The previous theoretical study has shown that pulse irradiation to the Mott insulating state in the Hubbard model can induce the enhancement of superconducting correlation due to the generation of $η$ pairs. Here, we show that the same mechanism can be applied to the Kondo lattice model, an effective model for heavy electron systems, by demonstrating that the pulse irradiation indeed enhances the $η$-pairing correlation. As in the case of the Hubbard model, the non-linear optical process is essential to increase the number of photoinduced $η$ pairs and thus the enhancement of the superconducting correlation. We also find the diffusive behavior of the spin dynamics after the pulse irradiation, suggesting that the increase of the number of $η$ pairs leads to the decoupling between the conduction band and the localized spins in the Kondo lattice model, which is inseparably related to the photodoping effect.
The Kondo effect, which originates from the screening of a localized magnetic moment by a spin-spin interaction, is widely observed in non-artificial magnetic materials, artificial quantum dots, and carbon nanotubes. In devices based on quantum dots or carbon nanotubes that target quantum information applications, the Kondo effect can be tuned by a gate voltage, a magnetic field, or light. However, the manipulation of the Kondo effect in non-artificial materials has not been thoroughly studied; in particular, the artificial creation of the Kondo effect remains unexplored. Per this subject stud…
We theoretically predict the emergence of 120-degree spin order as a nonequilibrium steady state in the photodriven Kondo-lattice model on a triangular lattice. In the system away from the half filling with ferromagnetic ground state, the photoexcitation of conduction electrons and the photoinduced renormalization of bandwidth cause reconstruction of band structure and subsequent redistribution of the electrons through relaxations, which result in an electronic structure similar to that in the half-filled system at equilibrium, where the 120-degree spin order is stabilized. In this photoinduce…
Universität Greifswald（格赖夫斯瓦尔德大学）；Deutsches Zentrum für Luft- und Raumfahrt (DLR)（德国航空航天中心）；Friedrich-Alexander-Universität Erlangen-Nürnberg（埃尔朗根-纽伦堡大学）
By employing unbiased numerical methods, we show that pulse irradiation can induce unconventional superconductivity even in the Mott insulator of the Hubbard model. The superconductivity found here in the photoexcited state is due to the $η$-pairing mechanism, characterized by staggered pair-density-wave oscillations in the off-diagonal long-range correlation, and is absent in the ground-state phase diagram; i.e., it is induced neither by a change of the effective interaction of the Hubbard model nor by simple photocarrier doping. Because of the selection rule, we show that the nonlinear optic…
We have examined the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor PuCoGa5. The deduced electron-phonon coupling constant is incompatible with the measured superconducting transition temperature Tc=18.5 K, which speaks against phonon-mediated superconducting pairing. Upon lowering the temperature, we observe an order-of-magnitude increase of the quasiparticle relaxation time in agreement with the phonon bottleneck scenario - evidence for the presence of a hybridization gap in the electronic density of states. The modification of photoinduced reflectance in the s…
Employing the density-matrix renormalization group technique in the matrix-product-state representation, we investigate the photoexcited superconducting correlations induced by the $η$-pairing mechanism in the half-filled Hubbard chain. We estimate the characteristic pair correlation function and verify the accuracy of our numerical results by comparison with exact-diagonalization data for small systems. The optimal parameter set of the pump that most enhances the $η$-pair correlations, is calculated in the strong-coupling regime. For such a pump, we explore the possibility of quasi-long-range…
Recent experiments in quantum critical heavy fermion metals have pushed to the fore the question about whether antiferromagnetic fluctuations can promote both spin-singlet and spin-triplet superconductivity. Here we address the issue through non-perturbative calculations in antiferromagnetically correlated Kondo systems. We identify Kondo-destruction quantum critical points in both the SU(2) symmetric and Ising-anisotropic cluster Bose-Fermi Anderson models. The spin-singlet pairing correlations are significantly enhanced near the quantum critical point in the SU(2) case; however, with adequat…
The Kondo lattice model enlarged by an antiferromagnetic coupling $J_{\rm AF}$ between the localized spins is here investigated using computational techniques. Our results suggest the existence of a d-wave superconducting phase close to half-filling mediated by antiferromagnetic fluctuations. This establishes a closer connection between theory and heavy fermion experiments than currently provided by the standard Kondo lattice model with $J_{\rm AF}$=0.
