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The spin-fermion (SF) model postulates that the dominant coupling between low-energy fermions in near critical metals is mediated by collective spin fluctuations (paramagnons) peaked at the Néel wave vector, Q_{N}, connecting hot spots on opposite sides of the Fermi surface. It has been argued that strong correlations at hot spots lead to a Fermi surface deformation (FSD) featuring flat regions and increased nesting. This conjecture was confirmed in the perturbative self-consistent calculations when the paramagnon propagator dependence on momentum deviation from Q_{N} is given by χ^{−1}∝|Δq|. Using diagrammatic Monte Carlo (diagMC) technique we show that such a dependence holds only at temperatures orders of magnitude smaller than any other energy scale in the problem, indicating that a different mechanism may be at play. Instead, we find that a χ^{−1}∝|Δq|^{2} dependence yields a robust finite-T scenario for achieving FSD. To link phenomenological and microscopic descriptions, we applied the connected determinant diagMC method to the (t−t^{′}) Hubbard model and found that at large U/t>5.5 before the formation of electron and hole pockets (i) the FSD defined as a maximum of the spectral function is not very pronounced; instead, it is the lines of zeros of the renormalized dispersion relation that deforms toward nesting, and (ii) the static spin susceptibility is well described by χ^{−1}∝|Δq|^{2}. Flat FS regions yield a nontrivial scenario for realizing a non-Fermi liquid state.

We develop a formalism to study the resonant inelastic x-ray scattering (RIXS) response in metals based on the diagrammatic expansion for its cross section. The standard approach to the solution of the RIXS problem relies on two key approximations: short-range potentials and noninteracting conduction electrons. However, these approximations are inaccurate for charged particles in metals, where the long-range Coulomb interaction and dynamic screening effects are very important. In this work, we study how to extract important information about collective excitations in the Coulomb plasma, plasmons, and electron-hole pairs from RIXS data. We find that single- and multi-plasmon excitations can easily be distinguished by positions of the corresponding peaks, singularities, and their intensities. We also discuss the hybrid processes, where plasmon emission is accompanied by excitation of electron-hole pairs and study how they manifest themselves.

We study the low energy physics of a Kondo chain where electrons from a one-dimensional band interact with magnetic moments via an anisotropic exchange interaction. It is demonstrated that the anisotropy gives rise to two different phases which are separated by a quantum phase transition. In the phase with easy plane anisotropy, Z2 symmetry between sectors with different helicity of the electrons is broken. As a result, localization effects are suppressed and the dc transport acquires (partial) symmetry protection. This effect is similar to the protection of the edge transport in time-reversal invariant topological insulators. The phase with easy axis anisotropy corresponds to the Tomonaga-Luttinger liquid with a pronounced spin-charge separation. The slow charge density wave modes have no protection against localization. [ABSTRACT FROM AUTHOR]

The search for Majorana bound states in solid-state physics has been limited to materials that display a gap in their bulk spectrum. We show that such unpaired states appear in certain quasi-one-dimensional Josephson-junction arrays with gapless bulk excitations. The bulk modes mediate a coupling between Majorana bound states via the Ruderman-Kittel-Yosida-Kasuya mechanism. As a consequence, the lowest energy doublet acquires a finite energy difference. For a realistic set of parameters this energy splitting remains much smaller than the energy of the bulk eigenstates even for short chains of length L~10. [ABSTRACT FROM AUTHOR]

We present a field-theory analysis of a model of two SU(2n)-invariant magnetic chains coupled by a generic interaction preserving time-reversal and inversion symmetry. Contrary to the SU(2)-invariant case the zero-temperature phase diagram of such two-leg spin ladder does not contain topological phases. Only generalized valence bond solid phases are stabilized when n>1 with different wave vectors and ground-state degeneracies. In particular, we find a phase which is made of a cluster of 2n spins put in an SU(2n) singlet state. For n=3, this cluster phase is relevant to 173Yb ultracold atoms, with an emergent SU(6) symmetry, loaded in a double-well optical lattice. [ABSTRACT FROM AUTHOR]

