A5035653147- Physics of Superconductivity and Magnetism
- Quantum and electron transport phenomena
- Advanced Condensed Matter Physics
- Quantum many-body systems
- Theoretical and Computational Physics
- Cold Atom Physics and Bose-Einstein Condensates
- Magnetic properties of thin films
- Magnetic and transport properties of perovskites and related materials
- Quantum Information and Cryptography
- Topological Materials and Phenomena
- Advanced NMR Techniques and Applications
- Spectroscopy and Quantum Chemical Studies
- Quantum Computing Algorithms and Architecture
- Quantum, superfluid, helium dynamics
- Semiconductor Quantum Structures and Devices
- Organic and Molecular Conductors Research
- Iron-based superconductors research
- NMR spectroscopy and applications
- Atomic and Subatomic Physics Research
- Magnetism in coordination complexes
- Quantum optics and atomic interactions
- Advanced Chemical Physics Studies
- Quantum chaos and dynamical systems
- Laser-Matter Interactions and Applications
- Rare-earth and actinide compounds
TU Dortmund University
2016-2025
UNSW Sydney
2009
University of Cologne
1997-2007
École Polytechnique Fédérale de Lausanne
2006-2007
Saarland University
2005-2006
Tohoku University
2004
Université Paris-Sud
1996-1997
Laboratoire de physique des Solides
1996-1997
RWTH Aachen University
1992-1994
FH Aachen
1992-1994
A general strategy to maintain the coherence of a quantum bit is proposed. The analytical result derived rigorously including all memory and backaction effects. It based on an optimized $\ensuremath{\pi}$-pulse sequence for dynamic decoupling extending Carr-Purcell-Meiboom-Gill cycle. very efficient, in particular, strong couplings environment.
The aim of dynamical decoupling consists in the suppression decoherence by appropriate coherent control a quantum register. Effectively, interaction with environment is reduced. In particular, sequence π pulses considered. Here we present exact results on coupling bit to its optimized sequences pulses. effect various cut-offs spectral density investigated. As result show that harder cut-off is, better an pulse can deal it. For which are neither completely hard nor very soft advocate iterated...
Abstract In equilibrium systems amplitude and phase collective modes are decoupled, as they mutually orthogonal excitations. The direct detection of these Higgs Leggett by linear-response measurements is not possible, because do couple directly to the electromagnetic field. this work, using numerical exact simulations we show for case two-gap superconductors, that optical pump–probe experiments excite both out equilibrium. We find non-adiabatic excitation process introduces a strong...
Motivated by recent measurements on CuGe${\mathrm{O}}_{3}$, the spectrum of magnetic excitations an antiferromagnetic $S=\frac{1}{2}$ chain with alternating coupling strength is investigated. Wave-vector-dependent magnons and a continuum square-root behavior at band edges are found. The spectral density continua calculated. Spin-rotation symmetry fixes gap to be twice elementary magnon gap. This in excellent agreement experimental results. existence (anti)bound states two predicted: below...
Bound states made from two triplet excitations on the Shastry-Sutherland lattice are investigated. Based perturbative unitary transformation by flow equations quantitative properties like dispersions and qualitative symmetries determined. The high order results [up to (J2/J1)(14)] permit one fix parameters of SrCu2(BO3)(2) precisely: J1 = 6.16(10) meV, x J2/J1 0.603(3), J( perpendicular) 1.3(2) meV. At border magnetic Brillouin zone a general double degeneracy is derived. An unexpected...
We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was separation of one-triplon two-triplon scattering. Two-triplon is observed quantitatively first time so access realized to important strong magnetic quantum fluctuations. The spin gap found be 26.4+/-0.3 meV. data are successfully modeled using continuous unitary transformation...
We study the magnetic-field-induced quantum phase transition from a gapped that has no magnetic long-range order into gapless in spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY). At temperatures below about 1 K, specific heat attains an asymptotic linear temperature dependence, characteristic of Tomonaga-Luttinger liquid. Inelastic neutron scattering and measurements both phases are good agreement with theoretical calculations, demonstrating DIMPY is first model...
Two recent developments in quantum control, concatenation and optimization of pulse intervals, are combined to yield a strategy suppress unwanted couplings systems high order. Longitudinal relaxation transverse dephasing can be suppressed so that with small splitting between their energy levels kept isolated from environment. The required number pulses grows exponentially the desired order but is only square root needed if used. An approximate scheme even brings down polynomial growth....
