arXiv subjects

Markus Ternes

Publications and source records attributed to Markus Ternes.

26 records · Page 2Linked to original sources

arXiv2016

Correlation Driven Transport Asymmetries Through Coupled Spins

Correlation is a fundamental statistical measure of order in interacting quantum systems. In solids, electron correlations govern a diverse array of material classes and phenomena such as heavy fermion compounds, Hunds metals, high-Tc superconductors, and the Kondo effect. Spin-spin correlations, notably investigated by Kaufman and Onsager in the 1940s 6, are at the foundation of numerous theoretical models but are challenging to measure experimentally. Reciprocal space methods can map correlations, but at the single atom limit new experimental probes are needed. Here, we determine the correlations between a strongly hybridized spin impurity and its electron bath by varying the coupling to a second magnetic impurity in the junction of a scanning tunneling microscope. Electronic transport through these coupled spins reveals an asymmetry in the differential conductance reminiscent of spin-polarized transport in a magnetic field. We show that at zero field, this asymmetry can be controlled by the coupling strength and is directly related to either ferromagnetic (FM) or antiferromagnetic (AFM) spin-spin correlations.

cond-mat.mes-hall↗

arXiv2015

Entanglement-induced Kondo screening in atomic spin chains

Quantum entanglement permeates the complex ground states of correlated electron materials defying single-particle descriptions. Coupled magnetic atoms have potential as model systems for entanglement in condensed matter giving the opportunity to create artificial many-body states which can be controlled by tuning the underlying interactions. They provide an avenue to unravel the complexities of correlated-electron materials. Here we use low temperature scanning tunnelling microscopy (STM) and atomic manipulation to tune entanglement in chains of magnetic atoms. We find that a Kondo singlet state can emerge from this entanglement. The many electron Kondo state is based on the screening of the entangled spin ground state of the chain by substrate electrons and can be engineered to envelop at least ten magnetic atoms. The concomitant Kondo resonance measured in the differential conductance enables the electric read-out of entanglement. By tuning composition and coupling strength within atomic chains it is possible to create model spin chains with defined entanglement. This lays the foundation for a new class of experiments to construct exotic correlated-electron materials atom by atom.

cond-mat.mes-hall↗

arXiv2015

Spin Excitations and Correlations in Scanning Tunneling Spectroscopy

In recent years inelastic spin-flip spectroscopy using a lowtemperature scanning tunneling microscope has been a very successful tool for studying not only individual spins but also complex coupled systems. When these systems interact with the electrons of the supporting substrate correlated manyparticle states can emerge, making them ideal prototypical quantum systems. The spin systems, which can be constructed by arranging individual atoms on appropriate surfaces or embedded in synthesized molecular structures, can reveal very rich spectral features. Up to now the spectral complexity has only been partly described. This manuscript shows that perturbation theory enables one to describe the tunneling transport, reproducing the differential conductance with surprisingly high accuracy. Well established scattering models, which include Kondo-like spin-spin and potential interactions, are expanded to enable calculation of arbitrary complex spin systems in reasonable time scale and the extraction of important physical properties. The emergence of correlations between spins and, in particular, between the localized spins and the supporting bath electrons are discussed and related to experimentally tunable parameters. These results might stimulate new experiments by providing experimentalists with an easily applicable modeling tool.

cond-mat.mes-hall↗

arXiv2015

Quantum Engineering of Spin and Anisotropy in Magnetic Molecular Junctions

Single molecule magnets and single spin centers can be individually addressed when coupled to contacts forming an electrical junction. In order to control and engineer the magnetism of quantum devices, it is necessary to quantify how the structural and chemical environment of the junction affects the spin center. Metrics such as coordination number or symmetry provide a simple method to quantify the local environment, but neglect the many-body interactions of an impurity spin when coupled to contacts. Here, we utilize a highly corrugated hexagonal boron nitride (h-BN) monolayer to mediate the coupling between a cobalt spin in CoHx (x=1,2) complexes and the metal contact. While the hydrogen atoms control the total effective spin, the corrugation is found to smoothly tune the Kondo exchange interaction between the spin and the underlying metal. Using scanning tunneling microscopy and spectroscopy together with numerical simulations, we quantitatively demonstrate how the Kondo exchange interaction mimics chemical tailoring and changes the magnetic anisotropy.

