Arno P. Kampf

Universität Augsburg, Augsberg, Bavaria, Germany

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Publications (140)434.88 Total impact

  • Source
    W. A. Atkinson · A. P. Kampf · S. Bulut
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    ABSTRACT: We study the emergence of charge ordered phases within a pi-loop current (piLC) model for the pseudogap based on a three-band model for underdoped cuprate superconductors. Loop currents and charge ordering are driven by distinct components of the short-range Coulomb interactions: loop currents result from the repulsion between nearest-neighbor copper and oxygen orbitals, while charge order results from repulsion between neighboring oxygen orbitals. We find that the leading piLC phase has an antiferromagnetic pattern similar to previously discovered staggered flux phases, and that it emerges abruptly at hole dopings p below the van Hove filling. Subsequent charge ordering tendencies in the piLC phase reveal that diagonal d-charge density waves (dCDW) are suppressed by the loop currents while axial order competes more weakly. In some cases we find a wide temperature range below the loop-current transition, over which the susceptibility towards an axial dCDW is large. In these cases, short-range axial charge order may be induced by doping-related disorder. A unique feature of the coexisting dCDW and piLC phases is the emergence of an incommensurate modulation of the loop currents. If the dCDW is biaxial (checkerboard) then the resulting incommensurate current pattern breaks all mirror and time-reversal symmetries, thereby allowing for a polar Kerr effect.
    Preview · Article · Dec 2015
  • Source
    S. Bulut · Arno P. Kampf · W. A. Atkinson
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    ABSTRACT: We present evidence for the existence of a spontaneous instability towards an orbital loop-current phase in a multiorbital Hubbard model for the CuO$_2$ planes in cuprates. Contrary to the previously proposed $\theta_{II}$ phase with intra-unit cell currents, the identified instability is towards a staggered pattern of intertwined current loops. The orbitally resolved current pattern thereby shares its staggered character with the proposal of d-density wave order. The current pattern will cause a Fermi surface reconstruction and the opening of a pseudogap. We argue that the pseudogap phase with time-reversal symmetry breaking currents is susceptible to further phase transitions and therefore offers a route to account for axial incommensurate charge order and a polar Kerr effect in underdoped cuprates.
    Full-text · Article · Mar 2015 · Physical Review B
  • W. A. Atkinson · A. P. Kampf
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    ABSTRACT: Many cuprate superconductors possess an unusual charge-ordered phase that is characterized by an approximate ${d}_{{x}^{2}$-${}{y}^{2}}$ intraunit cell form factor and a finite modulation wave vector ${\mathbf{q}}^{*}$. We study the effects of impurities on this charge-ordered phase via a single-band model in which bond order is the analog of charge order in the cuprates. Impurities are assumed to be pointlike and are treated within the self-consistent $t$-matrix approximation. We show that suppression of bond order by impurities occurs through the local disruption of the ${d}_{{x}^{2}$-${}{y}^{2}}$ form factor near individual impurities. Unlike $d$-wave superconductors, where the sensitivity of ${T}_{c}$ to impurities can be traced to a vanishing average of the ${d}_{{x}^{2}$-${}{y}^{2}}$ order parameter over the Fermi surface, the response of bond order to impurities is dictated by a few Fermi surface ``hotspots.'' The bond order transition temperature ${T}_{\mathrm{bo}}$ thus follows a different universal dependence on impurity concentration ${n}_{i}$ than does the superconducting ${T}_{c}$. In particular, ${T}_{\mathrm{bo}}$ decreases more rapidly than ${T}_{c}$ with increasing ${n}_{i}$ when there is a nonzero Fermi surface curvature at the hotspots. Based on experimental evidence that the pseudogap is insensitive to Zn doping, we conclude that a direct connection between charge order and the pseudogap is unlikely. Furthermore, the enhancement of stripe correlations in the La-based cuprates by Zn doping is evidence that this charge order is also distinct from stripes.
