arXiv subjects

Ki-Seok Kim

Publications and source records attributed to Ki-Seok Kim.

At least 73 records · Page 4Linked to original sources

arXiv2016

Emergent geometry from field theory: Wilson's renormalization group revisited

We find a geometrical description from a field theoretical setup based on Wilson's renormalization group in real space. We show that renormalization group equations of coupling parameters encode the metric structure of an emergent curved space, regarded to be an Einstein equation for the emergent gravity. Self-consistent equations of local order-parameter fields with an emergent metric turn out to describe low energy dynamics of a strongly coupled field theory, analogous to the Maxwell equation of the Einstein-Maxwell theory in the AdS$_{d+2}$/CFT$_{d+1}$ duality conjecture. We claim that the AdS$_{3}$/CFT$_{2}$ duality may be interpreted as Landau-Ginzburg theory combined with Wilson's renormalization group, which introduces vertex corrections into the Landau-Ginzburg theory in the large$-N_{s}$ limit, where $N_{s}$ is the number of fermion flavors.

hep-th↗

arXiv2016

Metal-Insulator Transitions and non-Fermi Liquid Behaviors in 5d Perovskite Iridates

Transition metal oxides, in particular, 3d or 4d perovskites have provided diverse emergent physics that originates from the coupling of various degrees of freedom such as spin, lattice, charge, orbital, and also disorder. 5d perovskites form a distinct class because they have strong spin-orbit coupling that introduces to the system an additional energy scale that is comparable to bandwidth and Coulomb correlation. Consequent new physics includes novel Jeff = 1/2 Mott insulators, metal-insulator transitions, spin liquids, and topological insulators. After highlighting some of the phenomena appearing in Ruddlesden-Popper iridate series Srn+1IrnO3n+1, we focus on the transport properties of perovskite SrIrO3. Using epitaxial thin films on various substrates, we demonstrate that metal-insulator transitions can be induced in perovskite SrIrO3 by reducing its thickness or by imposing compressive strain. The metal-insulator transition driven by thickness reduction is due to disorder, but the metal-insulator transition driven by compressive strain is accompanied by peculiar non-Fermi liquid behaviors, possibly due to the delicate interplay between correlation, disorder, and spin-orbit coupling. We examine various theoretical frameworks to understand the non-Fermi liquid physics and metal-insulator transition that occurs in SrIrO3 and offer the Mott-Anderson-Griffiths scenario as a possible solution.

cond-mat.str-el↗

arXiv2016

Distribution of critical temperature at Anderson localization

Based on a local mean-field theory approach at Anderson localization, we find a distribution function of critical temperature from that of disorder. An essential point of this local mean-field theory approach is that the information of the wave-function multifractality is introduced. The distribution function of the Kondo temperature ($T_{K}$) shows a power-law tail in the limit of $T_{K} \rightarrow 0$ regardless of the Kondo coupling constant. We also find that the distribution function of the ferromagnetic transition temperature ($T_{c}$) gives a power-law behavior in the limit of $T_{c} \rightarrow 0$ when an interaction parameter for ferromagnetic instability lies below a critical value. However, the $T_{c}$ distribution function stops the power-law increasing behavior in the $T_{c} \rightarrow 0$ limit and vanishes beyond the critical interaction parameter inside the ferromagnetic phase. These results imply that the typical Kondo temperature given by a geometric average always vanishes due to finite density of the distribution function in the $T_{K} \rightarrow 0$ limit while the typical ferromagnetic transition temperature shows a phase transition at the critical interaction parameter. We propose that the typical transition temperature serves a criterion for quantum Griffiths phenomena vs. smeared transitions: Quantum Griffiths phenomena occur above the typical value of the critical temperature while smeared phase transitions result at low temperatures below the typical transition temperature. We speculate that the ferromagnetic transition at Anderson localization shows the evolution from quantum Griffiths phenomena to smeared transitions around the critical interaction parameter at low temperatures.

cond-mat.dis-nn↗

arXiv2015

Interplay between disorder and inversion symmetry: Extreme enhancement of the mobility near the Weyl point in BiTeI

