arXiv subjects

Sung-Sik Lee

Publications and source records attributed to Sung-Sik Lee.

At least 73 records · Page 4Linked to original sources

arXiv2006

Amperean Pairing Instability in the U(1) Spin Liquid State with Fermi Surface and Application to $κ-(BEDT-TTF)_2 Cu_2 (CN)_3$

Recent experiments on the organic compound $κ-(BEDT-TTF)_2 Cu_2 (CN)_3$ raise the possibility that the system may be described as a quantum spin liquid. Here we propose a pairing state caused by the `Amperean' attractive interaction between spinons on a Fermi surface mediated by the U(1) gauge field. We show that this state can explain many of the observed low temperature phenomena and discuss testable consequences.

cond-mat.str-el↗

arXiv2006

Observation of emergent photon in Monte Carlo simulation of exciton condensate

Using Monte Carlo simulation, we studied fractionalized phases in a model of exciton bose condensate and found evidence for an emergent photon in a finite size system. The fractionalized phase is a meta-stable Coulomb phase where an emergent photon arises as a gapless collective excitation of the excitons. We also studied a possibility of spiral* phase where fractionalization and long range spiral order of the exciton condensate coexist.

cond-mat.str-el↗

arXiv2006

Vortex description of the fractionalized phase in exciton bose condensate

As a sequel to the previous work [Phys. Rev. B 72, 235104 (2005)] we present a vortex description of the fractionalized phase in exciton bose condensate. Magnetic flux line and monopole of the 3+1D emergent U(1) gauge theory are identified in the exciton picture. A bundle of vortex/anti-vortex pairs of all flavors of excitons corresponds to the magnetic flux line and a point at which the vortices and anti-vortices recombine is identified as magnetic monopole. This completes the magnetic sector of the low energy excitation in the fractionalized phase.

cond-mat.str-el↗

arXiv2005

A dual gravity study of the 2+1D compact U(1) gauge theory coupled with strongly interacting matter fields

We consider the D2-brane probe action in the gravity background dual to N coincident Dp-branes by treating the separation between the D2- and Dp-branes as a nondynamical parameter for $p=2,4,6$. The gauge coupling, the core size of a non-BPS instanton and the mass gap of the compact U(1) gauge theory in the D2-brane are determined as a function of the separation in the type IIA gravity region. The results are interpreted in terms of the 2+1D U(1) gauge theory coupled with the matter fields which are also strongly coupled with the p+1D SU(N) gauge field. It is shown that strong coupling of the matter fields to the SU(N) gauge field can drastically modify their screening of the U(1) gauge field. The non-perturbative dependence of the U(1) gauge coupling on the energy scale is obtained.

hep-th↗

arXiv2005

Emergent U(1) gauge theory with fractionalized boson/fermion from the bose condensation of exciton in multi-band insulator

Fractionalized phases are studied in a low energy theory of exciton bose condensate in a multi-band insulator. It is shown that U(1) gauge theory with either fractionalized boson or fermion can emerge out of a single model depending on the coupling constants. Both the statistics and spin of the fractionalized particles are dynamically determined, satisfying the spin-statistics theorem in the continuum limit. We present two mutually consistent descriptions for the fractionalization. In the first approach, it is shown that fractionalized degree of freedom emerges from reduced phase space constrained by strong interaction and that the U(1) gauge field arises as a collective excitation of the low energy modes. In the second approach, complimentary descriptions are provided for the fractionalization based on world line picture of the original excitons. The emergent gauge structure is identified from the fluctuating web of exciton world lines which, in turn, realizes the string net condensation in a space-time picture.

cond-mat.str-el↗

arXiv2005

U(1) Gauge Theory of the Hubbard Model : Spin Liquid States and Possible Application to k-(BEDT-TTF)_2 Cu_2 (CN)_3

