The Kondo effect, one of the oldest correlation phenomena known in condensed matter physics [1], has regained attention due to scanning tunneling spectroscopy (STS) experiments performed on single magnetic impurities [2,3]. Despite the sub-nanometer resolution capability of local probe techniques one of the fundamental aspects of Kondo physics, its spatial extension, is still subject to discussion. Up to now all STS studies on single adsorbed atoms have shown that observable Kondo features rapidly vanish with increasing distance from the impurity [4,5,6,7,8,9]. Here we report on a hitherto unobserved long range Kondo signature for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We present a theoretical interpretation of the measured signatures using a combined approach of band structure and many-body numerical renormalization group (NRG) calculations. These are in excellent agreement with the rich spatially and spectroscopically resolved experimental data. The interaction of a single magnetic impurity with the surrounding electron gas of a non-magnetic metal leads to fascinating phenomena in the low temperature limit, which are summarized by the term Kondo effect [1]. Such an impurity has a localized spin moment that interacts with the electrons of the conduction band. If the system is cooled below a characteristic temperature, the Kondo temperature T K , a correlated electronic state develops and the impurity spin is screened. The most prominent fingerprint of this many body singlet state is a narrow resonance at the Fermi energy ε F in the single particle spectrum of the impurity, called Kondo or Abrikosov-Suhl resonance. The existence of this Kondo resonance has been experimentally confirmed for dense systems with high resolution photoemission electron spectroscopy and inverse photoemission [10,11]. Due to their limited spatial resolution these measurements always probe a very large ensemble of magnetic atoms. With its capability to study local electronic properties with high spatial and energetic resolution, Scanning Tunneling Spectroscopy (STS) has paved the way to access individual impurities [2,3].A theoretical prediction for the local density of states (LDOS) -the key quantity measured in STS experiments -was first provided byÚjsághy et al [12]. According to their calculations the Kondo resonance induces strong spectroscopic signatures at the Fermi energy whose line shape is oscillatory with distance to the impurity. Since the first STS studies in 1998 [2, 3] a lot of experiments on magnetic atoms and molecules on metal surfaces were performed, all revealing Kondo fingerprints [5,6,7,8,9]. However, it is worth noting that all previous STS experiments on isolated ad atoms have reported that the Kondo signature rapidly vanishes within a few angstrom and no variation of the line shape occurs (for a review on ad atom Kondo systems see [13]).In the present work we follow a novel route and investigate single isolated magnetic impurities buried below the surface with a low temperature STM ope...

The interplay between the Ruderman-Kittel-Kasuya-Yosida interaction and the Kondo effect is expected to provide the driving force for the emergence of many phenomena in strongly correlated electron materials. Two magnetic impurities in a metal are the smallest possible system containing all these ingredients and define a bottom-up approach towards a longterm understanding of concentrated/dense systems. Here we report on the experimental and theoretical investigation of iron dimers buried below a Cu(100) surface by means of lowtemperature scanning tunnelling spectroscopy combined with density functional theory and numerical renormalization group calculations. The Kondo effect, in particular the width of the Abrikosov-Suhl resonance, is strongly altered or even suppressed due to magnetic coupling between the impurities. It oscillates as a function of dimer separation revealing that it is related to indirect exchange interactions mediated by the conduction electrons.

