a fermionic spinon for a spin degree of freedom and a bosonic rotor (chargon) for a charge degree of freedom. In the absence of disorder the Mott-Hubbard insulator is assumed to be the spin liquid Mott insulator in terms of gapless spinons near the Fermi surface and gapped chargons interacting via U(1) gauge fields. We found that the Mott-Hubbard critical point becomes unstable as soon as disorder is turned on. As a result, a disorder critical point appears to be identified with the spin liquid glass insulator to the Fermi liquid metal transition, where the spin liquid glass consists of the U(1) spin liquid and the chargon glass. We expect that glassy behaviors of charge fluctuations can be measured by the optical spectra in the insulating phase of an organic material $κ-(BEDT-TTF)_{2}Cu_{2}(CN)_{3}$. Furthermore, since the Mott-Anderson critical point depends on the spinon conductivity, universality in the critical exponents may not be found.

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arXiv2006

An effective Lagrangian for the continuous transition in an extended Kondo lattice model

We propose an effective Lagrangian for the continuous transition from the heavy fermion metal to the antiferromagnetic metal in an extended Kondo lattice model. Based on the slave-boson representation we introduce an additional new order parameter associated with difference of the chemical potential between conduction electrons $c_{iσ}$ and local spinons $f_{iσ}$. This order parameter allows pseudospin construction $T_{ix} = {1/2}<{c}_{iα}^{\dagger}f_{iα} + f_{iα}^{\dagger}c_{iα}>$, $T_{iy} = - \frac{i}{2}<{c}_{iα}^{\dagger}f_{iα} - f_{iα}^{\dagger}c_{iα}>$, and $T_{iz} = {1/2}<{c}_{iα}^{\dagger}c_{iα} - f_{iα}^{\dagger}f_{iα}>$, where $T_{i\pm} = T_{ix} \pm iT_{iy}$ corresponds to the usual hybridization order parameter in the slave-boson representation of the Kondo lattice model. The resulting effective action is shown to be an anisotropic pseudospin model with a Landau damping term for the screened-unscreened (XY$-$Ising) phase transition. To describe the emergence of antiferromagnetic order in the unscreened (Ising) phase, we phenomenologically introduce the antiferromagnetic Heisenberg model for the localized spins, where the effective coupling strength is given by $J_{eff} = J|<{T}_{iz}>|^{2}$. This ad-hoc construction allows the continuous transition from the heavy fermion phase to the antiferromagnetic phase because breakdown of Kondo screening ($<{T}_{i\pm}> = 0$ and $<{T}_{iz}> \not= 0$) causes effective exchange interactions between unscreened local moments.

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arXiv2005

Deconfinement in the presence of a Fermi surface

U(1) gauge theory of non-relativistic fermions interacting via compact U(1) gauge fields in the presence of a Fermi surface appears as an effective field theory in low dimensional quantum antiferromagnetism and heavy fermion liquids. We investigate deconfinement of fermions near the Fermi surface in the effective U(1) gauge theory. Our present analysis benchmarks the recent investigation of quantum electrodynamics in two space and one time dimensions ($QED_3$) by Hermele et al. [Phys. Rev. B {\bf 70}, 214437 (2004)]. Utilizing a renormalization group analysis, we show that the effective U(1) gauge theory with a Fermi surface has a stable charged fixed point. Remarkably, the renormalization group equation for an internal charge $e$ (the coupling strength between non-relativistic fermions and U(1) gauge fields) reveals that the conductivity $σ$ of fermions near the Fermi surface plays the same role as the flavor number $N$ of massless Dirac fermions in $QED_3$. This leads us to the conclusion that if the conductivity of fermions is sufficiently large, instanton excitations of U(1) gauge fields can be suppressed owing to critical fluctuations of the non-relativistic fermions at the charged fixed point. As a result a critical field theory of non-relativistic fermions interacting via noncompact U(1) gauge fields is obtained at the charged fixed point.

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arXiv2005

Deconfined quantum criticality in the two dimensional Kondo lattice model

We investigate the continuous quantum phase transition from an antiferromagnetic metal to a heavy fermion liquid based on the Kondo lattice model in two dimensions. We propose that antiferromagnetic spin fluctuations and conduction electrons fractionalize into neutral bosonic spinons and charged spinless fermions at the quantum critical point. This deconfined quantum criticality leads us to establish a critical field theory in terms of the fractionalized fields interacting via emergent U(1) gauge fields. The critical field theory not only predicts non-Fermi liquid physics near the quantum critical point but also recovers Fermi liquid physics away from the quantum critical point.