We formulate a U(1) gauge theory of the Hubbard model in the slave-rotor representation. From this formalism it is argued that spin liquid phases may exist near the Mott transition in the Hubbard model on triangular and honeycomb lattices at half filling. The organic compound k-(BEDT-TTF)_2 Cu_2 (CN)_3 is a good candidate for the spin liquid state on a triangular lattice. We predict a highly unusual temperature dependence for the thermal conductivity of this material.

cond-mat.str-el↗

arXiv2004

Studies of doping and temperature dependences of superfluid weight and spectral function; Universal scaling behavior of pseudogap

Using the improved U(1) and SU(2) slave-boson approaches of the t-J Hamiltonian that we developed recently, we study the doping and temperature dependence of superfluid weight and spectral function and discuss our finding of the universal scaling behavior of pseudogap. It is shown that at low hole doping concentrations and at low temperatures there exists a propensity of a linear decrease in the superfluid weight with temperature, and a tendency of doping independence in the linearly decreasing slopes of the superfluid weight with temperature in qualitative agreement with the experiments. It is also demonstrated that there exists the boomerang behavior, that is, both Tc and the superfluid weight increase with hole doping concentration x in the underdoped region, reaches a saturation(maximum) at a hole doping above optimal doping and decreases beyond the saturation point in the overdoped region in agreement with mu-SR measurements. We further investigate the doping and temperature dependence of spectral function and discuss our theoretical finding of a scaling behavior of pseudogap. In addition we discuss the cause of hump and quasi-particle peak in the observed spectral functions of high Tc cuprates. It is demonstrated that the sharpening of the observed quasi-particle peak below Tc is attributed to the bose condensation of holon pair in agreement with observations. From the predicted ratios of pseudogap to both the superconducting temperature Tc and the pseudogap temperature T* respectively as a function of hole doping concentration x, we find that there exists a universal scaling behavior (sample independence) of these ratios with doping, by showing a nonlinearly decreasing behavior of the former and a near doping independence of the latter.

cond-mat.supr-con↗

arXiv2004

Quantum phase transitions in $d-wave$ superconductors

Based on an effective Lagrangian obtained from the holon-pair boson theory of Lee and Salk [Phys. Rev. B {\bf 64}, 052501 (2001)] for high $T_c$ cuprates, we explore physical states involved with quantum phase transitions around a critical hole doping of $d-wave$ superconductivity. We find a new quantum phase transition, a confinement to deconfinement transition for internal gauge charge in the superconducting phase. An antiferro- to para- magnetic transition in the superconducting state is explained in the context of the confinement to deconfinement transition. We obtain effective field theories in each region associated with the phase transitions.

cond-mat.str-el↗

arXiv2004

Superconductivity in the presence of antiferromagnetism for high Tc cuprates

Using the U(1) holon pair slave boson theory [Phys. Rev. B 64, 052501 2001)], we derive a low energy field theory of dealing with both d-wave superconductivity and antiferromagnetism for underdoped cuprates by constructing both the Cooper pair field and the chargon pair field. In terms of the internal gauge field, the Cooper pair field carries no internal charge while the chargon pair field carries the internal charge. They are decoupled in the low energy limit. This allows us to separately treat the XY model of the Cooper pair field to describe superconductivity and the Abelian Higgs model of the chargon pair field to describe antiferromagnetism in the presence of Dirac fermions at and near the d-wave nodal points. Thus we find that the d-wave superconductivity can coexist with antiferromagnetism and that despite the coexistence, the antiferromagnetism can not affect the superconducting transition, thus allowing the XY universality class in the extreme type II limit.

cond-mat.supr-con↗

arXiv2004

Absence of time-reversal symmetry breaking in association with the order parameter of Cooper pair in high $T_c$ superconductivity

As an extension of our previous work on the holon pairing instability in the t-J Hamiltonian [Phys. Rev. B {\bf 66}, 054427 (2002)], we examine the orbital symmetries of holon pairing order parameters in high $T_c$ superconductivity by examining the energy poles of t-matrix. We find that both $s$- and d-wave symmetries in holon pair order parameter occur at low lying energy states corresponding to the higher energy poles of t-matrix while only the s-wave symmetry appears at the lowest energy pole and that this results in the d-wave symmetry in the Cooper pair order which is a composite of the holon pair of s-wave symmetry and the spinon pair of d-wave symmetry below $T_c$. Finally we demonstrate that there exists no time-reversal symmetry breaking in association with the Cooper pair order parameter.

