Events

Studies of disordered spin chains have recently experienced a renewed interest, inspired by the question to which extent the exact numerical calculations comply with the existence of a many-body localization phase transition. For the paradigmatic random field Heisenberg spin chains, many intriguing features were observed when the disorder is considerable compared to the spin interaction strength. Here, we introduce a phenomenological theory that may explain some of those features. The theory is based on the proximity to the noninteracting limit, in which the system is an Anderson insulator. Taking the spin imbalance as an exemplary observable, we demonstrate that the proximity to the local integrals of motion of the Anderson insulator determines the dynamics of the observable at infinite temperature. In finite interacting systems our theory quantitatively describes its integrated spectral function for a wide range of disorders .

We study dynamical correlation functions in the random-field Heisenberg chain, which probe the relaxation times at different length scales. Firstly, we show that the relaxation time associated with the dynamical imbalance (examining the relaxation at the smallest length scale) decreases with disorder much faster than the one determined by the dc conductivity (probing the global response of the system). We argue that the observed dependence of relaxation on the length scale originates from local nonresonant regions. The latter has particularly long relaxation times or remains frozen, allowing for nonzero dc transport via higher-order processes. Based on the numerical evidence, we introduce a toy model that suggests that the nonresonant regions' asymptotic dynamics are essential for the proper understanding of disordered chains. In addition, the toy model explains the exponential dependence of the transport properties on the disorder and the broad distribution of dc conductivity.

Watch the lecture:

Fundamental quasiparticle interactions in solids such as electron-electron and electron-phonon scattering are of fundamental importance e.g. for electronic and optical material properties. While for simple systems such as quasi-free electron gases, fairly good descriptions have been reached, for more complex solids, it remains highly challenging to unravel their intricate interplay, and to identify the dominant channels. Additional challenges arise from broken-symmetry ground states emerging due to strong correlations in complex quantum materials. A way to tackle this problem is by tracking a material’s dynamical response after ultrafast optical excitation, as this approach yields direct insight into energy dissipation and scattering processes on their intrinsic timescales. In addition, tailored photoexcitation also provides a way to dynamically control specific material properties, and thereby modify the material’s intrinsic interaction channels.

Here, we combine these two approaches, by tracking the fundamental quasiparticle interactions during a photoinduced insulator-to-metal phase transition in a charge-density-wave (CDW) material. Using time- and angle-resolved photoemission spectroscopy (trARPES), we study the transient electronic structure of the prototypical CDW compound TbTe3. By employing strong optical excitation, we suppress the CDW energy gap at the Fermi level, thereby driving the system into a transient metallic state, followed by a coherent evolution of the system within the transient CDW potential . Simultaneously, we track the relaxation of highly excited, hot quasiparticles, and find a highly unusual modulation of their relaxation rate. State-of-the-art calculations based on non-equilibrium Green’s functions provide a microscopic view onto the interplay of quasiparticle scattering and the (transiently modified) electronic band structure, which allow us to quantify the modification of particle-hole scattering processes due to the influence of the CDW energy gap .

.

For some time, cryo-computing has been severely limited by the absence of a suitable fast and energy efficient low-temperature memory making it an ideal platform for energy efficient memories. Conventional superconducting memories use an architecture based on Josephson junctions (JJs. Ideally, such memory should be compatible with single-flux quantum (SFQ) logic in terms of speed, switching energy and matching impedance. Here we present an implementation of non-volatile charge configuration memory (CCM)3,4 in a cryo-computing environment with a hybrid device incorporating a superconducting nanowire cryotron (nTron)5. The dynamical response of the device is modelled in terms of the superconducting order parameter in a confined channel of a current-controlled nanowire with a CCM shunt6geared towards understanding and controlling coherence and dissipation in nanowires. The dynamics is probed by measuring the evolution of the V-I characteristics and the distributions of switching and retrapping currents upon varying the shunt resistor and temperature. Theoretical analysis of the experiments indicates that as the value of the shunt resistance is decreased, the dynamics turns more coherent presumably due to stabilization of phase-slip centers in the wire and furthermore the switching current approaches the Bardeen’s prediction for equilibrium depairing current. By a detailed comparison between theory and experimental, we make headway into identifying regimes in which the quasi-one-dimensional wire can effectively be described by a zero-dimensional circuit model analogous to the RCSJ (resistively and capacitively shunted Josephson junction. Analysis of time-dynamics and current-voltage characteristics based on measured device parameters show that single flux quantum (SFQ)-level pulses can drive non-volatile CCM on the picosecond timescale. We also present first measured current-voltage characteristics and read operation of actual hybrid memory devices showing expected behaviour.

The inherent high energy efficiency and ultrahigh speed makes this hybrid device an ideal memory for use in cryo-computing and quantum computing peripheral devices.

Understanding how the dynamics of a given quantum system with many degrees of freedom is altered by the presence of a generic perturbation is a notoriously difficult question. Recent works predict that, in the overwhelming majority of cases, the unperturbed dynamics is just damped by a simple function, e.g., exponentially as expected from Fermi's Golden Rule. While these predictions rely on random-matrix arguments and typicality, they can only be verified for a specific physical situation by comparing to the actual solution or measurement. Crucially, it also remains unclear how frequent and under which conditions counterexamples to the typical behavior occur. In this work, we discuss this question from the perspective of projection-operator techniques, where exponential damping of a density matrix occurs in the interaction picture but not necessarily in the Schrödinger picture. We show that a nontrivial damping in the Schrödinger picture can emerge if the dynamics in the unperturbed system possesses rich features, for instance due to the presence of strong interactions. This suggestion has consequences for the time dependence of correlation functions. We substantiate our theoretical arguments by large-scale numerical simulations of charge transport in the extended Fermi-Hubbard chain, where the nearest-neighbor interactions are treated as a perturbation to the integrable reference system.