Parafermion zero energy modes are a vital element of fault-tolerant topological quantum computation. Although it is believed that such modes form on the border between topological and normal phases, this has been demonstrated only for Z2 (Majorana) and Z3 parafermions. I consider an integrable model of one-dimensional fermions where such a demonstration is possible for ZN parafermions with any N. The procedure is easily generalizable for more complicated symmetry groups. [ABSTRACT FROM AUTHOR]

We study the Ginzburg-Landau free energy functional for two coupled U(1) charge order parameters describing two nonequivalent charge orders with wave vector Q detected in x-ray and STM measurements of underdoped cuprates. We do not rely on a mean-field analysis, but rather utilize a field-theoretical technique suitable to study the interplay between vortex physics and discrete symmetry breaking in two-dimensional systems with U(1) symmetry. Our calculations support the idea that in the clean systems there are two transitions: from a high-temperature disordered state into a state with a composite charge order which breaks time-reversal symmetry, but leaves U( 1) fields disordered, and then into a state with quasi-long-range order in the U( 1) fields. [ABSTRACT FROM AUTHOR]

We discuss a possible realization of overscreened N-channel Kondo model in quasi-one-dimensional magnets. We suggest that it is feasible to have arrangements for N from 2 to 6 and discuss the related quantum chemistry problems. [ABSTRACT FROM AUTHOR]

We discuss a model Kondo-type Hamiltonian representing an analytically tractable version of the model used by Yin et a!. [Phys. Rev. B 86, 239904(E) (2012)] to explain the non-Fermi-liquid behavior of iron chalcogenides and ruthenates in an intermediate energy range. We consider a regime where a complete screening of the local degrees of freedom proceeds in two stages described by two characteristic energy scales TKorb ⪢ E0. The first scale marks a screening of the orbital degrees of freedom and the second one marks a crossover to the regime with coherent propagation of quasiparticles. We present analytical results for the specific heat and magnetic susceptibility at T ⪡ TKorb. [ABSTRACT FROM AUTHOR]

I employ a semiphenomenological model introduced by Tsvelik and Chubukov [Phys. Rev. Lett. 98, 237001 (2007)] to describe the state with coexisting superconductivity (SC) and charge density wave (CDW) recently discovered in YBa2Cu306+x (YBCO). The SC and the CDW order parameter fields are united in a single pseudospin and can be rotated into each other. It is suggested that disorder creates isolated pseudospins which become centers of inelastic scattering of electrons. It is suggested that this scattering is responsible for the logarithmic upturn in the resistivity p(T) ~ - In T observed at low doping. [ABSTRACT FROM AUTHOR]

It is shown that excitation spectra of generalized Gibbs ensembles of one-dimensional integrable models with isotopic symmetry contain universal features insensitive to details of the distribution. Namely, the low energy limit of the subsystem of isotopic (for instance, spin) excitations is described by the effective action of a ferromagnet at thermodynamic equilibrium with a single temperature and with the stiffness determined by the initial conditions. The condition of universality is that the entropy per excited particle is small. [ABSTRACT FROM AUTHOR]

An effective low-energy field theory is developed for a system of two chains. The main novelty of the approach is that it allows one to treat generic intrachain repulsive interactions of arbitrary strength. The chains are coupled by direct tunneling and four-fermion interactions. At low energies, the individual chains are described as Luttinger liquids with an arbitrary ratio of spin vs and charge vc velocities. A judicious choice of the basis for the decoupled chains greatly simplifies the description and allows one to separate high- and low-energy degrees of freedom. In a direct analogy to the bulk cuprates, the resulting effective field theory distinguishes between three qualitatively different regimes: (i) small doping (vc≪vs), (ii) optimal doping (vs≈vc), and (iii) large doping (vs≪vc). I discuss the excitation spectrum and derive expressions for the electron spectral function, which turns out to be highly incoherent. The degree of incoherence increases when one considers an array of ladders (stripe phase). [ABSTRACT FROM AUTHOR]