cond-mat.supr-con↗

arXiv2003

Scaling behavior in the optical conductivity of the two dimensional systems of strongly correlated electrons based on the U(1) slave-boson approach to the t-J Hamiltonian

The U(1) holon-pair boson theory of Lee and Salk(Phys. Rev. B 64, 052501 (2001)) is applied to investigate the quantum scaling behavior of optical conductivity in the two dimensional systems of strongly correlated electrons. We examine the role of both the gauge field fluctuations and spin pair excitations on the w/T scaling behavior of the optical conductivity. It is shown that the gauge field fluctuations but not the spin pair excitations are responsible for the scaling behavior in the low frequency region w/T <<1. Importance for the contribution of the nodal spinons to the Drude peak is discussed. It is shown that the w/T scaling behavior is manifest in the low frequency region at low hole concentrations close to a critical concentration at which superconductivity arises at T=0K.

cond-mat.supr-con↗

arXiv2003

On the origin of the hump structure in the in-plane optical conductivity of high Tc cuprates based on a SU(2) slave-boson theory

An improved version of SU(2) slave-boson approach is applied to study the in-plane optical conductivity of the two dimensional systems of high Tc cuprates. We investigate the role of fluctuations of both the phase and amplitude of order parameters on the (Drude) peak-dip-hump structure in the in-plane conductivity as a function of hole doping concentration and temperature. The mid-infrared(MIR) hump in the in-plane optical conductivity is shown to originate from the antiferromagnetic spin fluctuations of short range(the amplitude fluctuations of spin singlet pairing order parameters), which is consistent with our previous U(1) study. However the inclusion of both the phase and amplitude fluctuations is shown to substantially improve the qualitative feature of the optical conductivity by showing substantially reduced Drude peak widths for entire doping range. Both the shift of the hump position to lower frequency and the growth of the hump peak height with increasing hole concentration is shown to be consistent with observations.

cond-mat.supr-con↗

arXiv2003

Doping and temperature dependence of superfluid weights for high Tc cuprates

Using the improved slave-boson approach of the t-J Hamiltonian [Phys. Rev. B 64, 052501 (2001)] that we developed recently, we report the hole doping and temperature dependence of the superfluid weight. It is shown that at low hole doping concentration x and at low temperatures T there exists a propensity of a linear decrease of the superfluid weight $n_s/m^*$ with temperature, and a tendency of doping independence in the slope of $\frac{n_s}{m^*}(x,T)$ vs. T in accordance of the relation $\frac{n_s}{m^*}(x,T) = \frac{n_s}{m^*}(x,0) - αT$ with $α$, a constant. It is also demonstrated that Tc increases with hole doping concentration x, reaches a saturation(maximum) at optimal doping and decreases with increasing $x$ and $n_s/m^*$ in the overdoped region. Such a reflex (decreasing) behavior of Tc is attributed to the weakening of coupling between the spin(spinon pair order) and charge(holon pair order) degrees of freedom in the overdoped region. All of these findings are in agreement with $μ$SR measurements.

cond-mat.supr-con↗

arXiv2003

Doping dependence of bose condensation energy and correlations with spectral peak intensity and superfluid weight in high Tc cuprates

Based on our recent holon-pair boson theory of the t-J Hamiltonian (Phys. Rev. B 64, 052501 (2001)) we report the doping dependence of the bose condensation energy, superfluid weight and spectral peak intensity. We find a universality of doping dependence in these physical quantities, by equally showing an arch shape in the variations of their magnitudes with the hole doping concentration. We find that all of these physical quantities scale well with the positive charge carrier (hole) density x, but not with the electron density 1-x for the entire range of hole doping. It is shown that the doping dependence of the condensation energy U at T = 0 K is given by the relation, $U(0) \approx αx^2 |Δ_0|^2$ with $Δ_0$, the pairing gap at 0 K and $α$, a constant.

