Weyl fermions can arise in Weyl semimetals (WSMs) in which the energy bands are usually nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetic WSMs is scarce. Here, using photoemission spectroscopy, we observe the degeneracy of Bloch bands already lifted in the paramagnetic phase of EuCd2As2. We attribute this effect to the...
Micro-resonators fabricated by optical lithography can be combined with scanning transmission x-ray microscope (STXM) and a time-resolved detection scheme to measure magnetic excitations in ferromagnetic resonance (FMR) with ultimate spatio-temporal resolution of nominally 35 nm and a snapshot detection down to 17.4 ps [1]. Two perpendicular Permalloy micro-stripes were fabricated using ...
Spin pumping is an efficient mechanism for the inception of spin current and for its conversion into charge current in non-magnetic metals or semiconductors via spin Hall effects. The generation of spin current in bilayers Py/n-GaN:Si is here reported. In n-GaN:Si and for a layer thickness greater than the spin diffusion length - a condition not met in previous studies on e.g. n-ZnO - a spin...
The field of STM induced light emission (STM-LE), especially on single molecules, has grown rapidly in the past 25 years [2] with astounding spatial as well as energetic resolution [3]. Yet, combining structural and optical information on single molecules remains challenging.
We present first results of a combined AFM and STM-LE setup on single vanadyl-phthalocyanine (VOPc) molecules. This...
In this talk I present a new ab-initio theory for superconducting systems with non-local external potentials based on the one-particle reduced density matrix ρ(r,r′) and the anomalous density χ(r,r′). All the equilibrium properties of the system are determined uniquely by these two quantities. By replacing the local electronic density with the non-local one-particle reduced density matrix, our...
Coulomb phases can be constructed for degrees of freedom that obey an ice rule on the pyrochlore lattice. Using diffuse and single crystal neutron and powder x-ray scattering, we identify a structural Coulomb phase (charge ice) formed by the Ni$^{2+}$ and Cr$^{3+}$ cations in CsNiCrF$_6$. The cations form a configurational structural ice and dictate local distortions of the (Ni/Cr)F$_6$...
Semiconductor-Metal heterostructures act as energy filters for charge transport with promising applications in thermoelectric energy conversion. Using a scanning thermal microscope technique we measure the temperature distribution of operando Al-Ge-Al nanowire devices integrated in a back-gated field effect transistor. The Ge segments are contacted with self-aligned quasi one-dimensional...
The frustrated nearest-neighbour antiferromagnetic Ising model on the kagome lattice exhibits a macroscopic ground state degeneracy. We implement an MCMC algorithm to study the degeneracy lifting leading to the ground state of the dipolar model, focusing on models with up to fourth neighbour interactions.
The ground state of the J1-J2-J3 model exhibits five different phases as a function of...
The quantum magnet BaCuSi2O6, consisting of stacked spin dimer bilayers, undergoes an anomalous dimensional reduction from 3D to 2D close to the quantum critical point [1]. Mechanisms for this dimensional reduction were proposed based on inter-bilayer frustration resulting from an antiferromagnetic intra-bilayer exchange. Ab initio calculations propose a ferromagnetic intra-bilayer exchange...
Here, we have studied the magnetically ordered phase of Ca2RuO4 at T = 16 K, using O-K edge Resonant Inelastic X-ray Scattering technique. Four
excitations have been identifed- 2 low energy exci-
tations at 80 meV and 400 meV respectively and two high energy excitations at energies 1.3 eV and 2.2 eV . The low energy peaks are interpreted
to be arising from composite spin- orbital...
Whether the magnetic excitations in doped cuprates are described by paramagnons or the continuum of charge and spin excitations of correlated electrons is still controversial. Recent RIXS studies with azimuthal-dependent measurements for polarization analysis demonstrated how charge and spin nature of the low-energy excitations can be resolved, providing thereby a way to study their properties...