Building on previous developments [A. Taheridehkordi, S. H. Curnoe, and J. P. F. LeBlanc, Phys. Rev. B 99, 035120 (2019); A. Taheridehkordi, S. H. Curnoe, and J. P. F. LeBlancPhys. Rev. B101, 125109 (2020); A. Taheridehkordi, S. H. Curnoe, and J. P. F. LeBlancPhys. Rev. B102, 045115 (2020), B. Holm and U. von Barth, Phys. Rev. B 57, 2108 (1998), J. Vičičević and M. Ferrero, Phys. Rev. B 101, 075113 (2020)], we show that the diagrammatic Monte Carlo technique allows us to compute finite-temperature response functions directly on the real-frequency axis within any field-theoretical formulation of the interacting fermion problem. There are no limitations on the type and nature of the system's action or whether partial summation and self-consistent treatment of certain diagram classes are used. In particular, by eliminating the need for numerical analytic continuation from a Matsubara representation, our scheme allows us to study spectral densities of arbitrary complexity with controlled accuracy in models with frequency-dependent effective interactions. For illustrative purposes we consider the problem of the plasmon linewidth in a homogeneous electron gas (jellium). [ABSTRACT FROM AUTHOR]

It is shown that a junction of three critical quantum Ising chains (Delta junction) can be described as a two-channel Kondo model with a spin S=1/2 localized at the junction, which is composed of the respective Ising, zero energy boundary Majorana modes. [ABSTRACT FROM AUTHOR]

It is suggested that networks of Majorana-Cooper pair boxes connected by metallic nanowires can simulate various exotic states of matter. In this simulation Majorana-Cooper boxes play the role of effective spins S = 1/2 and the metallic connections generate the Kondo screening and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Depending on what prevails--whether it is the Kondo effect or the RKKY exchange, one will have either a Kondo lattice or an effective spin model. The list of exotic states includes the famous hexagonal Kitaev model, a generalization of this model for a Kondo lattice, and various spin models with three-spin interactions. Special emphasis is made on the discussion of the Kondo lattice scenario. [ABSTRACT FROM AUTHOR]

We discuss quasi-one-dimensional magnetic Mott insulators from the pyroxene family where spin and orbital degrees of freedom remain tightly bound. We analyze their excitation spectrum and outline the conditions under which the orbital degrees of freedom become liberated so that the corresponding excitations become dispersive and the spectral weight shifts to energies much smaller than the exchange integral. [ABSTRACT FROM AUTHOR]

We demonstrate that low dimensional Kondo-Heisenberg systems, consisting of itinerant electrons and localized magnetic moments (Kondo impurities), can be used as a principally new platform to realize scalar chiral spin order. The underlying physics is governed by a competition of the Ruderman-Kittel-Kosuya-Yosida (RKKY) indirect exchange interaction between the local moments with the direct Heisenberg one. When the direct exchange is weak and RKKY dominates, the isotropic system is in the disordered phase. A moderately large direct exchange leads to an Ising-type phase transition to the phase with chiral spin order. Our finding paves the way towards pioneering experimental realizations of the chiral spin liquid in systems with spontaneously broken time-reversal symmetry. [ABSTRACT FROM AUTHOR]

We suggest a class of two-dimensional lattice spin Hamiltonians describing non-Abelian SU(2) chiral spin liquids--spin analogs of fractional non-Abelian quantum Hall states--with gapped bulk and gapless chiral edge excitations described by the SU(2)n Wess-Zumino-Novikov-Witten conformal field theory. The models are constructed from an array of generalized spin-n/2 ladders with multi-spin-exchange interactions which are coupled by isolated spins. Such models allow a controllable analytic treatment starting from the one-dimensional limit and are characterized by a bulk gap and non-Abelian SU(2)n gapless edge excitations. [ABSTRACT FROM AUTHOR]

This theoretical paper discusses microscopic models giving rise to special types of order in which conduction electrons are bound together with localized spins to create composite order parameters. It is shown that composite order is related to the formation of a spin liquid with gapped excitations carrying quantum numbers that are a fraction of those of an electron. These spin liquids are special in the sense that their formation necessarily involves spin degrees of freedom of both the conduction and the localized electrons and can be characterized by nonlocal order parameters. A detailed description of such spin-liquid states is presented with a special care given to a demonstration of their robustness against local perturbations preserving the Lie group symmetry and the translational invariance. [ABSTRACT FROM AUTHOR]