    No preview · Article · Mar 2015 · Physical Review B
  • Source
    W. A. Atkinson · A. P. Kampf
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    ABSTRACT: Many cuprate superconductors possess an unusual charge-ordered phase that is characterized by an approximate $d_{x^2-y^2}$ intra-unit cell form factor and a finite modulation wavevector $\bq^\ast$. We study the effects impurities on this charge ordered phase via a single-band model in which bond order is the analogue of charge order in the cuprates. Impurities are assumed to be pointlike and are treated within the self-consistent t-matrix approximation (SCTMA). We show that suppression of bond order by impurities occurs through the local disruption of the $d_{x^2-y^2}$ form factor near individual impurities. Unlike $d$-wave superconductors, where the sensitivity of $T_c$ to impurities can be traced to a vanishing average of the $d_{x^2-y^2}$ order parameter over the Fermi surface, the response of bond order to impurities is dictated by a few Fermi surface "hotspots". The bond order transition temperature $T_\mathrm{bo}$ thus follows a different universal dependence on impurity concentration $n_i$ than does the superconducting $T_c$. In particular, $T_\mathrm{bo}$ decreases more rapidly than $T_c$ with increasing $n_i$ when there is a nonzero Fermi surface curvature at the hotspots. Based on experimental evidence that the pseudogap is insensitive to Zn doping, we conclude that a direct connection between charge order and the pseudogap is unlikely. Furthermore, the enhancement of stripe correlations in the La-based cuprates by Zn doping is evidence that this charge order is also distinct from stripes.
    Full-text · Article · Feb 2015
  • Source
    Florian Loder · Arno P. Kampf · Thilo Kopp
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    ABSTRACT: The verification of topological superconductivity has become a major experimental challenge. Apart from the very few spin-triplet superconductors with p-wave pairing symmetry, another candidate system is a conventional, two-dimensional (2D) s-wave superconductor in a magnetic field with a sufficiently strong Rashba spin-orbit coupling. Typically, the required magnetic field to convert the superconductor into a topologically non-trivial state is however by far larger than the upper critical field H_c2, which excludes its realization. In this article, we argue that this problem can be overcome by rotating the magnetic field into the superconducting plane. We explore the character of the superconducting state upon changing the strength and the orientation of the magnetic field and show that a topological state, established for a sufficiently strong out-of-plane magnetic field, indeed extends to an in-plane field orientation. We present a three-band model applicable to the superconducting interface between LaAlO_3 and SrTiO_3, which should fulfil the necessary conditions to realize a topological superconductor.
    Full-text · Article · Dec 2014 · Scientific Reports
  • Source
    W. A. Atkinson · A. P. Kampf · S. Bulut
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    ABSTRACT: In a multiorbital model of the cuprate high-temperature superconductors soft antiferromagnetic (AF) modes are assumed to reconstruct the Fermi surface to form nodal pockets. The subsequent charge ordering transition leads to a phase with a spatially modulated transfer of charge between neighboring oxygen p_x and p_y orbitals and also weak modulations of the charge density on the copper d_{x^2-y^2} orbitals. As a prime result of the AF Fermi surface reconstruction, the wavevectors of the charge modulations are oriented along the crystalline axes with a periodicity that agrees quantitatively with experiments. This resolves a discrepancy between experiments, which find axial order, and previous theoretical calculations, which find modulation wavevectors along the Brillouin zone (BZ) diagonal. The axial order is stabilized by hopping processes via the Cu4s orbital, which is commonly not included in model analyses of cuprate superconductors.
    Full-text · Article · Apr 2014 · New Journal of Physics
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    ABSTRACT: Using realistic multi-orbital tight-binding Hamiltonians and the T-matrix formalism, we explore the effects of a non-magnetic impurity on the local density of states in Fe-based compounds. We show that scanning tunneling spectroscopy (STS) has very specific anisotropic signatures that track the evolution of orbital splitting (OS) and antiferromagnetic gaps. Both anisotropies exhibit two patterns that split in energy with decreasing temperature, but for OS these two patterns map onto each other under 90 degree rotation. STS experiments that observe these signatures should expose the underlying magnetic and orbital order as a function of temperature across various phase transitions.