cond-mat.mes-hall↗

arXiv2014

The Spin-Polarization of the Split Kondo State

Spin-resolved scanning tunneling microscopy is employed to quantitatively determine the spin-polarization of the magnetic field-split Kondo state. Tunneling conductance spectra of a Kondo-screened magnetic atom are evaluated within a simple model taking into account inelastic tunneling due to spin excitations and two Kondo peaks positioned symmetrically around the Fermi energy. We fit the spin state of the Kondo-screened atom with a Spin Hamiltonian independent of the Kondo effect and account for Zeeman-splitting of the Kondo peak in the magnetic field. We find that the width and the height of the Kondo peaks scales with the Zeeman energy. Our observations are consistent with full spin-polarization of the Kondo peaks, i.e., a majority spin peak below the Fermi energy and a minority spin peak above.

cond-mat.str-el↗

arXiv2014

Tracking Temperature Dependent Relaxation Times of Individual Ferritin Nanomagnets with a Wide-band Quantum Spectrometer

We demonstrate the tracking of the spin dynamics of ensemble and individual magnetic ferritin proteins from cryogenic up to room temperature using the nitrogen-vacancy color center in diamond as magnetic sensor. We employ different detection protocols to probe the influence of the ferritin nanomagnets on the longitudinal and transverse relaxation of the nitrogen-vacancy center, which enables magnetic sensing over a wide frequency range from Hz to GHz. The temperature dependence of the observed spectral features can be well understood by the thermally induced magnetization reversals of the ferritin and enables the determination of the anisotropy barrier of single ferritin molecules.

cond-mat.mes-hall↗

arXiv2013

Temperature and magnetic field dependence of a Kondo system in the weak coupling regime

The Kondo effect arises due to the interaction between a localized spin and the electrons of a surrounding host. Studies of individual magnetic impurities by scanning tunneling spectroscopy have renewed interest in Kondo physics; however, a quantitative comparison with theoretical predictions remained challenging. Here we show that the zero-bias anomaly detected on an organic radical weakly coupled to a Au (111) surface can be described with astonishing agreement by perturbation theory as originally developed by Kondo 60 years ago. Our results demonstrate that Kondo physics can only be fully conceived by studying both temperature and magnetic field dependence of the resonance. The identification of a spin 1/2 Kondo system is of relevance not only as a benchmark for predictions for Kondo physics but also for correlated electron materials in general.

cond-mat.str-el↗

arXiv2008

The role of magnetic anisotropy in the Kondo effect

In the Kondo effect, a localized magnetic moment is screened by forming a correlated electron system with the surrounding conduction electrons of a non-magnetic host. Spin S=1/2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin. Larger spins are subject to the influence of magnetocrystalline anisotropy, which describes the dependence of the magnetic moment's energy on the orientation of the spin relative to its surrounding atomic environment. Here we demonstrate the decisive role of magnetic anisotropy in the physics of Kondo screening. A scanning tunnelling microscope is used to simultaneously determine the magnitude of the spin, the magnetic anisotropy and the Kondo properties of individual magnetic atoms on a surface. We find that a Kondo resonance emerges for large-spin atoms only when the magnetic anisotropy creates degenerate ground-state levels that are connected by the spin flip of a screening electron. The magnetic anisotropy also determines how the Kondo resonance evolves in a magnetic field: the resonance peak splits at rates that are strongly direction dependent. These rates are well described by the energies of the underlying unscreened spin states.

cond-mat.str-el↗