The Kondo-Heisenberg model is used as a controllable tool to demonstrate the existence of a peculiar metallic state with unbroken translational symmetry where the Fermi surface volume is not controlled by the total electron density. I use a nonperturbative approach where the strongest interactions are taken into account by means of exact solution, and corrections are controllable. In agreement with the general requirements formulated by T. Senthil et al. [Phys. Rev. Lett. 90, 216403 (2003)], the resulting metallic state represents a fractionalized Fermi liquid where well defined quasiparticles coexist with gapped fractionalized collective excitations. The system undergoes a phase transition to an ordered phase (charge density wave or superconducting), at the transition temperature which is parametrically small in comparison to the quasiparticle Fermi energy. [ABSTRACT FROM AUTHOR]

We study the low-energy sector of the Heisenberg necklace model. Using the field-theory methods, we estimate how the coupling of the electronic spins with the paramagnetic Kondo spins affects the overall spin dynamics and evaluate its dependence on a magnetic field. We are motivated by the experimental realizations of the spin-1/2 Heisenberg chains in SrCuO2 and Sr2CuO3 cuprates, which remain one-dimensional Luttinger liquids down to temperatures much lower than the in-chain exchange coupling J. We consider the perturbation of the energy spectrum caused by the interaction γ with nuclear spins (I=3/2) present on the same sites. We find that the resulting necklace model has a characteristic energy scale, Λ~J1/3(γI)2/3, at which the coupling between (nuclear) spins of the necklace and the spins of the Heisenberg chain becomes strong. This energy scale is insensitive to a magnetic field B. For μBB>Λ we find two gapless bosonic modes that have different velocities, whose ratio at strong fields approaches a universal number, 2√+1. [ABSTRACT FROM AUTHOR]

We present high resolution angle resolved photoemission spectroscopy (ARPES) data for K0.3MoO3 (blue bronze) and propose a theoretical description of these results based primarily on electron-electron interactions. The observed Fermi surface, with two quasi-one-dimensional sheets, is consistent with a ladder material with a weak interladder coupling. Hence, we base our description on spectral properties of one-dimensional ladders. The marked broadening of the ARPES line shape, a significant fraction of an eV, is interpreted in terms of spin-charge separation. A high-energy feature, which is revealed in the spectra near the Fermi momentum thanks to improved energy resolution, is seen as a signature of a higher-energy bound state of soliton excitations on a ladder. [ABSTRACT FROM AUTHOR]

We report a combined experimental and theoretical study of the unusual ferromagnetism in the one-dimensional copper-iridium oxide Sr3CuIrO6. Utilizing Ir L3 edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromagnetic superexchange, driven by the alternating strong and weak spin-orbit coupling on the 5d Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu2+O4 plaquettes and Ir4+O6 octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds. [ABSTRACT FROM AUTHOR]

We study the temperature dependence of the electrical resistivity in a system composed of critical spin chains interacting with three-dimensional conduction electrons and driven to criticality via an external magnetic field. The relevant experimental system is Yb2Pt2Pb, a metal where itinerant electrons coexist with localized moments of Yb ions which can be described in terms of effective S=1/2 spins with a dominantly one-dimensional exchange interaction. The spin subsystem becomes critical in a relatively weak magnetic field, where it behaves like a Luttinger liquid. We theoretically examine a Kondo lattice with different effective space dimensionalities of the two interacting subsystems. We characterize the corresponding non-Fermi liquid behavior due to the spin criticality by calculating the electronic relaxation rate and the dc resistivity and establish its quasilinear temperature dependence. [ABSTRACT FROM AUTHOR]

The high-temperature normal state of the unconventional cuprate superconductors has resistivity linear in temperature T, which persists to values well beyond the Mott-Ioffe-Regel upper bound. At low temperatures, within the pseudogap phase, the resistivity is instead quadratic in T, as would be expected from Fermi liquid theory. Developing an understanding of these normal phases of the cuprates is crucial to explain the unconventional superconductivity. We present a simple explanation for this behavior, in terms of the umklapp scattering of electrons. This fits within the general picture emerging from functional renormalization group calculations that spurred the Yang-Rice-Zhang ansatz: Umklapp scattering is at the heart of the behavior in the normal phase. [ABSTRACT FROM AUTHOR]

