arXiv subjects

Stefan Kirchner

Publications and source records attributed to Stefan Kirchner.

At least 19 recordsLinked to original sources

arXiv2026

Multi-Criticality and RG Topology in the Charge-Kondo-Breakdown Scenario in the Cuprates

In this paper, we examine the dynamical charge-Kondo-breakdown scenario proposed for cuprate superconductors within the perspective of renormalization group (RG) topology. By analyzing the coupled RG flow equations governing the effective low-energy theory, we determine the fixed-point structure, stability properties, and global organization of the flow. We find that the putative finite-coupling interacting fixed point is unstable against perturbations transverse to an invariant critical manifold. As a result, generic RG trajectories exhibit runaway behavior. To elucidate the global structure of the theory, we combine analytical solutions of the flow equations with numerical phase portraits and Poincaré compactification. This analysis reveals that the interacting fixed point exhibits a marginally relevant instability, generating an exponentially large but finite crossover scale. The resulting flow topology closely resembles anisotropy-driven runaway flows encountered in fluctuation-induced weakly first-order transitions. Within this framework, the apparent quantum-critical regime can be understood as an extended crossover controlled by a near-critical fixed point, rather than by asymptotic scale invariance. The results obtained indicate that the existence of extended scaling behavior does not, by itself, imply the presence of a stable interacting quantum critical state. More generally, our analysis demonstrates how RG topology can provide a powerful diagnostic for distinguishing genuine criticality from pseudo-critical behavior in theories of strongly correlated quantum matter.

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arXiv2026

Critical Charge and Current Fluctuations across a Voltage-Driven Phase Transition

We investigate bias-driven non-equilibrium quantum phase transitions in a paradigmatic quantum-transport setup: an interacting quantum dot coupled to non-interacting metallic leads. Using the Random Phase Approximation, which is exact in the limit of a large number of dot levels, we map out the zero-temperature non-equilibrium phase diagram as a function of interaction strength and applied bias. We focus our analysis on the behavior of the charge susceptibility and the current noise in the vicinity of the transition. Remarkably, despite the intrinsically non-equilibrium nature of the steady state, critical charge fluctuations admit an effective-temperature description, $T_{\text{eff}}(T,V)$, that collapses the steady-state behavior onto its equilibrium form. In sharp contrast, current fluctuations exhibit genuinely non-equilibrium features: the fluctuation-dissipation ratio becomes negative in the ordered phase, corresponding to a negative effective temperature for the current degrees of freedom. These results establish current noise as a sensitive probe of critical fluctuations at non-equilibrium quantum phase transitions and open new directions for exploring voltage-driven critical phenomena in quantum transport systems.

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arXiv2024

Voltage-Driven Breakdown of Electronic Order

The non-thermal breakdown of a Mott insulator has been a topic of great theoretical and experimental interest with technological relevance. Recent experiments have found a sharp non-equilibrium insulator-to-metal transition that is accompanied by hysteresis, a negative differential conductance and lattice deformations. However, a thorough understanding of the underlying breakdown mechanism is still lacking. Here, we examine a scenario in which the breakdown is induced by chemical pressure in a paradigmatic model of interacting spinless fermions on a chain coupled to metallic reservoirs (leads). For the Markovian regime, at infinite bias, we qualitatively reproduce several established results. Beyond infinite bias, we find a rich phase diagram where the nature of the breakdown depends on the coupling strength as the bias voltage is tuned up, yielding different current-carrying non-equilibrium phases. For weak to intermediate coupling, we find a conducting CDW phase with a bias-dependent ordering wave vector. At large interaction strength, the breakdown connects the system to a charge-separated insulating phase. We find instances of hysteretic behavior, sharp current onset and negative differential conductance. Our results can help to shed light on recent experimental findings that address current-induced Mott breakdown.