We show experimental and theoretical evidence that BiTeI hosts a novel disordered metallic state named diffusive helical Fermi liquid (DHFL), characterized by a pair of concentric spin-chiral Fermi surfaces with negligible inter-valley scattering. Key experimental observations are extreme disparity of the mobility between inner and outer helical Fermi surfaces near the Weyl point and existence of the so called universal scaling behavior for the Hall resistivity. Although the extreme enhancement of the inner-Fermi-surface mobility near the Weyl point is quantitatively explained within the self-consistent Born approximation, the existence of universal scaling in the Hall resistivity shows its breakdown, implying necessity of mass renormalization in the inner Fermi-surface beyond the independent electron picture.

cond-mat.str-el↗

arXiv2015

Emergence of non-Fermi liquid behaviors in 5d perovskite SrIrO3 thin films: interplay between correlation, disorder, and spin-orbit coupling

We investigate the effects of compressive strain on the electrical resistivity of 5d iridium based perovskite SrIrO3 by depositing epitaxial films of thickness 35 nm on various substrates such as GdScO3 (110), DyScO3 (110), and SrTiO3 (001). Surprisingly, we find anomalous transport behaviors in the tempeature dependent resistivity, where the temperature exponent evolves continuously from 4/5 to 1 and to 3/2 with an increase of compressive strain. Furthermore, magnetoresistance always remains positive irrespective of resistivity upturns at low temperatures. These observations imply that the delicate interplay between correlation and disorder in the presence of strong spin-orbit coupling is responsible for the emergence of the non-Fermi liquid behaviors in 5d perovskite SrIrO3 thin films. We offer a theoretical framework for the interpretation of the experimental results.

cond-mat.str-el↗

arXiv2015

Nanoscale manipulation of the Mott insulating state coupled to charge order in 1T-TaS2

Quantum states of strongly correlated electrons are of prime importance to understand exotic properties of condensed matter systems and the controllability over those states promises unique electronic devices such as a Mott memory. As a recent example, a ultrafast switching device was demonstrated using the transition between the correlated Mott insulating state and a hidden-order metallic state of a layered transition metal dichalcogenides 1T-TaS2. However, the origin of the hidden metallic state was not clear and only the macroscopic switching by laser pulse and carrier injection was reported. Here, we demonstrate the nanoscale manipulation of the Mott insulating state of 1T-TaS2. The electron pulse from a scanning tunneling microscope switches the insulating phase locally into a metallic phase which is textured with irregular domain walls in the charge density wave (CDW) order inherent to this Mott state. The metallic state is a novel correlated phase near the Mott criticality with a coherent feature at the Fermi energy, which is induced by the moderate reduction of electron correlation due to the decoherence in CDW. This work paves the avenue toward novel nanoscale electronic devices based on correlated electrons.

cond-mat.mtrl-sci↗

arXiv2015

Interplay of electron-electron and electron-phonon interactions in the low temperature phase of 1T-TaS2

We investigate the interplay of the electron-electron and electron-phonon interactions in the electronic structure of an exotic insulating state in the layered dichalcogenide 1T-TaS2, where the charge-density-wave (CDW) order coexists with a Mott correlation gap. Scanning tunneling microscopy and spectroscopy measurements with high spatial and energy resolution determine unambiguously the CDW and the Mott gap as 0.20-0.24 eV and 0.32 eV, respectively, through the real space electron phases measured across the multiply formed energy gaps. An unusual local reduction of the Mott gap is observed on the defect site, which indicates the renormalization of the on-site Coulomb interaction by the electron-phonon coupling as predicted by the Hubbard-Holstein model. The Mott-gap renormalization provides new insight into the disorder-induced quasi-metallic phases of 1T-TaS2.

cond-mat.mtrl-sci↗

arXiv2015

Entangled polymer complex as Higgs phenomena

We derive an effective Maxwell-London equation for entangled polymer complex under the topological constraint, borrowing the theoretical framework from the topological field theory. We find that the transverse current flux of the test polymer chain, surrounded with entangled chains, decays exponentially from its average position with finite penetration depth, which is analogous to the magnetic-field decay in a superconductor (SC). Like as the mass acquirement of photons in SC is the origin of the magnetic-field decay, the polymer earns uncrossible intersections along the chain due to the preserved linking number, which restricts the deviation of the transverse polymer current in the normal direction. Interestingly, this picture is well incorporated with the most successful phenomenological theory of the so called tube model, of which researchers have long pursued its microscopic origin. The correspondence of our equation of motion to the tube model claims that the confining tube potential is a consequence of the topological constraint (linking number). The tube radius is attributed to the decay length, and increasing the retracting force at intersections or increasing the number of intersections (linking number), the tube becomes narrow and tighter. It further shows that the probability of the tube leakage decays exponentially with the decay length of tube radius.