    No preview · Article · Aug 2013 · Physical Review B
  • Source
    Florian Loder · Arno P Kampf · Thilo Kopp
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    ABSTRACT: Two-dimensional electron systems at oxide interfaces are often influenced by a Rashba type spin-orbit coupling, which is tunable by a transverse electric field. Ferromagnetism near the interface can simultaneously induce strong local magnetic fields. This combination of spin-orbit coupling and magnetism leads to asymmetric two-sheeted Fermi surfaces, on which either intra- or inter-band pairing is favored. The superconducting order parameters are derived within a microscopic pairing model realizing both the Bardeen-Cooper-Schrieffer superconductor with inter-band pairing and a mixed parity state with finite-momentum intra-band pairing. We present a phase diagram for the superconducting groundstates and analyze the density of states, the spectra, and the momentum distribution functions of the different phases. The results are discussed in the context of superconductivity and ferromagnetism at LaAlO3-SrTiO3 interfaces and superconductors with broken inversion symmetry.
    Full-text · Article · Aug 2013 · Journal of Physics Condensed Matter
  • Source
    F Loder · A P Kampf · T Kopp
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    ABSTRACT: The magnetic flux threading a conventional superconducting ring is typically quantized in units of Φ_{0}=hc/2e. The factor of 2 in the denominator of Φ_{0} originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of Φ_{0}. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular, at multiples of Φ_{0}/2. In such states, condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where hc/4e flux periodicities were observed.
    Full-text · Article · Jul 2013 · Physical Review Letters
  • Source
    Kalpataru Pradhan · Arno P. Kampf
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    ABSTRACT: We use a two-orbital double-exchange model including Jahn-Teller lattice distortions, superexchange interactions, and long-range Coulomb (LRC) interactions to investigate the origin of magnetically disordered interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in FM/AFI superlattices. The induced magnetic moment in the AFI layer varies non-monotonically with increasing AFI layer width as seen in the experiment. We provide a framework for understanding this non-monotonic behavior which has a one-to-one correspondence with the magnetization of the FM interface. The obtained insights provide a basis for improving the tunneling magnetoresistance in FM/AFI manganite superlattices by avoiding a magnetic dead layer (MDL) in the FM manganite.
    Preview · Article · Jun 2013 · Physical review. B, Condensed matter
  • Source
    S. Bulut · W. A. Atkinson · A. P. Kampf
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    ABSTRACT: Charge order in cuprate superconductors is a possible source of anomalous electronic properties in the underdoped regime. Intra-unit cell charge ordering tendencies point to electronic nematic order involving oxygen orbitals. In this context we investigate charge instabilities in the Emery model and calculate the charge susceptibility within diagrammatic perturbation theory. In this approach, the onset of charge order is signalled by a divergence of the susceptibility. Our calculations reveal three different kinds of order: a commensurate ($q=0$) nematic order, and two incommensurate nematic phases with modulation wavevectors that are either axial or oriented along the Brillouin zone diagonal. We examine the nematic phase diagram as a function of the filling, the interaction parameters, and the band structure. We also present results for the excitation spectrum near the nematic instability, and show that a soft nematic mode emerges from the particle-hole continuum at the transition. The Fermi surface reconstructions that accompany the modulated nematic phases are discussed with respect to their relevance for magneto-oscillation and photoemission measurements. The modulated nematic phases that emerge from the three-band Emery model are compared to those found previously in one-band models.