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arXiv2024

Electronic conduction and superconducting properties of CoSi$_2$ films on silicon--an unconventional superconductor with technological potential

We report observations of unusual normal-state electronic conduction properties and superconducting characteristics of high-quality CoSi$_2$/Si films grown on silicon Si(100) and Si(111) substrates. A good understanding of these features shall help to address the underlying physics of the unconventional pairing symmetry recently observed in transparent CoSi$_2$/TiSi$_2$ heterojunctions [S. P. Chiu \textit{et al.}, Sci. Adv. \textbf{7}, eabg6569 (2021); Nanoscale \textbf{15}, 9179 (2023)], where CoSi$_2$/Si is a superconductor with a superconducting transition temperature $T_c \simeq$ (1.1--1.5) K, dependent on its dimensions, and TiSi$_2$ is a normal metal. In CoSi$_2$/Si films, we find a pronounced positive magnetoresistance caused by the weak-antilocalization effect, indicating a strong Rashba spin-orbit coupling (SOC). This SOC generates two-component superconductivity in CoSi$_2$/TiSi$_2$ heterojunctions. The CoSi$_2$/Si films are stable under ambient conditions and have ultralow 1/$f$ noise. Moreover, they can be patterned via the standard lithography techniques, which might be of considerable practical value for future scalable superconducting and quantum device fabrication.

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arXiv2023

Effects of spin orbit coupling on proximity induced superconductivity

We investigate the effect of spin orbit coupling on proximity induced superconductivity in a normal metal attached to a superconductor. Specifically, we consider a heterostructure where the presence of interfaces gives rise to a Rashba spin orbit coupling. The properties of the induced superconductivity in these systems are addressed within the tunneling Hamiltonian formalism. We find that the spin orbit coupling induces a mixture of singlet and triplet pairing and, under specific circumstances, an odd frequency, even parity, spin triplet pairs can arise. We also address the effect of impurity scattering on the induced pairs, and discuss our results in context of heterostructures consisting of materials with spin-momentum locking.

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arXiv2023

Quantum-interference origin and magnitude of 1/$f$ noise in Dirac nodal line IrO$_2$ nanowires at low temperatures

We present 1/$f$ noise measurements of IrO$_2$ nanowires from 1.7 to 350 K. Results reveal that the noise magnitude (represented by Hooge parameter $γ$) increases at low temperatures, indicating low-frequency resistance noise from universal conductance fluctuations. The cause of this noise is determined to be due to oxygen vacancies in the rutile structure of IrO$_2$. Additionally, the number density of these mobile defects can be calculated from the $\sqrt{T}$ resistance rise caused by the orbital two-channel Kondo effect in the Dirac nodal line metal IrO$_2$.

cond-mat.mes-hall↗

arXiv2023

Efficient quantum information probes of non-equilibrium quantum criticality

Quantum information-based approaches, in particular the fidelity, have been flexible probes for phase boundaries of quantum matter. A major hurdle to a more widespread application of fidelity and other quantum information measures to strongly correlated quantum materials is the inaccessibility of the fidelity susceptibility to most state-of-the-art numerical methods. This is particularly apparent away from equilibrium where, at present, no general critical theory is available and many standard techniques fail. Motivated by the usefulness of quantum information based measures we show that a widely accessible quantity, the single-particle affinity, is able to serve as a versatile instrument to identify phase transitions beyond Landau's paradigm. We demonstrate that it not only is able to signal previously identified non-equilibrium phase transitions but also has the potential to detect hitherto unknown phases in models of quantum matter far from equilibrium.

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arXiv2022

Topological semimetals without quasiparticles

The interplay between interactions and topology in quantum materials is of extensive current interest. Strong correlations are known to be important for insulating topological states, as exemplified by the fractional quantum Hall effect. For the metallic case, whether and how they can drive topological states that have no free-electron counterparts is an open and pressing question. We introduce a general framework for lattice symmetries to constrain single-particle excitations even when they are not quasiparticles, and substantiate it in a periodic Anderson model with two channels of conduction electrons. We demonstrate that symmetry constrains correlation-induced emergent excitations to produce non-Fermi liquid topological phases. The loss of quasiparticles in these phases is manifested in a non-Fermi liquid form of spectral and transport properties, whereas its topological nature is characterized by surface states and valley and spin Hall conductivities. We also identify candidate materials to realize the proposed phases. Our work opens a door to a variety of non-Fermi liquid topological phases in a broad range of strongly correlated materials.

