Kondo effect in strained layered materials

Nature Physics, 2008

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 1 . Spin S = 1/2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin 2 . 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 3,4 . 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.

Physical Review B, 2013

We investigate Kondo physics in a host with a strongly diverging density of states. This study is motivated by a recent work on vacancies in the graphene honeycomb lattice, whose density of states is enhanced at low energies due to potential scattering. The generalized quantum impurity model describing the vacancy is shown to support a spin-1/2 (doublet) Kondo phase. The special role played by a diverging host density of states is examined in detail, with distinctive signatures associated with the power law Kondo effect shown to appear in thermodynamic quantities and the scattering t-matrix, with a strongly enhanced Kondo temperature. Although the effective Kondo model supports a stable phase characterized by strong renormalized particle-hole asymmetry, we find that this phase cannot in fact be accessed in the full Anderson model. In the more realistic case, where the divergence in the host density of states is cut off at low energies, a crossover is generated between pristine power law Kondo physics and a regular Kondo strong coupling state.

Physical Review B, 2014

We study a magnetic impurity intercalated in bilayer graphene. A representative configuration generates a hybridization function with strong dependence on the conduction-electron energy, including a full gap with one hard and one soft edge. Shifts of the chemical potential via gating or doping drive the system between non-Kondo (free-moment) and Kondo-screened phases, with strong variation of the Kondo scale. Quantum phase transitions near the soft edge are of Kosterlitz-Thouless type, while others are first order. Near the hard edge, a bound-state singlet appears inside the gap; although of single-particle character, its signatures in scanning tunneling spectroscopy are very similar to those arising from a many-body Kondo resonance.

We investigate Kondo physics in a host with a strongly diverging density of states. This study is motivated by a recent work on vacancies in the graphene honeycomb lattice, whose density of states is enhanced at low energies due to potential scattering. The generalized quantum impurity model describing the vacancy is shown to support a spin-1/2 (doublet) Kondo phase. The special role played by a diverging host density of states is examined in detail, with distinctive signatures associated with the power law Kondo effect shown to appear in thermodynamic quantities and the scattering t matrix, with a strongly enhanced Kondo temperature. Although the effective Kondo model supports a stable phase characterized by strong renormalized particle-hole asymmetry, we find that this phase cannot in fact be accessed in the full Anderson model. In the more realistic case, where the divergence in the host density of states is cut off at low energies, a crossover is generated between pristine power law Kondo physics and a regular Kondo strong coupling state.

Physical Review Letters, 2006

We measure transport through gold grain quantum dots fabricated using electromigration, with magnetic impurities in the leads. A Kondo interaction is observed between dot and leads, but the presence of magnetic impurities results in a gate-dependent zero-bias conductance peak that is split due to an RKKY interaction between the spin of the dot and the static spins of the impurities. A magnetic field restores the single Kondo peak in the case of an antiferromagnetic RKKY interaction. This system provides a new platform to study Kondo and RKKY interactions in metals at the level of a single spin. PACS numbers: 75.30.Hx 72.15.Qm 73.63.Kv 73.23.-b 75.20.Hr

EPL (Europhysics Letters), 2009

Magnetic impurities bridging nanocontacts and break junctions of nearly magnetic metals may lead to permanent moments, analogous to the giant moments well known in the bulk case. A numerical renormalization group (NRG) study shows that, contrary to mean field based expectations, a permanent moment never arises within an Anderson model, which invariably leads to strong Kondo screening. By including in the model an additional ferromagnetic exchange coupling between leads and impurity, the NRG may instead stabilize a permanent moment through a ferromagnetic Kondo effect. The resulting state is a rotationally invariant spin, which differs profoundly from mean field. A sign inversion of the zero-bias anomaly and other spectroscopic signatures of the switch from regular to ferromagnetic Kondo are outlined.

Journal of Physics: Condensed Matter, 2007

The interplay of disorder and competing interactions is investigated in the carrier-induced ferromagnetic state of the Kondo lattice model within a numerical finite-size study in which disorder is treated exactly. Competition between impurity spin couplings, stability of the ferromagnetic state, and magnetic transition temperature are quantitatively investigated in terms of magnon properties for different models including dilution, disorder, and weakly-coupled spins. A strong optimization is obtained for Tc at hole doping p << x, highlighting the importance of compensation in diluted magnetic semiconductors. The estimated Tc is in good agreement with experimental results for Ga1−xMnxAs for corresponding impurity concentration, hole bandwidth, and compensation. Finite-temperature spin dynamics is quantitatively studied within a locally self-consistent magnon renormalization scheme, which yields a substantial enhancement in Tc due to spin clustering, and highlights the nearly-paramagnetic spin dynamics of weakly-coupled spins. The large enhancement in density of low-energy magnetic excitations due to disorder and competing interactions results in a strong thermal decay of magnetization, which fits well with the Bloch form M0(1 − BT 3/2) at low temperature, with B of same order of magnitude as obtained in recent squid magnetization measurements on Ga1−xMnxAs samples.

We study the local moment formation and the Kondo effect at single-atom vacancies in Graphene. We develop a model accounting for the vacancy reconstruction as well as non-planarity effects induced by strain and/or temperature. Thus, we find that the dangling σ orbital localized at the vacancy is allowed to strongly hybridize with the π-band since the scattering with the vacancy turns the hybridization into singular function of the energy (∼ [| | ln 2 /D] −1 , D ∼ the bandwidth). This leads to several new types of impurity phases, which control the magnitude of the vacancy magnetic moment and the possibility of Kondo effect depending on the strength of the local Coulomb interactions, the Hund's rule coupling, the doping level, and the degree of particle-symmetry breaking.

Physical Review Letters, 2012

We analyze the physics of a one-orbital Anderson impurity model in a two-dimensional electron gas in the presence of Rashba spin-orbit (RSO) interactions in the Kondo regime. The spin SU(2) symmetry breaking results in an effective two-band electron gas coupled to the impurity. The Kondo regime is obtained by a Schrieffer-Wolff transformation revealing the existence of a parity breaking term with the form of the Dzyaloshinsky-Moriya (DM) interaction. The DM term vanishes at the particle-hole symmetric point of the system, but it has important effects otherwise. Performing a renormalization group (RG) analysis we find that the model describes a two-channel Kondo system with ferro-and anti-ferromagnetic couplings. Furthermore, the DM term renormalizes the antiferromagnetic Kondo coupling producing an exponential enhancement of the Kondo temperature. We suggest that these effects can be observed in semiconducting systems, as well as in graphene and topological insulators.

Physical Review B, 1999

We consider the role of static disorder in the spin sector of the one-and two-channel Kondo models. The distribution functions of the disorder-induced effective energy splitting between the two levels of the Kondo impurity are derived to the lowest order in the concentration of static scatterers. It is demonstrated that the distribution functions are strongly asymmetric, with the typical splitting being parametrically smaller than the average rms value. We employ the derived distribution function of splittings to study the temperature dependence of the low-temperature conductance of a sample containing an ensemble of two-channel Kondo impurities. The results are used to analyze the consistency of the two-channel Kondo interpretation of the zero-bias anomalies observed in Cu/(Si:N)/Cu nanoconstrictions. 72.10.Fk, 75.20.Hr