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arXiv2005

Deconfined Quantum Criticality at the Quantum Phase Transition from Antiferromagnetism to Algebraic Spin Liquid

We investigate the quantum phase transition from antiferromagnetism ($AF$) to algebraic spin liquid ($ASL$). {\it We propose that spin 1/2 fermionic spinons in the $ASL$ fractionalize into spin 1/2 bosonic spinons and spinless fermions at the quantum critical point ($QCP$) between the $AF$ and the $ASL$}. Condensation of the bosonic spinons leads to the $AF$, where the condensed bosonic spinons are confined with the spinless fermions to form the fermionic spinons. These fermionic spinons are also confined to make antiferromagnons as elementary excitations in the $AF$. {\it Approaching the $QCP$ from the $AF$, spin 1 critical antiferromagnetic fluctuations are expected to break up into spin 1/2 critical bosonic spinons. Then, these bosonic spinons hybridize with spin 1/2 fermionic spinons, making spinless fermions}. As a result the fermionic spinons decay into the bosonic spinons and the spinless fermions. But, the spinless fermions are confined and thus, only the bosonic spinons emerge at the $QCP$. This coincides with the recent studies of {\it deconfined quantum criticality}\cite{Laughlin_deconfinement,Senthil_deconfinement,Kim1,Ichinose_de confinement}. When the bosonic spinons are gapped, the $ASL$ is realized. The bosonic spinons are confined with the spinless fermions to form the fermionic spinons. These fermionic spinons are deconfined to describe the $ASL$.

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arXiv2005

Existence of Fermion Zero Modes and Deconfinement of Spinons in Quantum Antiferromagnetism resulting from Algebraic Spin Liquid

We investigate the quantum antiferromagnetism arising from algebraic spin liquid via spontaneous chiral symmetry breaking. We claim that in the antiferromagnet massive Dirac spinons can appear to make broad continuum spectrum at high energies in inelastic neutron scattering. The mechanism of spinon deconfinement results from the existence of fermion zero modes in single monopole potentials. Neel vectors can make a skyrmion configuration around a magnetic monopole of compact U(1) gauge fields. Remarkably, in the monopole-skyrmion composite potential the Dirac fermion is shown to have a zero mode. The emergence of the fermion zero mode forbids the condensation of monopoles, resulting in deconfinement of Dirac spinons in the quantum antiferromagnet.

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arXiv2005

Deconfined quantum criticality of the O(3) nonlinear $σ$ model in two spacial dimensions: A renormalization group study

We investigate the quantum phase transition of the O(3) nonlinear $σ$ model {\it without Berry phase} in two spacial dimensions. Utilizing the $CP^{1}$ representation of the nonlinear $σ$ model, we obtain an effective action in terms of bosonic spinons interacting via compact U(1) gauge fields. Based on the effective field theory, we find that the bosonic spinons are deconfined to emerge at the quantum critical point of the nonlinear $σ$ model. It is emphasized that the deconfinement of spinons is realized {\it in the absence of Berry phase}. This is in contrast to the previous study of Senthil et al. [Science {\bf 303}, 1490 (2004)], where the Berry phase plays a crucial role, resulting in the deconfinement of spinons. It is the reason why the deconfinement is obtained even in the absence of the Berry phase effect that the quantum critical point is described by the XY ("neutral") fixed point, not the IXY ("charged") fixed point. The IXY fixed point is shown to be unstable against instanton excitations and the instanton excitations are proliferated. {\it At the IXY fixed point it is the Berry phase effect that suppresses the instanton excitations, causing the deconfinement of spinons}. {\it On the other hand, the XY fixed point is found to be stable against instanton excitations because an effective internal charge is zero at the neutral XY fixed point}. As a result the deconfinement of spinons occurs at the quantum critical point of the O(3) nonlinear $σ$ model in two dimensions.

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arXiv2005

Effect of nonmagnetic disorder on criticality in the "dirty" U(1) spin liquid

We investigate the effect of nonmagnetic disorder on the stability of the algebraic spin liquid ($ASL$) by deriving an effective field theory, nonlinear $σ$ model ($NLσM$). We find that the anomalous critical exponent characterizing the criticality of the $ASL$ causes an anomalous gradient in the $NLσM$. We show that the sign of the anomalous gradient exponent or the critical exponent of the $ASL$ determines the stability of the "dirty" $ASL$. A positive exponent results in an unstable fixed point separating delocalized and localized phases, which is consistent with our previous study [Phys. Rev. B {\bf 70}, 140405 (2004)]. We find power law suppression for the density of spinon states in contrast to the logarithmic correction in the free Dirac theory. On the other hand, a negative exponent destabilizes the $ASL$, causing the Anderson localization. We discuss the implication of our study in the pseudogap phase of high $T_c$ cuprates.

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arXiv2005

Role of doped holes in a U(1) spin liquid

In the context of the SU(2) slave boson theory we show that condensation of holons can result in the zero mode of a nodal spinon in a single instanton potential. Instanton contribution in the presence of the zero mode induces the 't Hooft effective interaction, here mass to the spinon. We find that the spinon mass is determined by the state of instantons in the presence of the zero mode. The mass corresponds to antiferromagnetic moment of the nodal spinon. Considering the state of instantons, we discuss the possibility of coexistence between antiferromagnetism and $d-wave$ superconductivity in underdoped cuprates.

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