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arXiv2022

Enhanced two-component superconductivity in CoSi2/TiSi2 heterojunctions

We report enhanced two-component superconductivity in (CoSi2/Si)/TiSi2 superconductor/normal-metal (S/N) heterojunctions. An enhanced superconducting transition temperature about twice that of CoSi2 and an upper critical field about 20 times bigger than that of epitaxial CoSi2/Si films were found. The tunneling spectra of three-terminal S/N junctions show pronounced zero-bias conductance peaks (ZBCPs) that signify penetration of odd-frequency, spin-triplet and even-parity Cooper pairs in TiSi2 from triplet dominant pairing in CoSi2/Si driven by symmetry reduction at the CoSi2/Si interface. Both the enhancement of the superconducting transition temperature and the ZBCPs are found to be more pronounced if TiSi2 is made more diffusive.

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arXiv2021

Kondomania

Originally the Kondo effect describes a scattering mechanism of electrons in metals that have defects with internal quantum mechanical degrees of freedom. The characteristic dynamic interplay between localized and itinerant states occurs in many different forms. It is now clear that the Kondo effect shapes the properties of various classes of quantum materials and is a key to understanding their unusual properties. In the following we outline a selection of the most important developments in this Kondomania.

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arXiv2021

Effects of spin orbit coupling in superconducting proximity devices -- application to $\mathrm{CoSi_2 / TiSi_2}$ heterostructures

Motivated by the recent findings of unconventional superconductivity in $\mathrm{CoSi_2 / TiSi_2}$ heterostructures, we study the effect of interface induced Rashba spin orbit coupling on the conductance of a three terminal "T" shape superconducting device. We calculate the differential conductance for this device within the quasi-classical formalism that includes the mixing of triplet-singlet pairing due to the Rashba spin orbit coupling. We discuss our result in the light of the conductance spectra reported by Chiu {\it et al.} for $\mathrm{CoSi_2 / TiSi_2}$ heterostructures.

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arXiv2020

Observation of triplet superconductivity in CoSi$_2$/TiSi$_2$ heterostructures

Unconventional superconductivity and in particular triplet superconductivity have been front and center of topological materials and quantum technology research. Here we report our observation of triplet superconductivity in nonmagnetic CoSi$_2$/TiSi$_2$ heterostructures on silicon. CoSi$_2$ undergoes a sharp superconducting transition at a critical temperature $T_c \approx$ 1.5 K, while TiSi$_2$ is a normal metal. We investigate conductance spectra of both two-terminal CoSi$_2$/TiSi$_2$ tunnel junctions and three-terminal T-shaped CoSi$_2$/TiSi$_2$ superconducting proximity structures. We report an unexpectedly large spin-orbit coupling in CoSi$_2$ heterostructures. Below $T_c$, we observe (1) a narrow zero-bias conductance peak on top of a broad hump, accompanied by two symmetric side dips in the tunnel junctions, (2) a narrow zero-bias conductance peak in T-shaped structures, and (3) hysteresis in the junction magnetoresistance. These three independent and complementary observations are indicative of chiral $p$-wave pairing in CoSi$_2$/TiSi$_2$ heterostructures. This chiral triplet superconductivity and the excellent fabrication compatibility of CoSi$_2$ and TiSi$_2$ with present-day silicon integrated-circuit technology facilitate full scalability for potential use in quantum-computing devices.

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arXiv2020

Oxygen vacancy-driven orbital multichannel Kondo effect in Dirac nodal line metals IrO2 and RuO2

Strong electron correlations have long been recognized as driving the emergence of novel phases of matter. A well recognized example is high-temperature superconductivity which cannot be understood in terms of the standard weak-coupling theory. The exotic properties that accompany the formation of the two-channel Kondo effect including the emergence of an unconventional metallic state in the low-energy limit also originate from strong electron interactions. Despite its paradigmatic role for the formation of non-standard metal behavior, the stringent conditions required for its emergence have made the observation of the nonmagnetic, orbital two-channel Kondo effect in real quantum materials difficult, if not impossible. We report the observation of orbital one- and two-channel Kondo physics in the symmetry-enforced Dirac nodal line metals IrO2 and RuO2 nanowires and show that the symmetries that enforce the existence of Dirac nodal lines also promote the formation of nonmagnetic Kondo correlations. Rutile oxide nanostructures thus form a versatile quantum matter platform to engineer and explore intrinsic, interacting topological states of matter.