Characterizing the delocalization transition in closed quantum systems with a many-body localized phase is a key open question in the field of nonequilibrium physics. We exploit the fact that localization of particles as realized in Anderson localization and standard many-body localization (MBL) implies Fock-space localization in single-particle basis sets characterized by a real-space index. Using a recently introduced quantitative measure for Fock-space localizationcomputed from the density distributions, the occupation distance , we systematically study its scaling behavior across delocalization transitions .We first identify critical points from scaling collapses of numerical data with an excellent agreement with literature results for the critical disorder strengths of noninteracting fermions, such as the one-dimensional Aubry-André model and the three-dimensional Anderson model. We then observe a distinctively different scaling behavior in the case of interacting fermions with random disorder consistent with a Kosterlitz-Thouless transition. Finally, we use our measure to extract the transition point as a function of filling for interactsing fermions .

An excited electron propagates in condensed matter with its momentum k at an energy E(k) and experiences elastic and inelastic scattering processes, which lead to electronic relaxation and energy transfer to microscopic excitations of the lattice and spin systems. Experiments employing femtosecond time-resolved photoelectron spectroscopy exploited so far very successfully the surface sensitivity of the method and probed such scattering processes locally at or near the surface in the time domain . Here, we report on experimental results which analyze the non-local dynamics of excited electrons in two-photon photoemission (2PPE) and demonstrate sensitivity to buried media . In these experiments one photon excites in Au/Fe/MgO(001) heterostructures electrons in Fe. Electron propagation through the layer stack to the Au surface is detected in 2PPE in back side pump – front side probe experiments in a time-of-flight like scheme. We observe pronounced differences between front and back side pumping of the heterostructure which are attributed to electron transport contributions through the layer stack. Furthermore, competition of e-e with e-ph scattering will be discussed in n heterostructures. Pump-probe experiments of element specific spectroscopy in combination with electron diffraction provide here unprecedented insights regarding the mechanism of energy transfer across interfaces and emphasize the importance of coupling hot electrons to non-thermalized interface phonons . Extension of these experimental tools to address effects of strong electron correlation and spin-dependent dynamics across interfaces will be discussed. This work was funded by the Deutsche Forschungsgemeinschaft through the Collaborative Research Center CRC 1242.

Controlling material properties by illumination with ultrashort optical pulses is a promising new pathway to extend the functionality of complex solids. Prominent examples of this rapidly growing research field include photostabilization of superconductivity and switching to metastable states not accessible in thermal equilibrium . A metastable state of particular interest is the optically or electrically induced hidden phase of 1T-TaS2, as it features an order-of-magnitude change in resistivity , which allows for novel energy-efficient high-speed memory devices . However, so far a clear understanding of the microscopic processes that govern the dynamic pathways to metastable states is still missing, limiting controllability due to empirical and unspecific switching protocols. Here, using time- and angle-resolved photoemission spectroscopy (trARPES), we investigate the electronic band structure and ultrafast photoinduced phase transition from commensurate charge-density-wave (CDW) ground state to the hidden state in 1T-TaS2. Mapping the band structure of the hidden state reveals suppression of correlation effects and confirms metallization, suggesting a critical role of interlayer stacking order of the TaS2 sheets in the hidden state. Next, we track the fluence-dependent electron dynamics upon photoexcitation and find strong evidence that the CDW amplitude mode governs a collective, ultrafast switching pathway to the hidden state. This is further corroborated by demonstrating coherent control of the switching efficiency into the hidden phase by controlling the CDW amplitude mode using a multi-pump-pulse excitation scheme. We envision that the amplitude-mode governed transition applies to a range of CDW compounds .

We report a time-resolved ultrafast quasiparticle dynamics investigation of cubic boron arsenide (c-BAs), which is a recently discovered highly thermal conducting material. The excited state ultrafast relaxation channels dictated by the electron-phonon coupling (EPC), phonon-phonon scattering, and radiative electron-hole recombination have been unambiguously identified, along with their typical interaction times . Significantly, the EPC strength is obtained from the dynamics, with a value of = 0.008, demonstrating an unusually weak coupling between the electrons and phonons. As a comparison, an ultraweak EPC strength for graphene is also expected. Notably, during our analysis we have generalized the fluence-dependence method for obtaining the EPC strength to room temperature, which can be applied to many other types of quantum materials in the future. Time permits, I will also talk about two other recent progresses: (1) We have conceived and constructed an on-site in situ high-pressure ultrafast pump-probe spectroscopy instrument that facilitates ultrafast pump-probe dynamics measurements under high pressure condition . (2) We demonstrate that second harmonic generation (SHG) is a local microscopic process—it does not rely on macroscopic broken inversion symmetry . An AB-type superstructure can generate SHG easily and an ABC-type super-structure is not necessary. Our results introduce a scheme for SHG, extending the generation and control of SHG in nano-photonics to nearly all the accessible centrosymmetric materials.

.

Watch the lecture:

Watch the lecture:

Using the dynamical mean field theory we investigate the magnetic field dependence of dc conductivity in the Hubbard model on the square lattice, fully taking into account the orbital effects of the field introduced via the Peierls substitution. In addition to the conventional Shubnikov–de Haas quantum oscillations, associated with the coherent cyclotron motion of quasiparticles and the presence of a well-defined Fermi surface, we find an additional oscillatory component with a higher frequency that corresponds to the total area of the Brillouin zone. These paradigm-breaking oscillations appear at elevated temperature. This finding is in excellent qualitative agreement with the recent experiments on graphene superlattices. We elucidate the key roles of the off-diagonal elements of the current vertex and the incoherence of electronic states, and explain the trends with respect to temperature and doping.

.