    Full-text · Article · May 2013 · Physical Review B
  • Source
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    ABSTRACT: We study oxygen K-edge x-ray absorption spectroscopy (XAS) and investigate the validity of the Zhang-Rice singlet (ZRS) picture in overdoped cuprate superconductors. Using large-scale exact diagonalization of the three-orbital Hubbard model, we observe the effect of strong correlations manifesting in a dynamical spectral weight transfer from the upper Hubbard band to the ZRS band. The quantitative agreement between theory and experiment highlights an additional spectral weight reshuffling due to core-hole interaction. Our results confirm the important correlated nature of the cuprates and elucidate the changing orbital character of the low-energy quasiparticles, but also demonstrate the continued relevance of the ZRS even in the overdoped region.
    Full-text · Article · Apr 2013 · Physical review. B, Condensed matter
  • Sinan Bulut · William A. Atkinson · Arno Kampf
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    ABSTRACT: Recent experimental evidence for charge order in cuprates is a possible source of anomalous electronic properties in the underdoped regime. Intra-unit cell charge ordering tendencies point to electronic nematic order involving oxygen orbitals. In this context we investigate charge instabilities in the Emery model. The charge susceptibilities reveal three different kinds of nematic order. The first is an intra-unit cell (q=0) nematic order. The second and the third are incommensurate charge orders with wavevectors that are either uniaxial or oriented along the Brillouin zone diagonal. The two latter charge patterns correspond to a spatially modulated nematic phase. The selection of the leading instability depends on the filling, the interaction parameters, and details of the band structure. For these candidate charge orderings we discuss their possible relevance for the charge ordering signatures in X-ray and STM experiments.
    No preview · Article · Mar 2013
  • Kalpataru Pradhan · Arno P. Kampf
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    ABSTRACT: We investigate interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long range Coulomb interactions. In FM/AFI heterostructures the magnetic and the transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent FM and AFI manganites have the same electron density n. For n=0.6, the induced ferromagnetic moment in the AFI layers sandwiched between FM manganites decreases monotonically with increasing layer width. For n=0.5 instead, the induced ferromagnetic moment varies non-monotonously with the layer width. These differences for n=0.6 and n=0.5 originate from different charge-transfer profiles and magnetic reconstructions at the FM/AFI interfaces. The width of the AFI layers furthermore controls the magnitude of the magnetoresistance and the metal to insulator transition of the FM/AFI heterostructure. These results are discussed in the context of recent experiments on LSMO/PCMO [1] and LCMO/PCMO superlattices [2].[4pt] [1] D. Niebieskikwiat et al., Phys. Rev. Lett. 99, 247207 (2007).[0pt] [2] H. Li et al. Appl. Phys. Lett. 80, 628 (2002).
    No preview · Article · Mar 2013
  • Florian Loder · Arno Kampf · Thilo Kopp
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    ABSTRACT: The magnetic flux threading a conventional superconducting ring is typically quantized in units of φ0=hc/2e. The factor 2 in the denominator of φ0 originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of φ0. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular at multiples of φ0/2. In such states condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where hc/4e flux periodicities as well as uniaxially modulated superconducting states were observed.
    No preview · Article · Mar 2013
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    ABSTRACT: In iron-based superconductors, nematicity has been reported in transport measurements and a broad range of spectroscopies, including angle-resolved photoemission, neutron scattering, and scanning tunneling spectroscopy (STS). Several theories have attributed these observed anisotropies of broken tetragonal symmetry to either pure spin physics or unequal occupation of the iron d-electron orbitals, referred to as orbital ordering. We use realistic multi-orbital tight-binding Hamiltonians and T-matrix formalism to explore the effects of non-magnetic impurities in an orbitally split and spin density wave (SDW) state. In each of these, the local density of states around the impurity in both position space and Fourier-transformed quasiparticle interference (QPI) have very specific signatures that may be observable in STS. These allow one to identify and track the evolution of orbital splitting and SDW gaps in regimes that have not previously been explored.