We numerically prove photoinduced $η$-pairing in a half-filled fermionic Hubbard chain at both zero and finite temperature. The result, obtained by combining the matrix-product-state based infinite time-evolving block decimation technique and the purification method, applies to the thermodynamic limit. Exciting the Mott insulator by a laser electric field docked on via the Peierls phase, we track the time-evolution of the correlated many-body system and determine the optimal parameter set for which the nonlocal part of the $η$-pair correlation function becomes dominant during the laser pump at…
The Kondo lattice model plays a key role in our understanding of quantum materials, but a lack of small parameters has posed a long-standing problem. We present a 3 dimensional S= 1/2 Kondo lattice model describing a spin liquid within an electron sea. Strong correlations in the spin liquid are treated exactly, enabling a controlled analytical approach. Like a Peierls or BCS phase, a logarithmically divergent susceptibility leads to an instability into a new phase at arbitrarily small Kondo coupling. Our solution captures a plethora of emergent phenomena, including odd-frequency pairing, pair …
By employing the time-dependent exact diagonalization method, we investigate the photoexcited states of the excitonic insulator in the extended Falicov-Kimball model (EFKM). We here show that the pulse irradiation can induce the interband electron-electron pair correlation in the photoexcited states, while the excitonic electron-hole pair correlation in the initial ground state is strongly suppressed. We also show that the photoexcited states contains the eigenstates of the EFKM with a finite number of interband electron-electron pairs, which are responsible for the enhancement of the electron…
In the two-dimensional Kondo-Heisenberg lattice model away from half-filled, the local antiferromagnetic exchange coupling can provide the pairing mechanism of quasiparticles via the Kondo screening effect, leading to the heavy fermion superconductivity. We find that the pairing symmetry \textit{strongly} depends on the Fermi surface (FS) structure in the normal metallic state. When $J_{H}/J_{K}$ is very small, the FS is a small hole-like circle around the corner of the Brillouin zone, and the s-wave pairing symmetry has a lower ground state energy. For the intermediate coupling values of $J_{…
The Kondo lattice model describes a quantum phase transition between the antiferromagnetic state and heavy-fermion states. Applying the dual-fermion approach, we explore possible superconductivities emerging due to the critical antiferromagnetic fluctuations. The d-wave pairing is found to be the leading instability only in the weak-coupling regime. As the coupling is increased, we observe a change of the pairing symmetry into a p-wave spin-singlet pairing. The competing superconductivities are ascribed to crossover between small and large Fermi surfaces, which occurs with formation of heavy q…
Giant Resonant Enhancement of Photoinduced Dynamical Cooper Pairing, far above $T_c$
Harvard University（哈佛大学）；ETH Zürich（苏黎世联邦理工学院）；Max Planck Institute for the Physics of Complex Systems（马克斯·普朗克复杂系统物理研究所）；Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质结构动力学研究所）；University of Oxford（牛津大学）
Magnetic molecules adsorbed on a superconductor give rise to a local competition of Cooper pair and Kondo singlet formation inducing subgap bound states. For Manganese-phthalocyanine molecules on a Pb(111) substrate, scanning tunneling spectroscopy resolves pairs of subgap bound states and two Kondo screening channels. We show in a combined approach of scaling and numerical renormalization group calculations that the intriguing relation between Kondo screening and superconducting pairing is solely determined by the hybridization strength with the substrate. We demonstrate that an effective one…
Motivated by the unconventional superconductivity observed in heavy-fermion metals, we investigate pairing susceptibilities near a continuous quantum phase transition of the Kondo-destruction type. We solve two-impurity Bose-Fermi Anderson models with Ising and Heisenberg forms of the interimpurity exchange interaction using continuous-time quantum Monte-Carlo and numerical renormalization-group methods. Each model exhibits a Kondo-destruction quantum critical point separating Kondo-screened and local-moment phases. For antiferromagnetic interimpurity exchange interactions, singlet pairing is …
Inspired by the recent experimental evidence of antiferromagnetism and superconductivity coexistence in heavy fermion CeRhIn$_5$, we propose a fully microscopic approach based on the idea of real space pairing within the Anderson-Kondo lattice model. We present an overall phase diagram incorporating the emergence of a quantum critical point, where Kondo insulating (KI), antiferromagnetic (AF) and superconducting (SC) phases meet. We also obtain the Kondo insulating state with totally compensated magnetic moments as the parental state for the emerging SC phases. Furthermore, the coexistent (AF+…
The nature of unconventional superconductivity is intimately linked to the microscopic nature of the pairing interactions. In this work, motivated by cubic heavy fermion compounds with embedded multipolar moments, we theoretically investigate superconducting instabilities instigated by multipolar Kondo interactions. Employing multipolar fluctuations (mediated by RKKY interaction) coupled to conduction electrons via two-channel Kondo and novel multipolar Kondo interactions, we uncover a variety of superconducting states characterized by higher-angular momentum Cooper pairs, $J=0,1,2,3$. We demo…
Superconductivity and magnetism are usually the conflicting (competing) phenomena. We show, however, that in nanoscopic objects the electron pairing may promote the magnetic ordering. Such situation is possible at low temperatures in the quantum dots placed between the conducting and superconducting reservoirs, where the proximity induced electron pairing cooperates with the correlations enhancing the spin-exchange interactions. The emerging Kondo resonance, which is observable in the Andreev conductance, can be significantly enhanced by the coupling to superconducting lead. We explain this in…
We analyse here the role electrons on Anderson-U impurities play in superconductivity in a metal alloy. We find that phonon coupling at impurities counteracts the traditional effects which dominate T_c suppression in the non-magnetic limit. In some cases, we findt hat non-magnetic impurities can enhance T_c. In the Kondo limit, a Fermi liquid analysis reveals that it is the enhancement in the density of states arising from the Kondo resonancethat counteracts pair-weakening.