Dynamical mean-field theory is used to study the quantum critical point (QCP) in the doped Hubbard model on a square lattice. The QCP is characterized by a universal scaling form of the self-energy and a spin density wave instability at an incommensurate wave vector. The scaling form unifies the low-energy kink and the high-energy waterfall feature in the spectral function, while the spin dynamics includes both the critical incommensurate and high-energy antiferromagnetic paramagnons. We use the frequency-dependent four-point correlation function of spin operators to calculate the momentum-dependent correction to the electron self-energy. By comparing with the calculations based on the spin-fermion model, our results indicate the frequency dependence of the quasiparticle-paramagnon vertices is an important factor to capture the momentum dependence in quasiparticle scattering. [ABSTRACT FROM AUTHOR]

In this paper we calculate the nonlinear susceptibility and the resonant Raman cross section for the paramagnetic phase of the ferromagnetic quantum Ising model in one dimension. In this region the spectrum of the Ising model has a gap m. The Raman cross section has a strong singularity when the energy of the outgoing photon is at the spectral gap ?f?m and a square root threshold when the frequency difference between the incident and outgoing photons ?i-?f?2m. The latter feature reflects the fermionic nature of the Ising model excitations. [ABSTRACT FROM AUTHOR]

We study the multichannel Kondo impurity dynamics realized in a mesoscopic superconducting island connected to metallic leads. The effective "impurity spin" is nonlocally realized by Majorana bound states and strongly coupled to lead electrons by non-Fermi liquid correlations. We explore the spin dynamics and its observable ramifications near the low-temperature fixed point. The topological protection of the system raises the perspective to observe multichannel Kondo impurity dynamics in experimentally realistic environments. [ABSTRACT FROM AUTHOR]

The local structure of the spinel LiRh2O4 has been studied using atomic-pair distribution function analysis of powder x-ray diffraction data. This measurement is sensitive to the presence of short Rh-Rh bonds that form due to dimerization of Rh4+ ions on the pyrochlore sublattice, independent of the existence of long-range order. We show that structural dimers exist in the low-temperature phase, as previously supposed, with a bond shortening of Δr~0.15 Å. The dimers persist up to 350 K, well above the insulator-metal transition, with Δr decreasing in magnitude on warming. Such behavior is inconsistent with the Fermi-surface nesting-driven Peierls transition model. Instead, we argue that LiRh2O4 should properly be described as a strongly correlated system. [ABSTRACT FROM AUTHOR]

The zero-field excitation spectrum of the strong-leg spin ladder (C7H10N)2CuBr4 is studied with a neutron time-of-flight technique. The spectrum is decomposed into its symmetric and asymmetric parts with respect to the rung momentum and compared with theoretical results obtained by the density matrix renormalization group method. Additionally, the calculated dynamical correlations are shown for a wide range of rung and leg coupling ratios in order to point out the evolution of arising excitations, as, e.g., of the two-magnon bound state from the strong to the weak coupling limit. [ABSTRACT FROM AUTHOR]

We propose a model for the intrinsic quantum criticality of β-YbAlB4, in which a vortex in momentum space gives rise to a new type of Fermi surface singularity. The unquenched angular momentum of the ∣J=7/2, mJ=±5/2⟩ Yb 4f states generates a momentum-space line defect in the hybridization between 4f and conduction electrons, leading to a quasi-two-dimensional Fermi surface with a k⊥4 dispersion and a singular density of states proportional to E-1/2. We discuss the implications of this line node in momentum space for our current understanding of quantum criticality and its interplay with topology. [ABSTRACT FROM AUTHOR]