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arXiv2020

Nonequilibrium phases and phase transitions of the XY-model

We obtain the steady-state phase diagram of a transverse field XY spin chain coupled at its ends to magnetic reservoirs held at different magnetic potentials. In the long-time limit, the magnetization bias across the system generates a current-carrying non-equilibrium steady-state. We characterize the different non-equilibrium phases as functions of the chain's parameters and magnetic potentials, in terms of their correlation functions and entanglement content. The mixed-order transition, recently observed for the particular case of a transverse field Ising chain, is established to emerge as a generic out-of-equilibrium feature and its critical exponents are determined analytically. Results are also contrasted with those obtained in the limit of Markovian reservoirs. Our findings should prove helpful in establishing the properties of non-equilibrium phases and phase transitions of extended open quantum systems.

cond-mat.mes-hall↗

arXiv2020

Quantum criticality in the spin-isotropic pseudogap Bose-Fermi Kondo model: entropy, scaling, and the g-theorem

We study the behavior of the entropy of the pseudogap Bose-Fermi Kondo model within a dynamical large-$N$ limit, where $N$ is related to the symmetry group of the model. This model is a general quantum impurity model that describes a localized level coupled to a fermionic bath having a density of states that vanishes in a powerlaw fashion near the Fermi energy and to a bosonic bath possessing a powerlaw spectral density below a cutoff energy. As a function of the couplings to the baths various quantum phase transitions can occur. We study how the impurity entropy changes across these zero-temperature transitions and compare our results with predictions based on the g-theorem. This is accomplished by an analysis of the leading and sub-leading scaling behavior. Our analysis shows that the $g$-theorem does not apply to the pseudogap Bose-Fermi Kondo model at the large-N level. This inapplicability originates from an anomalous contribution to the scaling function in the hydrodynamic regime where $k_B T>\hbar ω$ which is absent in the quantum coherent regime, i.e., for $k_B T<\hbar ω$. We also compare our results with those obtained for the Sachdev-Ye-Kitaev model.

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arXiv2020

Heavy-electron quantum criticality and single-particle spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) have become indispensable tools in the study of correlated quantum materials. Both probe complementary aspects of the single-particle excitation spectrum. Taken together, ARPES and STM have the potential to explore properties of the electronic Green's function, a central object of many-body theory. This review explicates this potential with a focus on heavy-electron quantum criticality, especially the role of Kondo destruction. A discussion on how to probe the Kondo destruction effect across the quantum-critical point using ARPES and STM measurements is presented. Particular emphasis is placed on the question of how to distinguish between the signatures of the initial onset of hybridization-gap formation, which is the "high-energy" physics to be expected in all heavy-electron systems, and those of Kondo destruction, which characterizes the low-energy physics and, hence, the nature of quantum criticality. Recent progress and possible challenges in the experimental investigations are surveyed, the STM and ARPES spectra for several quantum-critical heavy-electron compounds are compared, and the prospects for further advances are outlined.

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arXiv2019

Dynamical scaling of charge and spin responses at a Kondo destruction quantum critical point

Quantum critical points often arise in metals perched at the border of an antiferromagnetic order. The recent observation of singular and dynamically scaling charge conductivity in an antiferromagnetic quantum critical heavy fermion metal implicates beyond-Landau quantum criticality. Here we study the charge and spin dynamics of a Kondo destruction quantum critical point (QCP), as realized in an SU(2)-symmetric Bose-Fermi Kondo model. We find that the critical exponents and scaling functions of the spin and single-particle responses of the QCP in the SU(2) case are essentially the same as those of the large-N limit, showing that $1/N$ corrections are subleading. Building on this insight, we demonstrate that the charge responses at the Kondo destruction QCP are singular and obey $ω/T$ scaling. This property persists at the Kondo destruction QCP of the SU(2)-symmetric Kondo lattice model.

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arXiv2019

Classical and quantum liquids induced by quantum fluctuations

Geometrically frustrated interactions may render classical ground-states macroscopically degenerate. The connection between classical and quantum liquids and how the degeneracy is affected by quantum fluctuations is, however, less well understood. We study a simple model of coupled quantum and classical degrees of freedom, the so-called Falicov-Kimball model, on a triangular lattice and away from half-filling. For weak interactions the phase diagram features a charge disordered state down to zero temperature. We provide compelling evidence that this phase is a liquid and show that it is divided by a crossover line that terminates in a quantum critical point. Our results offer a new vantage point to address how quantum liquids can emerge from their classical counterparts.

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