Utilizing the time-dependent density-matrix renormalization group technique, we numerically prove photoinduced pairing states in the extended Falicov-Kimball model (EFKM) at half filling, both with and without internal SU(2) symmetry. In the time-dependent photoemission spectra an extra band appears above the Fermi energy after pulse irradiation, indicating an insulator-to-metal transition. Even in the absence of the SU(2) structure, the pair correlations are enhanced during the pump, and afterwards decrease over time. This implies the possible metallization of Ta$_2$NiSe$_5$, a strong candida…
The appearance of unconventional superconductivity near heavy-fermion quantum critical points (QCPs) motivates investigation of pairing correlations close to a ``beyond Landau'' Kondo-destruction QCP. We focus on a two-Anderson-impurity cluster in which Kondo destruction is induced by a pseudogap in the conduction-electron density of states. Analysis via continuous-time quantum Monte-Carlo and the numerical renormalization group reveals a previously unstudied QCP that both displays the critical-local moment fluctuations characteristic of Kondo destruction and leads to a strongly enhanced singl…
Real-time dynamics techniques have proven increasingly useful in understanding strongly correlated systems both theoretically and experimentally. By employing unbiased time-resolved exact diagonalization, we study pump dynamics in the two-dimensional plaquette Hubbard model, where distinct hopping integrals $t_h$ and $t_h^\prime$ are present within and between plaquettes. In the intermediate coupling regime, a significant enhancement of $d$-wave superconductivity is observed and compared with that obtained by simple examination of expectation values with the eigenstates of the Hamiltonian. Our…
Mapping reservoir-enhanced superconductivity to near-long-range magnetic order in the undoped 1D Anderson- and Kondo-lattices
Heriot-Watt University（赫瑞-瓦特大学）；University of Geneva（日内瓦大学）；Weizmann Institute of Science（魏茨曼科学研究所）；Max-Planck-Institut für Physik komplexer Systeme（马克斯·普朗克复杂系统物理研究所）
The nature of superconductivity in heavy-fermion materials is a subject under intense debate, and controlling this many-body state is central for its eventual understanding. Here, we examine how proximity effects may change this phenomenon, by investigating the effects of an additional metallic layer on the top of a Kondo-lattice, and allowing for pairing in the former. We analyze a bilayer Kondo Lattice Model with an on-site Hubbard interaction, $-U$, on the additional layer, using a mean-field approach. For $U=0$, we notice a drastic change in the density-of-states due to multiple-orbital si…
The normal state of high-Tc cuprates has been considered one of the essential topics in high-temperature superconductivity research. However, compared to the high magnetic fields study of it, understanding a photoinduced normal state remains elusive. Here, we explore a photoinduced normal state of YBa$_2$Cu$_3$O$_{6.67}$ (YBCO) through a charge density wave (CDW) with time-resolved resonant soft x-ray scattering, as well as a high-magnetic field x-ray scattering. In the non-equilibrium state in which people predict a quenched superconducting state based on the previous optical spectroscopies, …
We apply a Gutzwiller-like variational technique to study Josephson conduction across a quantum dot with an odd number of electrons connected to two superconducting leads. Our method projects out all states on the dot but the Kondo singlet and is valid when Kondo correlations are dominant and no Andreev bound states localized at the dot are available for Kondo screening. In these conditions superconducting pairing is a competing effect and the junction is $π-$like, to optimize antiferromagnetic correlations on the dot. As the superconducting gap increases, the Josephson current also increases,…
We report results of low-temperature thermodynamic and transport measurements of Pb_{1-x}Tl_{x}Te single crystals for Tl concentrations up to the solubility limit of approximately x = 1.5%. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature T_K ~ 6 K. The observation of such an effect supports an electronic pairing mechanism …
We investigate the pairing symmetry of the Kondo-Heisenberg model on triangular lattice, which is believed to capture the core competition of Kondo screening and local magnetic exchange interaction in heavy electron compounds. On the dominant background of the heavy fermion state, the introduction of the Heisenberg antiferromagnetic interaction ($J_H$) leads to superconducting pairing instability. Depending on the strength of the interactions, it is found that the pairing symmetry prefers an extended $s$-wave for small $J_H$ and high conduction electron density but a chiral $d_{x^2-y^2}+id_{xy…
