arXiv subjects

Matthias Vojta

Publications and source records attributed to Matthias Vojta.

At least 199 records · Page 11Linked to original sources

arXiv2002

Quantum phase transitions in models of coupled magnetic impurities

We discuss models of interacting magnetic impurities coupled to a metallic host. If twice the sum of the impurity spins is larger than the total number of host screening channels, the system shows one or more quantum phase transitions where the ground-state spin changes as a function of the inter-impurity couplings. The simplest example is realized by two spin-1/2 Kondo impurities coupled to a single orbital of the host; this model exhibits a singlet-doublet transition. We investigate the phase diagram and crossover behavior of this model and present Numerical Renormalization Group results together with general arguments showing that the quantum phase transition is either of first order or of the Kosterlitz-Thouless type, depending on the symmetry of the Kondo couplings. Connections to other models and possible applications are discussed.

cond-mat.mes-hall↗

arXiv2002

Pinning of dynamic spin density wave fluctuations in the cuprate superconductors

We present a theory of the pinning of dynamic spin density wave (SDW) fluctuations in a d-wave superconductor by local imperfections which preserve spin-rotation invariance, such as impurities or vortex cores. The pinning leads to static spatial modulations in spin-singlet observables, while the SDW correlations remain dynamic: these are the `Friedel oscillations' of a spin-gap antiferromagnet. We connect the spectrum of these modulations as observed by scanning tunnelling microscopy to the dynamic spin structure factor measured by inelastic neutron scattering.

cond-mat.str-el↗

arXiv2001

Magnetic properties and spin waves of bilayer magnets in a uniform field

The two-layer square lattice quantum antiferromagnet with spins 1/2 shows a zero-field magnetic order-disorder transition at a critical ratio of the inter-plane to intra-plane couplings. Adding a uniform magnetic field tunes the system to canted antiferromagnetism and eventually to a fully polarized state; similar behavior occurs for ferromagnetic intra-plane coupling. Based on a bond operator spin representation, we propose an approximate ground state wavefunction which covers all phases by means of a unitary transformation. The excitations can be efficiently described as independent bosons; in the antiferromagnetic phase these reduce to the well-known spin waves, whereas they describe gapped spin-1 excitations in the singlet phase. We compute the spectra of these excitations as well as the magnetizations throughout the whole phase diagram.

cond-mat.str-el↗

arXiv2001

Magnetic field tuning of charge and spin order in the cuprate superconductors

Recent neutron scattering, nuclear magnetic resonance, and scanning tunneling microscopy experiments have yielded valuable new information on the interplay between charge and spin density wave order and superconductivity in the cuprate superconductors, by using an applied perpendicular magnetic field to tune the ground state properties. We compare the results of these experiments with the predictions of a theory which assumed that the ordinary superconductor was proximate to a quantum transition to a superconductor with co-existing spin/charge density wave order.

cond-mat.str-el↗

arXiv2001

Kondo effect of impurity moments in d-wave superconductors: Quantum phase transition and spectral properties

We discuss the dynamics of magnetic moments in d-wave superconductors, in particular we focus on moments induced by doping non-magnetic impurities into cuprates. The interaction of such moments with the Bogoliubov quasiparticles of the superconductors can be decribed by variants of the pseudogap Kondo model, characterized by a power-law density of states at the Fermi level. The Numerical Renormalization Group technique is employed to investigate this Kondo problem for realistic band structures and particle-hole asymmetries, both at zero and finite temperatures. In particular, we study the boundary quantum phase transition between the local-moment and the asymmetric strong-coupling phases, and argue that this transition has been observed in recent nuclear magnetic resonance experiments. We determine the spectral properties of both phases, the location of the critical point as function of Kondo coupling and doping, and discuss the quantum-critical cross-overs near this phase transition. In addition, the changes in the local density of states around the impurity are calculated as function of temperature, being relevant to scanning tunneling microscopy experiments.

cond-mat.str-el↗

arXiv2001

Multichannel pseudogap Kondo model: Large-N solution and quantum-critical dynamics

We discuss a multichannel SU(N) Kondo model which displays non-trivial zero-temperature phase transitions due to a conduction electron density of states vanishing with a power law at the Fermi level. In a particular large-N limit, the system is described by coupled integral equations corresponding to a dynamic saddle point. We exactly determine the universal low-energy behavior of spectral densities at the scale-invariant fixed points, obtain anomalous exponents, and compute scaling functions describing the crossover near the quantum-critical points. We argue that our findings are relevant to recent experiments on impurity-doped d-wave superconductors.

cond-mat.str-el↗

arXiv2001

Multiple beam interference in a quadrupolar glass fiber

Motivated by the recent observation of periodic filter characteristics of an oval-shaped micro-cavity, we study the possible interference of multiple beams in the far field of a laser-illuminated quadrupolar glass fiber. From numerical ray-tracing simulations of the experimental situation we obtain the interference-relevant length-difference spectrum and compare it with data extracted from the experimental filter results. Our analysis reveals that different polygonal cavity modes being refractively output-coupled in the high-curvature region of the fiber contribute to the observed far-field interference.