Phenomena that are highly sensitive to magnetic fields can be exploited in sensors and non-volatile memories [1]. The scaling of such phenomena down to the single molecule level [2, 3] may enable novel spintronic devices [4]. Here we report magnetoresistance in a single molecule junction arising from negative differential resistance that shifts in a magnetic field at a rate two orders of magnitude larger than Zeeman shifts. This sensitivity to the magnetic field produces two voltage-tunable forms of magnetoresistance, which can be selected via the applied bias. The negative differential resistance is caused by transient charging [5][6][7] of an iron phthalocyanine (FePc) molecule on a single layer of copper nitride (Cu 2 N) on a Cu(001) surface, and occurs at voltages corresponding to the alignment of sharp resonances in the filled and empty molecular states with the Cu(001) Fermi energy. An asymmetric voltage-divider effect enhances the apparent voltage shift of the negative differential resistance with magnetic field, which inherently is on the scale of the Zeeman energy [8]. These results illustrate the impact that asymmetric coupling to metallic electrodes can have on transport through molecules, and highlight how this coupling can be used to develop molecular spintronic applications.Research into magnetoresistance [9, 10] has been driven by the widespread use of giant magnetoresistance (GMR) sensors in hard drives as well as other applications such as magnetoresistive random access memory (MRAM) [1]. To reach even higher storage densities, research has begun to concentrate on magnetoresistance at the atomic scale [2, 3, 11]. For a single molecule, however, the small area for enclosing flux and modest energy scales associated with electronic Zeeman shifts typically make it difficult to tune magnetoresistive phenomena with an external magnetic field.Another electron transport phenomenon with technological relevance is negative differential resistance (NDR) [5, 7,[12][13][14][15][16][17][18][19], in which an increase in voltage causes a decrease in current. Commercial devices, such as the resonant tunnelling diode, utilise these regions in specialised applications [20, 21]. Various mechanisms cause NDR at the atomic scale [5, 7,[12][13][14][15][16][17][18][19], though none are expected to have a magnetic field dependence that would shift the NDR on a scale larger than the Zeeman energy.Using low temperature scanning tunnelling microscopy (STM) (see Supplementary Methods), we observe an NDR effect for FePc molecules placed in a vacuum junction on top of 2 a Cu(001) surface capped with a single layer of Cu 2 N (Fig. 1). Cu 2 N is a thin insulator that can decouple the spins of magnetic atoms from the underlying surface [22]; FePc is a magnetic molecule that can be easily sublimed [23][24][25] and is observed to have interesting magnetic properties on thin insulating layers [26]. On Cu 2 N, FePc is centred above both Cu and N sites. The two binding sites can be differentiated using atomically resolved imaging a...

We investigate single Fe and Co atoms buried below a Cu(100) surface using low temperature scanning tunneling spectroscopy. By mapping the local density of states of the itinerant electrons at the surface, the Kondo resonance near the Fermi energy is analyzed. Probing bulk impurities in this well-defined scattering geometry allows separating the physics of the Kondo system and the measuring process. The line shape of the Kondo signature shows an oscillatory behavior as a function of depth of the impurity as well as a function of lateral distance. The oscillation period along the different directions reveals that the spectral function of the itinerant electrons is anisotropic.PACS numbers: 68.37. Ef,72.10.Fk,72.15.Qm, For a long time it has been well known that a localized spin degree of freedom-a magnetic impurity-in a nonmagnetic host metal significantly alters the scattering behavior of the conduction band electrons of the host as compared to nonmagnetic impurities. This results in a variety of thermodynamic anomalies, which are summarized by the term Kondo effect [1]. The most prominent macroscopic hallmark is the resistance minimum at low temperatures observed for metals with magnetic impurities. From a microscopic point of view the impurity interacts with the surrounding electron gas-the itinerant electrons. Below a characteristic temperature, the Kondo temperature T k , a narrow many-body resonance named Kondo or Abrikosov-Suhl resonance builds up in the spectral function of the impurity at the chemical potential and the impurity spin is effectively screened. Electric transport is dominated by electrons near the Fermi energy for low temperature. Hence the Kondo resonance leads to a strong scattering of electrons at the impurity and an increase of resistivity with decreasing temperature.Microscopic properties of single impurity Kondo systems on the atomic scale regained interest by recent scanning tunneling microscopy (STM) experiments [2-7] on single magnetic ad-atoms and molecules deposited on noble metal surfaces (for a review see [8]). In these works the Kondo effect manifests itself as a sharp signature in the STM differential conductance around zero bias. Studies which investigate the dependence of the Kondo signature on the lateral distance [2,5] showed that the signal rapidly vanishes and the line shape nearly remains constant. This is in contrast to theory [9-11] which predicts a long range visibility and oscillatory behavior of the spectral function of the itinerant electrons -the local density of states (LDOS). Models for tunneling through ad-atom systems have to treat the tip, its coupling to the localized impurity state, its coupling to the itinerant bulk electrons as well as surface states, and all interferences between alternative transport paths [10,11].During recent years STM was refined to investigate nano structures [12][13][14][15][16] or even single impurity atoms [17][18][19][20] which are buried below a metal surface. The interference pattern on the surface caused by the sub surface s...