We investigate the internal structure of the domain walls (DWs) in Tm3Fe5O12 (TmIG) and TmIG/Pt bilayers and demonstrate their efficient manipulation by spin-orbit torques with velocities of up to 400 m/s and current threshold for DW flow of 5x10^6 A/cm^2. Pt current lines patterned on extended TmIG films allow controlling DW propagation and magnetization switching in selected regions....
Co$_x$Zn$_{1-x}$O – Permalloy (Py) heterostructures were investigated with frequency-dependent ferromagnetic resonance (FMR), x-ray magnetic circular dichroism (XMCD) and SQUID magnetometry. At low temperatures Co$_x$Zn$_{1-x}$O is an uncompensated antiferromagnet showing a narrowly opened hysteresis and a vertical exchange-bias effect [1,2]. By means of SQUID a static interaction is...
La$_2$NiMnO$_6$ (LNMO) is an insulating double perovskite oxide with ferromagnetic Curie temperature around 280K driven by the oxygen-mediated super exchange interaction between long-range ordered Ni$^{2+}$ and Mn$^{4+}$ ions. An insulating ferromagnet would be ideal for novel spintronic devices but only a few attempts of growing ultrathin films have been reported.
Here we show that the...
Since many years, rare earth nickelates attract the researchers interest due to their huge variety of fascinating physical properties which are tunable by the interplay of electron correlations and crystal structure. Further, these systems show dimensionality-driven transitions (e.g. LaNiO3 (LNO) thin films), and strain-induced transitions (e.g. NdNiO3 heterostructures).
We investigate the...
Many perovskite transition metal oxides (TMOs) exhibit metal-insulator transitions whose complex physics is not fully understood. We use soft-X-ray ARPES to visualize how the electrons delocalize and couple to bosonic lattice excitations in CaMnO3 upon its doping with Ce. We show the progressive development of a complex Fermi surface where mobile electrons weakly coupled to lattice coexist...
SrMnO3 can be strain-engineered to be multiferroic, with coexisting and tuneable magnetic and ferroelectric (FE) order. We recently showed, using first principles calculations, that the ferroic strain-temperature phase diagram of SrMnO3 accommodates a tetracritical point (TCP) with coinciding magnetic and FE ordering temperatures.
Here, we construct a Landau theory...
We show electronic transport data on a particular interesting carbon system: Two vertically stacked layers of graphene that are twisted with respect to each other. With the twist angle, the properties change fundamentally. The two layers are decoupled at large and strongly coupled at small angles. It is even possible to achieve superconductivity at a certain twist angle.
To probe the system...
We investigate the electronic structure of IrTe$_2$ to elucidate the origin of its charge-ordered phase transitions. Here, we present an X-ray photoemission spectroscopy study as a function of temperature across the IrTe$_2$ phase transitions. Our surface sensitive measurements reveal new results on the specific nature of the transitions in contradiction with the literature. According to our...
Novel properties and exciting perspectives are offered by two-dimensional magnetic materials, like binary MX2 and ternary MYZ3 (M is a metal element; X is a halogen; Y = Si, Ge or P; Z is a chalcogen). Various complementary growth techniques are employed to produce these materials in crystalline form, namely the Chemical Vapor Transport and the high temperature solution (flux) growth. Here we...
The electronic structure of 2D materials undergoes significant changes as their thickness is reduced down to the atomic limit. In few layer black phosphorus (BP) crystals, a promising semiconductor for optoelectronic and electronic applications, the bandgap increases drastically and the effective mass at the valence band and conduction band edges changes significantly. Here, we present the...
We will report on a templated electrochemical technique for patterning arrays of single-crystalline Si nanowires with feature dimensions down to 5 nm. This technique, termed three-dimensional electrochemical axial lithography (3DEAL),[1] allows the design and parallel fabrication of hybrid silicon nanowire arrays decorated with complex metal nano-ring architectures in a flexible and modular...
We have previously demonstrated a novel approach for the growth of III-V nanowires on Si, using focused ion beam (FIB) -implanted Ga as nucleation points for self-catalysed GaAs nanowire growth. In this work, we have further investigated the possibility of growing optically active nanowires using this technique, via the growth of GaAs nanowires containing single InGaAs quantum wells in the...