cond-mat.soft↗

arXiv2015

The Kondo effect revisited: RG-improved perturbation theory based on the Schwinger-boson representation

Resorting to the Schwinger-boson representation for the description of a localized magnetic-impurity state, we develop an RG-improved (renormalization group) perturbation theory for the Kondo effect. This Schwinger-boson based RG-improved perturbation theory covers the whole temperature range from a decoupled local moment state to a local Fermi-liquid state through the crossover temperature regime, shown from the specific heat and spin susceptibility of the magnetic impurity. The Schwinger-boson based RG-improved perturbation theory makes the strong coupling fixed point at IR (infrared) accessible from the gaussian one at UV (ultraviolet) within the perturbation framework, regarded to be complementary to the Schwinger-boson based NCA (non-crossing approximation) self-consistent theory [Phys. Rev. Lett. {\bf 96}, 016601 (2006)]. The existence of the perturbatively accessible strong coupling fixed point implies the nature on the statistics of spinons, not determined by hands but chosen by the nature of strongly coupled systems.

cond-mat.str-el↗

arXiv2015

Critical field theory for ferromagnetic quantum criticality in the strong coupling regime of Hertz-Moriya-Millis theory

We develop U(1) slave spin-rotor theory, suggesting a metal-metal transition from Landau's Fermi-liquid state to a bad metal phase, as U(1) slave charge-rotor theory [Phys. Rev. B {\bf 70}, 035114 (2004)] describes a metal-insulator transition from Landau's Fermi-liquid state to a spin-liquid phase. U(1) slave spin-rotor formulation allows us to generalize Hertz-Moriya-Millis theory for ferromagnetic quantum phase transitions, replacing Landau's Fermi-liquid state with an incoherent metallic phase. As a result, we argue that localized magnetic moments emerge to govern quantum critical physics in bad metals.

cond-mat.str-el↗

arXiv2015

Emergent gauge fields and their nonperturbative effects in correlated electrons

The history of modern condensed matter physics may be regarded as the competition and reconciliation between Stoner's and Anderson's physical pictures, where the former is based on momentum-space descriptions focusing on long wave-length fluctuations while the latter is based on real-space physics emphasizing emergent localized excitations. In particular, these two view points compete with each other in various nonperturbative phenomena, which range from the problem of high T$_{c}$ superconductivity, quantum spin liquids in organic materials and frustrated spin systems, heavy-fermion quantum criticality, metal-insulator transitions in correlated electron systems such as doped silicons and two-dimensional electron systems, the fractional quantum Hall effect, to the recently discussed Fe-based superconductors. An approach to reconcile these competing frameworks is to introduce topologically nontrivial excitations into the Stoner's description, which appear to be localized in either space or time and sometimes both, where scattering between itinerant electrons and topological excitations such as skyrmions, vortices, various forms of instantons, emergent magnetic monopoles, and etc. may catch nonperturbative local physics beyond the Stoner's paradigm. In this review article we discuss nonperturbative effects of topological excitations on dynamics of correlated electrons. ......