physics.optics↗

arXiv2001

Quantum phase transitions and collective modes in d-wave superconductors

Fluctuations near second-order quantum phase transitions in d-wave superconductors can cause strong damping of fermionic excitations, as observed in photoemission experiments. The damping of the gapless nodal quasiparticles can arise naturally in the quantum-critical region of a transition with an additional spin-singlet, zero momentum order parameter; we argue that the transition to a d_x^2-y^2 + i d_xy pairing state is the most likely possibility in this category. On the other hand, the gapped antinodal quasiparticles can be strongly damped by the coupling to antiferromagnetic spin fluctuations arising from the proximity to a Neel-ordered state. We review some aspects of the low-energy field theories for both transitions and the corresponding quantum-critical behavior. In addition, we discuss the spectral properties of the collective modes associated with the proximity to a superconductor with d_x^2-y^2 + i d_xy symmetry, and implications for experiments.

cond-mat.supr-con↗

arXiv2001

Spin orthogonality catastrophe in two-dimensional antiferromagnets and superconductors

We compute the spectral function of a spin S hole injected into a two-dimensional antiferromagnet or superconductor in the vicinity of a magnetic quantum critical point. We show that, near van Hove singularities, the problem maps onto that of a static vacancy carrying excess spin S. The hole creation operator is characterized by a new boundary anomalous dimension and a vanishing quasiparticle residue at the critical point. We discuss possible relevance to photoemission spectra of cuprate superconductors near the anti-nodal points.

cond-mat.str-el↗

arXiv2001

Impurity in a d-wave superconductor: Kondo effect and STM spectra

We present a theory for recent STM studies of Zn impurities in the superconductor BSCCO, using insights from NMR experiments which show that there is a net S=1/2 moment on the Cu ions near the Zn. We argue that the Kondo spin dynamics of this moment is the origin of the low bias peak in the differential conductance, rather than a resonance in a purely potential scattering model. The spatial and energy dependence of the STM spectra of our model can also fit the experiments.

cond-mat.str-el↗

arXiv2000

Non-magnetic impurities as probes of insulating and doped Mott insulators in two dimensions

We characterize paramagnetic Mott insulators by their response to static, non-magnetic impurities. States with spinon deconfinement (or spin-charge separation) are distinguished from those with spinon confinement by distinct impurity susceptibilities and finite-size spectra. We discuss the evolution of physical properties upon doping to a d-wave superconductor, and argue that a number of recent experiments favor spinon confinement in the reference Mott insulating state.

cond-mat.str-el↗

arXiv2000

Phase diagram of the quarter-filled extended Hubbard model on a two-leg ladder

We investigate the ground-state phase diagram of the quarter-filled Hubbard ladder with nearest-neighbor Coulomb repulsion V using the Density Matrix Renormalization Group technique. The ground-state is homogeneous at small V, a ``checkerboard'' charge--ordered insulator at large V and not too small on-site Coulomb repulsion U, and is phase-separated for moderate or large V and small U. The zero-temperature transition between the homogeneous and the charge-ordered phase is found to be second order. In both the homogeneous and the charge-ordered phases the existence of a spin gap mainly depends on the ratio of interchain to intrachain hopping. In the second part of the paper, we construct an effective Hamiltonian for the spin degrees of freedom in the strong-coupling charge-ordered regime which maps the system onto a frustrated spin chain. The opening of a spin gap is thus connected with spontaneous dimerization.

cond-mat.str-el↗

arXiv2000

Competing orders and quantum criticality in doped antiferromagnets

We use a number of large-N limits to explore the competition between ground states of square lattice doped antiferromagnets which break electromagnetic U(1), time-reversal, or square lattice space group symmetries. Among the states we find are d-, (s+id)-, and (d+id)-wave superconductors, Wigner crystals, Wigner crystals of hole pairs, orbital antiferromagnets (or staggered-flux states), and states with spin-Peierls and bond-centered charge stripe order. In the vicinity of second-order quantum phase transitions between the states, we go beyond the large-N limit by identifying the universal quantum field theories for the critical points, and computing the finite temperature, quantum-critical damping of fermion spectral functions. We identify candidate critical points for the recently observed quantum-critical behavior in photoemission experiments on BSCCO by Valla et al. (Science 285, 2110 (1999)). These involve onset of a charge density wave, or of broken time-reversal symmetry with (d+id) or (s+id) pairing, in a d-wave superconductor. It is not required (although it is allowed) that the stable state in the doped cuprates to be anything other than the d-wave superconductor--the other states need only be stable nearby in parameter space. At finite temperatures, fluctuations associated with these nearby states lead to the observed fermion damping in the vicinity of the nodal points in the Brillouin zone. The cases with broken time-reversal symmetry are appealing because the order parameter is not required to satisfy any special commensurability conditions. The observed absence of inelastic damping of quasiparticles with momenta (pi,k), (k,pi) (with 0 < k < pi) also appears very naturally for the case of a transition to (d+id) order.