Graphene is a promising candidate for nano-electronic devices, due to the expected long spin lifetimes and high carrier mobilities. Improvements in fabrication for graphene nanostructures have leveraged their quality to such an extent, that quantum dots in bilayer graphene are comparable to the best devices in gallium arsenide [1].
We use finite bias spectroscopy to identify the...
Bilayer-Graphene based Nanostructures promise unique opportunities in the field of quantum electronics. However, the endeavor to form quantum electronic building blocks such as Quantum Point Contacts (QPCs) and Quantum Dots (QDs) is significantly hampered by the presence of disorder.
To understand the influence of disorder on the formation of QPCs and QDs in Graphene, we employ Scanning...
The mid-infrared spectral region is referred to as fingerprint region since molecules have their unique fundamental absorption features there. Addressing this optical regime, quantum cascade technology provides innovative optoelectronic devices to significantly improve integration and performance in chemical sensing. In this work, we present a room-temperature monolithically-integrated quantum...
Quantum cascade laser (QCL) has established itself as the main laser source in the mid-infrared portion of the electromagnetic spectrum. Due to a substantial third-order non-linearity in the laser active region, QCLs can work in a self-starting frequency comb regime, making them interesting for spectroscopic applications. A model based on Maxwell-Bloch equations was developed in order to study...
We present a dual-comb spectrometer consisting of a free running frequency comb quantum cascade laser (QCL) and its Doppler shifted counterpart reflected from a fast scanning mirror. The stable multi-heterodyne signal is centered at $\sim$ 400 kHz and well defined by the linear scanning velocity of the reflector. This dual comb spectrometer features higher stability than the standard dual...
Quantum cascade lasers (QCL) are a compact and electrically pumped source of coherent mid-infrared light. Recently, it was discovered that QCLs can operate as frequency combs whose output is characterized by suppression of amplitude modulation and strong frequency modulation. However, the generation of short pulses by mode-locking of mid-infrared QCLs remains challenging to date due to their...
Frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts. We present an overview of our current research on mid-infrared frequency comb generation using interband and quantum cascade lasers (ICLs and QCLs). Our work ranges from fundamental laser physics to the realization of monolithic devices. We will highlight similarities and differences between these...
THz radiation is subject to a wide range of research and technological efforts, but it is limited by a lack of compact and powerful THz sources. A promising candidate is the quantum cascade laser (QCL), although it currently requires cryogenics since they only operate below 200 K. We present the first THz QCL operating on a thermoelectric cooler, up to a record-high temperature of 210 K. The...
We present the first interband cascade lasers fabricated into ring-shaped cavities emitting in continuous wave operation. A second order distributed feedback grating is used for single mode emission and light outcoupling in vertical direction through the GaSb substrate. In addition, the implementation of an epitaxial-side down mounting scheme facilitates improved heat transport from the active...
The performance of state-of-the-art GaAs-based THz-QCLs is limited by parasitic LO phonon transitions, preventing above-200 K operation. This can be overcome by using material systems with higher LO-phonon energies like ZnO, for which above-room-temperature operation in THz-QCLs is predicted. Using novel optoelectronic materials like wurzite Zn(Mg)O with no internal fields in the m-plane...
Exploiting intersubband transitions in Ge/SiGe quantum cascade devices provides a way to integrate terahertz light emitters into silicon-based technology. To date all electroluminescence demonstrations of Si-based heterostructures have been p-type using hole-hole transitions. In the pathway of realizing an n-type Ge/SiGe terahertz quantum cascade laser, we present electroluminescence...
Superfluorescence is a many-body collective coupling phenomenon, where coherence is established through spontaneously triggered correlations of quantum fluctuations from initially fully uncorrelated excited emitters. Here, we investigate densely packed cuboidal arrays of fully inorganic cesium lead halide perovskite quantum dots, known as superlattices and we observe key signatures of...