cond-mat.str-el↗

arXiv2014

Metal insulator transitions in perovskite SrIrO3 thin films

Understanding of metal insulator transitions in a strongly correlated system, driven by Anderson localization (disorder) and/or Mott localization (correlation), is a long standing problem in condensed matter physics. The prevailing fundamental question would be how these two mechanisms contrive to accomplish emergent anomalous behaviors. Here, we have grown high quality perovskite SrIrO3 thin films, containing a strong spin orbit coupled 5d element Ir, on various substrates such as GdScO3 (110), DyScO3 (110), SrTiO3 (001), and NdGaO3 (110) with increasing lattice mismatch, in order to carry out a systematic study on the transport properties. We found that metal insulator transitions can be induced in this system; by either reducing thickness (on best lattice matched substrate) or changing degree of lattice strain (by lattice mismatch between film and substrates) of films. Surprisingly these two pathways seek two distinct types of metal insulator transitions; the former falls into disorder driven Anderson type whereas the latter turns out to be of unconventional Mott-Anderson type with the interplay of disorder and correlation. More interestingly, in the metallic phases of SrIrO3, unusual non-Fermi liquid characteristics emerge in resistivity as the resistivity exponent evolves from from 4/5 to 1 to 3/2 with increasing lattice strain. We discuss theoretical implications of these phenomena to shed light on the metal insulator transitions.

cond-mat.str-el↗

arXiv2014

Anomalous transport phenomena in Weyl metal beyond the Drude model for Landau's Fermi liquids

Landau's Fermi-liquid theory is the standard model for metals, characterized by the existence of electron quasiparticles near a Fermi surface as long as Landau's interaction parameters lie below critical values for instabilities. Recently, this fundamental paradigm has been challenged by physics of strong spin-orbit coupling although the concept of electron quasiparticles remains valid near the Fermi surface, where the Landau's Fermi-liquid theory fails to describe electromagnetic properties of this novel metallic state, referred to as Weyl metal. A novel ingredient is that such a Fermi surface encloses a Weyl point with definite chirality, referred to as a chiral Fermi surface, which can arise from breaking of either time reversal or inversion symmetry in systems with strong spin-orbit coupling, responsible for both Berry curvature and chiral anomaly. As a result, electromagnetic properties of the Weyl metallic state are described not by conventional Maxwell equations but by axion electrodynamics, where Maxwell equations are modified with a topological-in-origin spatially modulated $θ(\bm{r}) \bm{E} \cdot \bm{B}$ term. This novel metallic state has been realized recently in Bi$_{1-x}$Sb$_{x}$ around $x \sim 3%$ under magnetic fields, where the Dirac spectrum appears around the critical point between the normal semiconducting ($x < 3%$) and topological semiconducting phases ($x > 3%$) and the time reversal symmetry breaking perturbation causes the Dirac point to split into a pair of Weyl points along the direction of the applied magnetic field for such a strong spin-orbit coupled system. In this review article, we discuss how the topological structure of both the Berry curvature and chiral anomaly (axion electrodynamics) gives rise to anomalous transport phenomena in Bi$_{1-x}$Sb$_{x}$ around $x \sim 3%$ under magnetic fields, modifying the Drude model of Landau's Fermi liquids.

cond-mat.mes-hall↗

arXiv2014

Role of axion electrodynamics in Weyl metal: Violation of Wiedemann-Franz law

Recently, enhancement of the longitudinal magneto-electrical conductivity (LMEC) has been observed in Bi$_{1-x}$Sb$_{x}$ around $x \sim 3\%$ under $\bm{E} \parallel \bm{B}$ ($\bm{E}$: external electric field and $\bm{B}$: external magnetic field) [Phys. Rev. Lett. {\bf 111}, 246603 (2013)], where an enhancement factor proportional to $B^{2}$ is suggested to result from the $\bm{E} \cdot \bm{B}$ term. In the present study, we show that this $B^{2}$ enhancement is not limited on the LMEC, where both the Seebeck and thermal conductivities in the longitudinal setup ($\bm{E} \parallel \bm{B}$) are predicted to show essentially the same enhancement proportional to $B^{2}$. In particular, the $B^{2}$ enhancement factor of the LMEC turns out to differ from that of the longitudinal thermal conductivity, responsible for breakdown of Wiedemann-Franz (WF) law, which means that anomalous currents flowing through the dissipationless channel differ from each other. Since the breakdown of the WF law appears in spite of the existence of electron quasiparticles, regarded to be a purely topological character (chiral anomaly), the Weyl metallic state cannot be identified with the Landau's Fermi-liquid fixed point. We propose the violation of the WF law as another hallmark of the Weyl metallic phase, which originates from axion electrodynamics.

cond-mat.str-el↗

arXiv2014

Dilute magnetic topological semiconductors: What's new beyond the physics of dilute magnetic semiconductors?