cond-mat.str-el↗

arXiv2000

Damping of collective modes and quasiparticles in d-wave superconductors

The two-dimensional d-wave superconducting state of the high temperature superconductors has a number of different elementary excitations: the spin-singlet Cooper pairs, the spin S=1/2 fermionic quasiparticles, and a bosonic S=1 resonant collective mode, phi_{alpha}, at the antiferromagnetic wavevector. Although the phi_{alpha} quanta are strongly coupled to the gapped quasiparticles near the (pi,0), (0,pi) wavevectors (the "hot spots"), they are essentially decoupled from the low energy quasiparticles near the nodes of the superconducting gap. Consequently, distinct and independent low energy quantum field theories can be constructed for the phi_{alpha} and nodal quasiparticle excitations. We review recent work introducing a 2+1 dimensional boundary conformal field theory for the damping of the phi_{alpha} excitations by non-magnetic impurities, which is built on the proximity to a magnetic ordering transition at which the phi_{alpha} condense; the results are compared with neutron scattering experiments. Photoemission and THz conductivity measurements indicate that the nodal quasiparticles undergo strong inelastic scattering at low temperatures; we propose that this is due to fluctuations near a quantum phase transition, and critically analyze candidate order parameters and field theories.

cond-mat.str-el↗

arXiv2000

Quantum impurity dynamics in two-dimensional antiferromagnets and superconductors

We present the universal theory of arbitrary, localized impurities in a confining paramagnetic state of two-dimensional antiferromagnets with global SU(2) spin symmetry. The energy gap of the host antiferromagnet to spin-1 excitations, Δ, is assumed to be significantly smaller than a typical nearest neighbor exchange. In the absence of impurities, it was argued in earlier work (Chubukov et al. cond-mat/9304046) that the low-temperature quantum dynamics is universally and completely determined by the values of Δand a spin-wave velocity c. Here we establish the remarkable fact that no additional parameters are necessary for an antiferromagnet with a dilute concentration of impurities, n_{imp} - each impurity is completely characterized by a integer/half-odd-integer valued spin, S, which measures the net uncompensated Berry phase due to spin precession in its vicinity. We compute the impurity-induced damping of the spin-1 collective mode of the antiferromagnet: the damping occurs on an energy scale Γ= n_{imp} (\hbar c)^2/Δ, and we predict a universal, asymmetric lineshape for the collective mode peak. We argue that, under suitable conditions, our results apply unchanged (or in some cases, with minor modifications) to d-wave superconductors, and compare them to recent neutron scattering experiments on YBCO by Fong et al. (cond-mat/9812047). We also describe the universal evolution of numerous measurable correlations as the host antiferromagnet undergoes a quantum phase transition to a Neel ordered state.

cond-mat.str-el↗

arXiv2000

Quantum Impurity in a Nearly Critical Two Dimensional Antiferromagnet

We describe the spin dynamics of an arbitrary localized impurity in an insulating two dimensional antiferromagnet, across the host transition from a paramagnet with a spin gap to a Neel state. The impurity spin susceptibility has a Curie-like divergence at the quantum-critical coupling, but with a universal, effective spin which is neither an integer nor a half-odd-integer. In the Neel state, the transverse impurity susceptibility is a universal number divided by the host spin stiffness (which determines the energy cost to slow twists in the orientation of the Neel order). These, and numerous other results for the thermodynamics, Knight shift, and magnon damping have significant applications to experiments on layered transition metal oxides.

cond-mat.str-el↗

arXiv2000

Impurity spin dynamics in 2D antiferromagnets and superconductors

We discuss the universal theory of localized impurities in the paramagnetic state of 2D antiferromagnets where the spin gap is assumed to be significantly smaller than a typical exchange energy. We study the impurity spin susceptibility near the host quantum transition from a gapped paramagnet to a Neel state, and we compute the impurity-induced damping of the spin-1 mode of the gapped antiferromagnet. Under suitable conditions our results apply also to d-wave superconductors.

cond-mat.str-el↗

arXiv1999

Hole dynamics and photoemission in a t-J model for SrCu_2(BO_3)_2

The motion of a single hole in a t-J model for the two-dimensional spin-gap compound SrCu_2(BO_3)_2 is investigated. The undoped Heisenberg model for this system has an exact dimer eigenstate and shows a phase transition between a dimerized and a Neel phase at a certain ratio of the magnetic couplings. We calculate the photoemission spectrum in the disordered phase using a generalized spin-polaron picture. By varying the inter-dimer hopping parameters we find a cross-over between a narrow quasiparticle band regime known from other strongly correlated systems and free-fermion behavior. The hole motion in the Neel-ordered phase is also briefly considered.

cond-mat.str-el↗