    No preview · Article · Mar 2013
  • Source
    Kalpataru Pradhan · Arno P. Kampf
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    ABSTRACT: We investigate the electronic reconstruction at the interface between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in superlattices using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long range Coulomb interactions. The magnetic and the transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent manganites have the same electron density n = 0.6. The induced ferromagnetic moment in the AFI layers decreases monotonically with increasing layer width, and the electron-density profile and the magnetic structure in the center of the AFI layer gradually return to the bulk limit. The width of the AFI layers and the charge-transfer profile at the interfaces control the magnitude of the magnetoresistance and the metal-insulator transition of the FM/AFI superlattices.
    Preview · Article · Feb 2013 · Physical review. B, Condensed matter
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study oxygen K-edge x-ray absorption spectroscopy (XAS) and investigate the validity of the Zhang-Rice singlet (ZRS) picture in overdoped cuprate superconductors. Using large-scale exact diagonalization of the three-orbital Hubbard model, we observe the effect of strong correlations manifesting in a dynamical spectral weight transfer from the upper Hubbard band to the ZRS band. The quantitative agreement between theory and experiment highlights an additional spectral weight reshuffling due to core-hole interaction. Our results confirm the important correlated nature of the cuprates and elucidate the changing orbital character of the low-energy quasi-particles, but also demonstrate the continued relevance of the ZRS even in the overdoped region.
    Full-text · Article · Nov 2012
  • Source
    Markus Schmid · Florian Loder · Arno P. Kampf · Thilo Kopp
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    ABSTRACT: Stripe phases are observed experimentally in several copper-based high-Tc superconductors near 1/8 hole doping. However, the specific characteristics may vary depending on the degree of dopant disorder and the presence or absence of a low- temperature tetragonal phase. On the basis of a Hartree-Fock decoupling scheme for the t-J model we discuss the diverse behavior of stripe phases. In particular the effect of inhomogeneities is investigated in two distinctly different parameter regimes which are characterized by the strength of the interaction. We observe that small concen- trations of impurities or vortices pin the unidirectional density waves, and dopant disorder is capable to stabilize a stripe phase in parameter regimes where homogeneous phases are typically favored in clean systems. The momentum-space results exhibit universal features for all coexisting density-wave solutions, nearly unchanged even in strongly disordered systems. These coexisting solutions feature generically a full energy gap and a particle-hole asymmetry in the density of states.
    Full-text · Article · Aug 2012 · New Journal of Physics
  • Source
    Florian Loder · Arno P. Kampf · Thilo Kopp
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    ABSTRACT: The periodic response of a metallic or a superconducting ring to an external magnetic flux is one of the most evident manifestations of quantum mechanics. It is generally understood that the oscillation period hc/2e in the superconducting state is half the period hc/e in the metallic state, because the supercurrent is carried by Cooper pairs with a charge 2e. On the basis of the Bardeen-Cooper-Schrieffer theory we discuss, in which cases this simple interpretation is valid and when a more careful analysis is needed. In fact, the knowledge of the oscillation period of the current in the ring provides information on the electron interactions. In particular, we analyze the crossover from the hc/e periodic normal current to the hc/2e periodic supercurrent upon turning on a pairing interaction in a metal ring. Further, we elaborate on the periodicity crossover when cooling a metallic loop through the superconducting transition temperature Tc.
    Full-text · Article · Jun 2012

Publication Stats

3k Citations
434.88 Total Impact Points

Institutions

  • 1997-2015
    • Universität Augsburg
      • Institute of Physics
      Augsberg, Bavaria, Germany
  • 1992-2007
    • Forschungszentrum Jülich
      Jülich, North Rhine-Westphalia, Germany
  • 1987-1997
    • University of Cologne
      • Institute for Theoretical Physics
      Köln, North Rhine-Westphalia, Germany
  • 1991
    • University of California, Santa Barbara
      • Department of Physics
      Santa Barbara, California, United States