It is well established that a correlated quantum impurity embedded in a metallic host can form the many-body Kondo state with itinerant electrons due to the effective antiferromagnetic coupling. Such effect is manifested spectroscopically by a narrow Abrikosov-Suhl peak appearing at the Fermi level below a characteristic temperature T_K. Recent experiments using nanoscopic heterojunctions where the correlated quantum impurities (dots) are coupled to superconducting reservoirs revealed that the Kondo-type correlations are substantially weaker because: i) the single-particle states of supercondu…
The Andreev transport properties of double quantum dot based Cooper pair splitters with one superconducting and two normal leads are studied theoretically in the Kondo regime. The influence of the superconducting pairing correlations on the local density of states, Andreev transmission coefficient and Cooper pair splitting efficiency is thoroughly analyzed. It is shown that finite superconducting pairing potential quickly suppresses the SU(2) Kondo effect, which can however reemerge for relatively large values of coupling to superconductor. In the SU(4) Kondo regime, a crossover from the SU(4)…
We propose to use residual parafermions of the overscreened Kondo effect for topological quantum computation. A superconducting proximity gap of $Δ<T_K$ can be utilized to isolate the parafermion from the continuum of excitations and stabilize the non-trivial fixed point. We use weak-coupling renormalization group, dynamical large-N technique and bosonization to show that the residual entropy of multichannel Kondo impurities survives in a superconductor. We find that while (in agreement with recent numerical studies) the non-trivial fixed point is unstable against intra-channel pairing, it is …
The explanation of heavy-fermion superconductivity is a long-standing challenge to theory. It is commonly thought to be connected to non-local fluctuations of either spin or charge degrees of freedom and therefore of unconventional type. Here we present results for the Kondo-lattice model, a paradigmatic model to describe heavy-fermion compounds, obtained from dynamical mean-field theory which captures local correlation effects only. Unexpectedly, we find robust s-wave superconductivity in the heavy-fermion state. We argue that this novel type of pairing is tightly connected to the formation o…
The possible heavy fermion superconductivity is carefully reexamined in the two-dimensional Kondo lattice model with an antiferromagnetic Heisenberg superexchange between local magnetic moments. In order to establish an effective mean field theory in the limit of the paramagnetic heavy Fermi liquid and near the half-filling case, we find that the spinon singlet pairing from the local antiferromagnetic short-range correlations can reduce the ground state energy substantially. In the presence of the Kondo screening effect, the Cooper pairs between the conduction electrons is induced. Depending o…
Emergence of odd-frequency s-wave superconductivity is demonstrated in the two-channel Kondo lattice by means of the dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method. Around half filling of the conduction bands, divergence of an odd-frequency pairing susceptibility is found, which signals instability toward the superconductivity. The corresponding order parameter is equivalent to a staggered composite-pair amplitude with even frequencies, which involves both localized spins and conduction electrons. A model wave function is constructed for the composite …
A new type of Kondo effect peculiar to unconventional superconductors is studied theoretically by using the Wilson's numerical renormalization group method. In this case, an angular momentum of a Cooper pair plays an important role in the Kondo effect. It produces multichannel exchange couplings with a local spin. Here we focus on a $p_x +i p_y$-wave state which is a full gap system. The calculated impurity susceptibility shows that the local spin is almost quenched by the Kondo effect in the strong coupling region ($T_{\rm K}/Δ\to \infty$), while the ground state is always a spin doublet over…
We study the two-dimensional Kondo lattice model with an additional Heisenberg exchange between localized spins. In a first step we use mean-field theory with two order parameters. The first order parameter is a complex pairing amplitude between conduction electrons and localized spins which describes condensation of Kondo (or Zhang-Rice) singlets. A nonvanishing value implies that the localized spins contribute to the Fermi surface volume. The second order parameter describes singlet-pairing between the localized spins and competes with the Kondo-pairing order parameter. Reduction of the carr…
Fractionalized Fermi liquids (FL$^\ast$) have been introduced as non-Fermi-liquid metallic phases, characterized by coexisting electron-like charge carriers and local moments which itself form a fractionalized spin liquid. Here we investigate a Kondo lattice model on the honeycomb lattice with compass interactions among the local moments, a concrete model hosting FL$^\ast$ phases based on Kitaev's $\mathbb{Z}_2$ spin liquid. We characterize the FL$^\ast$ phases via perturbation theory, and we employ a Majorana-fermion mean-field theory to map out the full phase diagram. Most remarkably we find…