Role of localized magnetic moments in metal-insulator transitions lies at the heart of modern condensed matter physics, for example, the mechanism of high T$_{c}$ superconductivity, the nature of non-Fermi liquid physics near heavy fermion quantum criticality, the problem of metal-insulator transitions in doped semiconductors, and etc. Dilute magnetic semiconductors have been studied for more than twenty years, achieving spin polarized electric currents in spite of low Curie temperatures. Replacing semiconductors with topological insulators, we propose the problem of dilute magnetic topological semiconductors. Increasing disorder strength which corresponds to the size distribution of ferromagnetic clusters, we suggest a novel disordered metallic state, where Weyl metallic islands appear to form inhomogeneous mixtures with topological insulating phases. Performing the renormalization group analysis combined with experimental results, we propose a phase diagram in $(λ_{so},Γ,T)$, where the spin-orbit coupling $λ_{so}$ controls a topological phase transition from a topological semiconductor to a semiconductor with temperature $T$ and the distribution for ferromagnetic clusters $Γ$ gives rise to a novel insulator-metal transition from either a topological insulating or band insulating phase to an inhomogeneously distributed Weyl metallic state with such insulating islands. Since electromagnetic properties in Weyl metal are described by axion electrodynamics, the role of random axion electrodynamics in transport phenomena casts an interesting problem beyond the physics of percolation in conventional disorder-driven metal-insulator transitions. We also discuss how to verify such inhomogeneous mixtures based on atomic force microscopy.

cond-mat.dis-nn↗

arXiv2014

Emergence of localized magnetic moments near antiferromagnetic quantum criticality

We revisit an antiferromagnetic quantum phase transition with $\bm{Q} = 2 \bm{k}_{F}$, where $\bm{Q}$ is an ordering wave vector and $\bm{k}_{F}$ is a Fermi momentum. Reformulating the Hertz-Moriya-Millis theory within the strong coupling approach to diagonalize the spin-fermion coupling term and performing the scaling analysis for an effective field theory with quantum corrections in the Eliashberg approximation, we propose a novel interacting fixed point for this antiferromagnetic quantum phase transition, where antiferromagnetic spin fluctuations become locally critical to interact with renormalized electrons. The emergence of local quantum criticality suggests a mechanism of $ω/T$ scaling for antiferromagnetic quantum criticality, generally forbidden in the context of the Hertz-Moriya-Millis theory.

cond-mat.str-el↗

arXiv2014

Boltzmann-equation approach to anomalous transport in a Weyl metal

Weyl metal is regarded as a platform toward interacting topological states of matter, where its topological structure gives rise to anomalous transport phenomena, referred to as chiral magnetic effect and "negative" magneto-resistivity, the origin of which is chiral anomaly. Recently, the negative magneto-resistivity has been observed with the signature of weak anti-localization at $x = 3 \sim 4 ~ \%$ in Bi$_{1-x}$Sb$_{x}$, where magnetic field is applied in parallel with electric field. Based on the Boltzmann-equation approach, we find the negative magneto-resistivity in the presence of weak anti-localization. An essential ingredient is to introduce the topological structure of chiral anomaly into the Boltzmann-equation approach, resorting to semi-classical equations of motion with Berry curvature.

cond-mat.str-el↗

arXiv2013

Stoner instability revisited: Emergence of local quantum criticality?

We revisit Stoner instability, an old problem but in a modern point of view. An idea is to extract out dynamics of directional fluctuations of spins explicitly, resorting to the CP$^{1}$ representation and integrating over their amplitude fluctuations. As a result, we derive an effective field theory for ferromagnetic quantum phase transitions in terms of bosonic spinons and fermionic holons. We show that this effective field theory reproduces overdamped spin dynamics in a paramagnetic Fermi liquid and magnon spectrum in a ferromagnetic Fermi liquid. An interesting observation is that the velocity of spinons becomes zero, approaching the ferromagnetic quantum critical point, which implies emergence of local quantum criticality. Based on this scenario, we predict the $ω/T$ scaling behavior near ferromagnetic quantum criticality beyond the conventional scenario of the weak-coupling approach.

cond-mat.str-el↗