The electronic structure of Sr3CuIrO6, a model system for the 5d Ir ion in an octahedral environment, is studied through a combination of resonant inelastic x-ray scattering and theoretical calculations. Resonant inelastic x-ray scattering spectra at the Ir L3 edge reveal an Ir t2g manifold that is split into three levels, in contrast to the expectations of the strong spin-orbit-coupling limit. Effective Hamiltonian and ab inito quantum chemistry calculations find a strikingly large noncubic crystal field splitting comparable to the spin-orbit coupling, which results in a strong mixing of the jeff = 1/2 and jeff = 3/2 states and modifies the isotropic wave functions on which many theoretical models are based. [ABSTRACT FROM AUTHOR]

We consider the effects of umklapp processes in doped two-leg fermionic ladders. These may emerge either at special band fillings or as a result of the presence of external periodic potentials. We show that such umklapp processes can lead to profound changes of physical properties and in particular stabilize pair-density wave phases [ABSTRACT FROM AUTHOR]

Our inelastic neutron scattering study of spin excitations in iron telluride reveals remarkable thermal evolution of the collective magnetism. In the temperature range relevant for the superconductivity in FeTe1-xSex materials, where the local-moment behavior is dominated by liquidlike correlations of emergent spin plaquettes, we observe unusual, marked increase of magnetic fluctuations upon heating. The effective spin per Fe at T ≈ 10 K, in the phase with weak antiferromagnetic order, corresponds to S ≈ 1, consistent with the recent analyses that emphasize importance of Hund's coupling [K. Haule and G. Kotliar, New J. Phys. 11, 025021 (2009).]. However, it grows to S ≈ 3/2 in the high-T disordered phase, suggestive of the Kondo-type behavior, where local magnetic moments are entangled with the itinerant electrons. [ABSTRACT FROM AUTHOR]

We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb- lattice magnet Na2IrO3, a candidate for a realization of a gapless spin liquid. Using resonant x-ray magnetic scattering at the Ir L3 edge, we find three-dimensional long-range antiferromagnetic order below TN = 13.3 K. From the azimuthal dependence of the magnetic Bragg peak, the ordered moment is determined to be predominantly along the a axis. Combining the experimental data with first-principles calculations, we propose that the most likely spin structure is a zig-zag structure. [ABSTRACT FROM AUTHOR]

We use first-principles methods to demonstrate that, in ZrTe5, a layered van der Waals material like graphite, atomic displacements corresponding to five of the six zone-center Ag (symmetry-preserving) phonon modes can drive a topological transition from a strong to a weak topological insulator with a Dirac semimetal state emerging at the transition, giving rise to a Dirac topology surface in the multidimensional space formed by the Ag phonon modes. This implies that the topological transition in ZrTe5 can be realized with many different settings of external stimuli capable of penetrating through the phonon-space Dirac surface without breaking the crystallographic symmetry. Furthermore, we predict that domains with effective mass of opposite signs can be created by laser pumping and will host Weyl modes of opposite chirality propagating along the domain boundaries. Studying phonon-space topology surfaces provides a new route to understanding and utilizing the exotic physical properties of ZrTe5 and related quantum materials. [ABSTRACT FROM AUTHOR]

We show that the coupling of homogeneous Heisenberg spin-1/2 ladders in different phases leads to the formation of interfacial zero energy Majorana bound states. Unlike Majorana bound states at the interfaces of topological quantum wires, these states are void of topological protection and generally susceptible to local perturbations of the host spin system. However, a key message of our Letter is that, in practice, they show a high degree of resilience over wide parameter ranges which may make them interesting candidates for applications. [ABSTRACT FROM AUTHOR]

We present a two-dimensional lattice model for quantum spin-1/2 for which the low-energy limit is governed by four flavors of strongly interacting Majorana fermions. We study this low-energy effective theory using two alternative approaches. The first consists of a mean-field approximation. The second consists of a random phase approximation (RPA) for the single-particle Green's functions of the Majorana fermions built from their exact forms in a certain one-dimensional limit. The resulting phase diagram consists of two competing chiral phases, one with Abelian and the other with non-Abelian topological order, separated by a continuous phase transition. Remarkably, the Majorana fermions propagate in the two-dimensional bulk, as in the Kitaev model for a spin liquid on the honeycomb lattice. We identify the vison fields, which are mobile (they are static in the Kitaev model) domain walls propagating along only one of the two space directions. [ABSTRACT FROM AUTHOR]