The transition from dimerized to uniform phases is studied in terms of spectral weights for spin chains using continuous unitary transformations. the S=1 channel are computed perturbatively around limit strong dimerization. We find that weight concentrated mainly subspaces with a small number elementary triplets (triplons), even vanishing So, besides spinons, triplons may be used as excitations chains. conclude there no necessity use fractional low-dimensional, undoped, or doped quantum...
We study a generalized Shastry-Sutherland model for the material ${\mathrm{SrCu}}_{2}({\mathrm{BO}}_{3}{)}_{2}$. Along line in parameter space, we show rigorously that has first-order phase transition between dimerized and N\'eel-ordered ground states. Furthermore, when magnetic field is applied phase, magnetization plateaus develop at commensurate values of magnetization. also discuss various aspects diagram properties this away from exactly soluble line, which include gap-closing...
Based on a two-dimensional model of coupled two-leg spin ladders, we derive unified picture recent neutron scattering data stripe-ordered La15/8Ba1/8CuO4, namely, the low-energy magnons around superstructure satellites and triplon excitations at higher energies. The resonance peak antiferromagnetic wave vector Q(AF) in phase corresponds to saddle point dispersion magnetic excitations. Quantitative agreement with is obtained for J=130-160 meV Jcyc/J=0.2-0.25.
We analyze the suppression of decoherence by means dynamical decoupling in pure-dephasing spin-boson model for baths with power law spectra. The sequence ideal $\ensuremath{\pi}$ pulses is optimized according to bath. expand function and separate canceling divergences from relevant terms. proposed chosen be one minimizing function. By construction, it provides best performance. analytically derive conditions that must satisfied. resulting equations are solved numerically. solutions very...
We theoretically study the pump-probe response of nonequilibrium BCS superconductors coupled to optical phonons. For ultrashort pump pulses a nonadiabatic regime emerges, which is characterized by oscillations superconducting order parameter as well generation coherent Using density-matrix formalism, we compute in Bogoliubov quasiparticle-phonon system and determine signatures phonon conductivity. find that dynamics superconductor reflects itself functions delay time $\ensuremath{\delta}t$...
A quantitative description of magnons in long-range ordered quantum antiferromagnets is presented which consistent from low to high energies. It illustrated for the generic $S=1/2$ Heisenberg model on square lattice. The approach based a continuous similarity transformation momentum space using scaling dimension as truncation criterion. Evidence found significant magnon-magnon attraction inducing Higgs resonance. high-energy roton minimum magnon dispersion appears be induced by strong...
The Heisenberg model for S=1/2 describes the interacting spins of electrons localized on lattice sites due to strong repulsion. It is simplest strong-coupling in condensed matter physics with wide-spread applications. Its relevance has been boosted further by discovery curate high-temperature superconductors. In leading order, their undoped parent compounds realize square-lattices. Much known about model, but mostly at small wave vectors, i.e., long-range processes, where governed spin waves...
Recent findings of new Higgs modes in unconventional superconductors require a classification and characterization the allowed by nontrivial gap symmetry. Here we develop theory for tailored nonequilibrium quantum quench to excite all possible oscillation symmetries superconducting condensate. We show that both finite momentum transfer symmetry allow an identification resulting oscillations. These serve as fingerprint ground state provide scheme these oscillations based on group underlying...
Phonon-assisted two-magnon absorption is studied in the spin- 1/2 two-leg ladders of (Ca,La)(14)Cu(24)O(41) for E parallel c (legs) and a (rungs). We verify theoretically predicted existence singlet bound states, which give rise to peaks at approximately equal 2140 2800 cm(-1). The continuum observed 4000 Two different theoretical approaches (Jordan-Wigner fermions perturbation theory) describe data very well J 1020-1100 cm(-1), parallel/J perpendicular 1-1.2. At high energies, magnetic...
Spectral densities are computed in unprecedented detail for quantum antiferromagnetic spin 1/2 two-leg ladders. These results were obtained due to a major methodical advance achieved by optimally chosen unitary transformations. The approach is based on dressed integer excitations. Considerable weight found at high energies the two-particle sector. Precursors of fractional spinon physics occur supporting conclusion that there no necessity resort excitations order describe features higher energies.
We investigate the instability of saturated ferromagnetic ground state (Nagaoka state) in Hubbard model on various lattices dimensions $d=2$ and $d=3$. A variational resolvent approach is developed for Nagaoka both $U=\ensuremath{\infty}$ $U<\ensuremath{\infty}$ which can easily be evaluated thermodynamic limit all common lattices. Our results significantly improve former bounds a possible regime ground-state phase diagram model. show that pronounced particle-hole asymmetry density states...