We discuss the odd-frequency pairing correlations discovered by Zachar, Kivelson and Emery (ZKE) in a one dimensional Kondo lattice. A lattice model that realizes the continuum theory of ZKE is introduced and the correlations it gives rise to are identified as odd-frequency singlet pairing. The excitation spectrum is found to contain a spin gap, and a much lower energy band of spinless excitations. We discuss how the power-law correlations realized in the ZKE model evolve into true long-range order when Kondo chains are weakly coupled together and tentatively suggest a way in which the higher …
We examine the internal structure of the heavy fermion condensate, showing that it necessarily involves a d-wave pair of quasiparticles on neighboring lattice sites, condensed in tandem with a composite pair of electrons bound to a local moment, within a single unit cell. These two components draw upon the antiferromagnetic and Kondo interactions to cooperatively enhance the superconducting transition temperature. The tandem condensate is electrostatically active, with a small electric quadrupole moment coupling to strain that is predicted to lead to a superconducting shift in the NQR frequenc…
We investigate in detail antiferromagnetic (AF) and superconducting (SC) phases as well as their coexistence in the two-dimensional Kondo lattice model on a square lattice, which is a paradigmatic model for heavy fermion materials. The results presented are mainly obtained using the variational cluster approximation (VCA) and are complemented by analytical findings for the equations of motion of pairing susceptibilities. A particularly interesting aspect is the possibility to have s-wave SC near half filling as reported by Bodensiek \textit{et al.} [Phys. Rev. Lett. \textbf{110}, 146406 (2013)…
Transverse-field muon spin rotation measurements in the vortex-lattice mixed state of the heavy-fermion (HF) superconductor PrOs4Sb12 yield a temperature dependence of the penetration depth indicative of an isotropic or nearly isotropic energy gap. This is not seen to date in any other HF superconductor and is a signature of isotropic pairing symmetry, possibly related to a novel nonmagnetic "quadrupolar Kondo" HF mechanism in PrOs4Sb12. The T = 0 relaxation rate σ_s(0) = 0.91(1) μs^-1 yields an estimated penetration depth λ(0) = 3440(20) Å, which is considerably shorter than in other HF super…
Quantum impurity models are prevalent throughout many body physics, providing some prime examples of strongly correlated systems. Aside from being of great interest in themselves they can provide deep insight into the effects of strong correlations in general. The classic example is the Kondo model wherein a magnetic impurity is screened at low energies by a non interacting metallic bath. Here we consider a magnetic impurity coupled to a quantum wire with pairing interaction which dynamically generates a mass gap. Using Bethe Ansatz we solve the system exactly finding that it exhibits both scr…
We study the non-local superconducting pairing of two interacting Anderson impurities, which has an instability near the quantum critical point from the competition between the Kondo effect and an antiferromagnetic inter-impurity spin exchange interaction. As revealed by the dynamics over the whole energy range, the superconducting pairing fluctuations acquire considerable strength from an energy scale much higher than the characteristic spin fluctuation scale while the low energy behaviors follow those of the staggered spin susceptibility. We argue that the glue to the superconducting pairing…
Spin-flip scattering from magnetic impurities has a strong pair-breaking effect in $s$-wave superconductors where increasing the concentration of impurities rapidly destroys superconductivity. For small Kondo temperature $T_K$ the destruction of superconductivity is preceded by the reentrant superconductivity at finite temperature range $T_{c2} < T < T_{c1}$, while the normal phase reappears at $T<T_{c2} \sim T_K$. Here we explore the superconducting phase in a periodic system modeled as the Anderson lattice with additional attractive on-site (Hubbard) interaction $g$ acting on the conduction …
We study the low energy spectrum of a correlated quantum dot embedded between the normal conducting and superconducting reservoirs and hybridized with the topological superconducting nanowire, hosting the Majorana end-modes. We investigate the leaking Majorana quasiparticle and inspect its interplay with the proximity induced on-dot pairing and correlations. In particular, we focus on the subgap Kondo effect near the quantum phase transition/crossover from the spinfull (doublet) to the spinless (BCS-type singlet) configurations. Treating the correlations perturbatively and within the NRG appro…