Current theories of superfluidity are based on the idea of a coherent quantum state with topologically protected quantized circulation. When this topological protection is absent, as in the case of 3He-A, the coherent quantum state no longer supports persistent superflow. Here, we argue that the loss of topological protection in a superconductor gives rise to an insulating ground state. We specifically introduce the concept of a Skyrme insulator to describe the coherent dielectric state that results from the topological failure of superflow carried by a complex-vector order parameter. We apply this idea to the case of SmB6, arguing that the observation of a diamagnetic Fermi surface within an insulating bulk can be understood as a realization of this state. Our theory enables us to understand the linear specific heat of SmB6 in terms of a neutral Majorana Fermi sea and leads us to predict that in low fields of order a Gauss, SmB6 will develop a Meissner effect. [ABSTRACT FROM AUTHOR]

We present a theoretical analysis of quasiparticle poisoning in Coulomb-blockaded Majorana fermion systems tunnel-coupled to normal-conducting leads. Taking into account finite-energy quasiparticles, we derive the effective low-energy theory and present a renormalization group analysis. We find qualitatively new effects when a quasiparticle state with very low energy is localized near a tunnel contact. For M=2 attached leads, such "dangerous" quasiparticle poisoning processes cause a spin S=1/2 single-channel Kondo effect, which can be detected through a characteristic zero-bias anomaly conductance peak in all Coulomb blockade valleys. For more than two attached leads, the topological Kondo effect of the unpoisoned system becomes unstable. A strong-coupling bosonization analysis indicates that at low energy the poisoned lead is effectively decoupled and hence, for M>3, the topological Kondo fixed point re-emerges, though now it involves only M-1 leads. As a consequence, for M=3, the low-energy fixed point becomes trivial corresponding to decoupled leads. [ABSTRACT FROM AUTHOR]

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 three-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 density wave formation and order fractionalization. The ground-state state is a pair density wave with a fractionalized charge e, S=1/2 order parameter, formed between electrons and Majorana fermions.

The local structure of NaTiSi_{2}O_{6} is examined across its Ti-dimerization orbital-assisted Peierls transition at 210 K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that, on warming, the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up to at least 490 K. The ODL state is correlated over the length scale spanning ∼6 sites of the Ti zigzag chains. Results imply that the ODL phenomenology extends to strongly correlated electron systems.

We use first-principles methods to demonstrate that, in ZrTe_{5}, a layered van der Waals material like graphite, atomic displacements corresponding to five of the six zone-center A_{g} (symmetry-preserving) phonon modes can drive a topological transition from a strong to a weak topological insulator with a Dirac semimetal state emerging at the transition, giving rise to a Dirac topology surface in the multidimensional space formed by the A_{g} phonon modes. This implies that the topological transition in ZrTe_{5} can be realized with many different settings of external stimuli capable of penetrating through the phonon-space Dirac surface without breaking the crystallographic symmetry. Furthermore, we predict that domains with effective mass of opposite signs can be created by laser pumping and will host Weyl modes of opposite chirality propagating along the domain boundaries. Studying phonon-space topology surfaces provides a new route to understanding and utilizing the exotic physical properties of ZrTe_{5} and related quantum materials.

We report a combined experimental and theoretical study of the unusual ferromagnetism in the one-dimensional copper-iridium oxide Sr(3)CuIrO(6). Utilizing Ir L(3) edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromagnetic superexchange, driven by the alternating strong and weak spin-orbit coupling on the 5d Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu(2+)O(4) plaquettes and Ir(4+)O(6) octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds.

We propose a model for the intrinsic quantum criticality of β-YbAlB(4), in which a vortex in momentum space gives rise to a new type of Fermi surface singularity. The unquenched angular momentum of the |J=7/2,m(J)=±5/2> Yb 4f states generates a momentum-space line defect in the hybridization between 4f and conduction electrons, leading to a quasi-two-dimensional Fermi surface with a k(⊥)(4) dispersion and a singular density of states proportional to E(-1/2). We discuss the implications of this line node in momentum space for our current understanding of quantum criticality and its interplay with topology.