cond-mat.supr-con↗

arXiv2002

Universal scaling behavior of pseudogap with doping in high Tc cuprates; temperature and doping dependence of spectral intensity

Based on our improved SU(2) slave-boson approach (Phys. Rev. B 64, 052501(2001)) to the t-J Hamiltonian, we report a scaling behavior of pseudogap with doping and the temperature and doping dependence of spectral functions. In addition we discuss the cause of hump and quasi-particle peak in the observed spectral functions of high Tc cuprates. It is demonstrated that the sharpening of the observed quasi-particle peak below Tc is attributed to the bose condensation of holon pair. From the computed ratios of pseudogap $Δ_0$ to both the superconducting temperature Tc and the pseudogap temperature $T^*$ as a function of hole doping concentration x, we find that there exists a universal scaling of these ratios with doping, that is, the hyperbolic scaling behavior of the former, $\frac{2 Δ_0}{k_B T_c} \sim x^{-α}$ with $α\sim 2$ and near doping independence of the latter, $\frac{2 Δ_0}{k_B T^*} \approx 4 \sim 6$ are found.

cond-mat.supr-con↗

arXiv2001

Phase Separation in the Two-Dimensional Systems of Strongly Correlated electrons; the Role of Spin Singlet Pairs on Hole Pairing Contribution to Hole-rich Phase

By paying attention to the hole-doped two-dimensional systems of antiferromagnetically (strongly) correlated electrons, we discuss the cause of hole-rich phase formation in association with phase separation. We show that the phase diagram obtained from the Maxwell's construction in the plane of temperature vs. hole density is consistent with one derived from the evaluation of hole-rich and electron-rich phases in real space. We observe that the formation of a hole-rich phase is attributed to the aggregation of hole pairs induced by spin singlet pairs present in the pseudogap phase and that a direct involvement of correlations between hole pairs are not essential for phase separation.

cond-mat.str-el↗

arXiv2001

On the Origin of Peak-dip-hump Structure in the In-plane Optical Conductivity of the High $T_C$ Cuprates; Role of Antiferromagnetic Spin Fluctuations of Short Range Order

An improved U(1) slave-boson approach is applied to study the optical conductivity of the two dimensional systems of antiferromagnetically correlated electrons over a wide range of hole doping and temperature. Interplay between the spin and charge degrees of freedom is discussed to explain the origin of the peak-dip-hump structure in the in-plane conductivity of high $T_C$ cuprates. The role of spin fluctuations of short range order(spin singlet pair) is investigated. It is shown that the spin fluctuations of the short range order can cause the mid-infrared hump, by exhibiting a linear increase of the hump frequency with the antiferromagnetic Heisenberg coupling strength.

cond-mat.supr-con↗

arXiv2000

Superfluid weight vs. superconducting temperature based on a U(1) slave-boson approach to the t-J Hamiltonian

Based on an improved U(1) slave-boson approach to the t-J Hamiltonian, we invest igate a relationship between the superfluid weight $n_s/m^*$(the superconducting charge carrier density/the effective mass of the charge carrier) and the superc onducting temperature $T_c$. From the present study we find a linear increase of $n_s/m^*$ with $T_c$ with th e doping concentration in the underdoped region, a saturation around the optimal doping and a decrease in both $n_s/m^*$ and $T_c$ in the overdoped region. Such a trend of the `boomerang' shaped locus in $n_s/m^*$ vs. $T_c$ with increas ing doping concentration from the underdoped to the heavily overdoped region is predicted to be in complete agreement with muon-spin-relaxation measurements. The boomerang behavior is found to occur in correlation with reduction in the sp in singlet pairing(spinon pairing) order in the heavily overdoped region.

cond-mat.supr-con↗