Atomar aufgelöste Dynamik von korrelierten Quantensystemen

In this thesis ultra-fast phenomena are investigated with a scanning tunneling microscope (STM). The real space influence of atomic sized defects on ultra-fast dynamics in correlated systems is one of the great mysteries in experimental research of solid states and is investigated in this thesis using two different approaches. In the first part an artificially built few atom magnet is investigated whose dynamic prop-erties are slowed down by placing it on a decoupling layer. The dynamic properties have a direct impact on spin-dependent transport and lead to the appearance of negative dif-ferential resistance. The effects can be astonishingly well described by a rate equation model which allows a deep insight into the processes occurring. The dynamics investigated in the first part are in the microsecond to nanosecond regime, as much faster processes cannot be measured by a conventional STM. To break this barri-er, in the second part of the thesis the development of a new unconventional STM is pre-sented. By coupling picosecond free-space terahertz (THz) laser pulses into the tunnel junction and inducing ultra-fast voltage pulses this new instrument enables pump-probe experiments with femtosecond time resolution on the atomic scale. In the last part of the thesis the dynamics of the charge density wave (CDW) system 2H-NbSe2 are investigated with the new THz-STM. A complex dynamic response is thereby observed consisting of a 600 fs decay and oscillating features with THz frequencies. The data can be explained by the excitation of the electronic system by a strong screening cur-rent which leads to the launch of collective modes of the CDW system. Further spatial resolved measurements indicate a clear link of the dynamic response with atomic defects.In dieser Arbeit werden ultra-schnelle Phänomene mit einem Rastertunnelmikro-skop (STM) untersucht. Der lokale Einfluss von atomaren Defekten auf ultra-schnelle Phänomene in korrelierten Systemen ist eines der großen Mysterien der experimentellen Erforschung von Festkörpern. Diesem Thema wird sich in Rahmen dieser Arbeit aus zwei unterschiedlichen Richtungen angenähert. Im ersten Teil wird ein aus wenigen Atomen künstlich gebauter Magnet untersucht, des-sen dynamische Eigenschaften durch das Platzieren auf einer Entkopplungsschicht ver-langsamt wurden. Die dynamischen Eigenschaften haben einen direkten Einfluss auf die elektrischen Transport Eigenschaften und führen zu einem negativen differentiellen Wie-derstand. Die auftretenden Effekte können außergewöhnlich präzise mit einem Model basierend auf einer Raten-Gleichung beschrieben werden, das einen tiefen Einblick in die auftretenden Prozesse erlaubt. Während die dynamischen Prozesse des ersten Teils sich im Mikrosekunden bis in das Nanosekunden Regime abspielen, lassen sich schnellere Prozesse nicht mehr mit konven-tionellen STM messen. Um diese Barriere zu durchbrechen, wird im zweiten Teil die Ent-wicklung eines neuen unkonventionellen STM vorgestellt. Durch die Kopplung von Piko-sekunden Freiraum Terahertz Pulsen in den Tunnel Kontakt und dem induzieren von ultra-schnellen Spannungspulsen erlaubt das neue Instrument auf der atomaren Skala stroboskopische Messungen mit einer Zeitauflösung kleiner als Pikosekunden. Mit dem neuen Instrument wird im letzten Kapitel das Ladungsträgerdichtewellen-(CDW)-system 2H-NbSe2 untersucht. Dabei wird eine dynamische Antwort bestehend aus einem 600 fs Abklingen und Schwingung mit THz Frequenzen beobachtet. Die Mes-sungen können durch eine elektronische Anregung erklärt werden, die durch einen star-ken Schirmungsstrom hervorgerufen wird, und zur Anregung von kollektiven Moden der CDW führt. Die Verbindung zwischen der dynamischen Antwort und atomaren Defekten wird in weiteren räumlich aufgelösten Messungen deutlich gezeigt

Minimally invasive spin sensing with scanning tunneling microscopy

Minimizing the invasiveness of scanning tunneling measurements is paramount for observation of the magnetic properties of unperturbed atomic-scale objects. We show that the invasiveness of STM inspection on few-atom spin systems can be drastically reduced by means of a remote detection scheme, which makes use of a sensor spin weakly coupled to the sensed object. By comparing direct and remote measurements we identify the relevant perturbations caused by the local probe. For direct inspection we find that tunneling electrons strongly perturb the investigated object even for currents as low as 3 pA. Electrons injected into the sensor spin induce perturbations with much reduced probability. The sensing scheme uses standard differential conductance measurements, and is decoupled both by its non-local nature, and by dynamic decoupling due to the significantly different time scales at which the sensor and sensed object evolve. The latter makes it possible to effectively remove static interactions between the sensed object and the spin sensor while still allowing the spin sensing. In this way we achieve measurements with a reduction in perturbative effects of up to 100 times relative to direct scanning tunneling measurements, which enables minimally invasive measurements of a few-atom magnet's fragile spin states with STM

Local Density of States at Metal-Semiconductor Interfaces: An Atomic Scale Study

We investigate low temperature grown, abrupt, epitaxial, nonintermixed, defect-free n-type and p-type Fe/GaAs(110) interfaces by cross-sectional scanning tunneling microscopy and spectroscopy with atomic resolution. The probed local density of states shows that a model of the ideal metal-semiconductor interface requires a combination of metal-induced gap states and bond polarization at the interface which is nicely corroborated by density functional calculations. A three-dimensional finite element model of the space charge region yields a precise value for the Schottky barrier height

Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy

Broadband THz pulses enable ultrafast electronic transport experiments on the nanoscale by coupling THz electric fields into the devices with antennas, asperities, or scanning probe tips. Here, we design a versatile THz source optimized for driving the highly resistive tunnel junction of a scanning tunneling microscope. The source uses optical rectification in lithium niobate to generate arbitrary THz pulse trains with freely adjustable repetition rates between 0.5 and 41 MHz. These induce subpicosecond voltage transients in the tunnel junction with peak amplitudes between 0.1 and 12 V, achieving a conversion efficiency of 0.4 V/(kV/cm) from far-field THz peak electric field strength to peak junction voltage in the STM. Tunnel currents in the quantum limit of less than one electron per THz pulse are readily detected at multi-MHz repetition rates. The ability to tune between high pulse energy and high signal fidelity makes this THz source design effective for exploration of ultrafast and atomic-scale electron dynamics

Closing the superconducting gap in small Pb nanoislands with high magnetic fields

Superconducting properties change in confined geometries. Here we study the effects of strong confinement in nanosized Pb islands on Si(111) 7×7. Small hexagonal islands with diameters less than 50 nm and a uniform height of seven atomic layers are formed by depositing Pb at low temperature and annealing at 300 K. We measure the tunneling spectra of individual Pb nanoislands using a low-temperature scanning tunneling microscope operated at 0.6 K and follow the narrowing of the superconducting gap as a function of magnetic field. We find the critical magnetic field, at which the superconducting gap vanishes, reaches several Tesla, which represents a greater than 50-fold enhancement compared to the bulk value. By independently measuring the size of the superconducting gap, and the critical magnetic field that quenches superconductivity for a range of nanoislands, we can correlate these two fundamental parameters and estimate the maximal achievable critical field for 7 ML Pb nanoislands to be 7 T

Three-Dimensional Mapping of Single-Atom Magnetic Anisotropy

Magnetic anisotropy plays a key role in the magnetic stability and spin-related quantum phenomena of surface adatoms. It manifests as angular variations of the atom’s magnetic properties. We measure the spin excitations of individual Fe atoms on a copper nitride surface with inelastic electron tunneling spectroscopy. Using a three-axis vector magnet we rotate the magnetic field and map out the resulting variations of the spin excitations. We quantitatively determine the three-dimensional distribution of the magnetic anisotropy of single Fe atoms by fitting the spin excitation spectra with a spin Hamiltonian. This experiment demonstrates the feasibility of fully mapping the vector magnetic properties of individual spins and characterizing complex three-dimensional magnetic systems

Control of quantum magnets by atomic exchange bias

Mixing of discretized states in quantum magnets has a radical impact on their properties. Managing this effect is key for spintronics in the quantum limit. Magnetic fields can modify state mixing and, for example, mitigate destabilizing effects in single-molecule magnets. The exchange bias field has been proposed as a mechanism for localized control of individual nanomagnets. Here, we demonstrate that exchange coupling with the magnetic tip of a scanning tunnelling microscope provides continuous tuning of spin state mixing in an individual nanomagnet. By directly measuring spin relaxation time with electronic pump–probe spectroscopy, we find that the exchange interaction acts analogously to a local magnetic field that can be applied to a specific atom. It can be tuned in strength by up to several tesla and cancel external magnetic fields, thereby demonstrating the feasibility of complete control over individual quantum magnets with atomically localized exchange coupling

Dynamical Negative Differential Resistance in Antiferromagnetically Coupled Few-Atom Spin Chains

We present the appearance of negative differential resistance (NDR) in spin-dependent electron transport through a few-atom spin chain. A chain of three antiferromagnetically coupled Fe atoms (Fe trimer) was positioned on a Cu2N/Cu(100) surface and contacted with the spin-polarized tip of a scanning tunneling microscope, thus coupling the Fe trimer to one nonmagnetic and one magnetic lead. Pronounced NDR appears at the low bias of 7 mV, where inelastic electron tunneling dynamically locks the atomic spin in a long-lived excited state. This causes a rapid increase of the magnetoresistance between the spin-polarized tip and Fe trimer and quenches elastic tunneling. By varying the coupling strength between the tip and Fe trimer, we find that in this transport regime the dynamic locking of the Fe trimer competes with magnetic exchange interaction, which statically forces the Fe trimer into its high-magnetoresistance state and removes the NDR

Atomic-scale sensing of the magnetic dipolar field from single atoms

Spin resonance provides the high-energy resolution needed to determine biological and material structures by sensing weak magnetic interactions1. In recent years, there have been notable achievements in detecting2 and coherently controlling3,4,5,6,7 individual atomic-scale spin centres for sensitive local magnetometry8,9,10. However, positioning the spin sensor and characterizing spin–spin interactions with sub-nanometre precision have remained outstanding challenges11,12. Here, we use individual Fe atoms as an electron spin resonance (ESR) sensor in a scanning tunnelling microscope to measure the magnetic field emanating from nearby spins with atomic-scale precision. On artificially built assemblies of magnetic atoms (Fe and Co) on a magnesium oxide surface, we measure that the interaction energy between the ESR sensor and an adatom shows an inverse-cube distance dependence (r−3.01±0.04). This demonstrates that the atoms are predominantly coupled by the magnetic dipole–dipole interaction, which, according to our observations, dominates for atom separations greater than 1 nm. This dipolar sensor can determine the magnetic moments of individual adatoms with high accuracy. The achieved atomic-scale spatial resolution in remote sensing of spins may ultimately allow the structural imaging of individual magnetic molecules, nanostructures and spin-labelled biomolecules