Kondo Theory Research Articles

Origin of the magnetic field enhancement of the spin signal in metallic nonlocal spin transport devices

The nonlocal spin valve (NLSV) enables unambiguous study of spin transport, owing to its ability to isolate pure spin currents. A key principle of NLSV operation is that the ``spin signal'' is invariant under application of in-plane magnetic fields (above the ferromagnetic contact saturation field). Yet, for certain ferromagnet/normal metal pairings in NLSVs, an unexpected field enhancement of the spin signal occurs, presenting a challenge that has, thus far, been difficult to resolve with existing models. By correlating the extracted spin transport parameters with material, temperature, and field dependencies, in this work we identify field quenching of magnetic impurity scattering as the origin of this effect, confirmed by excellent agreement between our results and field-dependent Kondo theory. In addition to addressing this long-standing mystery, our findings highlight a potential systematic underestimation of spin transport parameters. By identifying signature field and temperature dependencies, we provide here a relatively simple means to isolate and quantify this additional relaxation mechanism.

Spin–orbit interaction and Kondo scattering at the PrAlO3/SrTiO3 interface: effects of oxygen content

We report the effects of oxygen pressure during growth () on the electronic and magnetic properties of PrAlO3 films grown on -terminated SrTiO3 substrates. Resistivity measurements show an increase in the sheet resistance as is increased. The saturation of the sheet resistance down to 0.3 K is consistent with Kondo theory for torr. Resistivity data fits indicate Kondo temperatures of 16–18 K. For the sample, we measured a moderate positive magnetoresistance (MR) due to a strong spin–orbit (SO) interaction at low magnetic fields that evolves into a larger negative MR at high fields due to the Kondo effect. Analysis of the MR data permitted the extraction of the SO interaction critical field for the torr interface ( T). We observed high positive MR for the least oxygenated sample, where a fraction of the n-type carriers are derived from oxygen vacancies and possible cation interdiffusion; for this torr sample, Hall effect data indicate a thick conducting layer. Its extremely high MR (∼) is attributed to classical behavior due to a distribution of mobilities.

Enhanced spin–orbit coupling and charge carrier density suppression in LaAl1−xCrxO3/SrTiO3 hetero-interfaces

We report a gradual suppression of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface on substitution of chromium at the Al sites. The sheet carrier density at the interface (n□) drops monotonically from ∼2.2 × 1014 cm−2 to ∼2.5 × 1013 cm−2 on replacing ≈60% of the Al sites by Cr and the sheet resistance (R□) exceeds the quantum limit for localization (h/2e2) in the concentrating range 40–60% of Cr. The samples with Cr ⩽40% show a distinct minimum (Tm) in metallic R□(T) whose position shifts to higher temperatures on increasing the substitution. Distinct signatures of Rashba spin–orbit interaction (SOI) induced magnetoresistance (MR) are seen in R□ measured in out of plane field (H⊥) geometry at T ⩽ 8 K. Analysis of these data in the framework of Maekawa–Fukuyama theory allows extraction of the SOI critical field (HSO) and time scale (τSO) whose evolution with Cr concentration is similar as with the increasing negative gate voltage in LAO/STO interface. The MR in the temperature range 8 K ⩽ T ⩽ Tm is quadratic in the field with a +ve sign for H⊥ and −ve sign for H∥. The behaviour of H∥ MR is consistent with Kondo theory which in the present case is renormalized by the strong Rashba SOI at T < 8 K.

Theory of the spin Hall effect, and its inverse, in a ferromagnetic metal near the Curie temperature

We give a theory of the inverse spin Hall effect (ISHE) in ferromagnetic metals based on skew scattering via collective spin fluctuations. This extends Kondo's theory of the anomalous Hall effect (AHE) to include short-range spin-spin correlations. We find a relation between the ISHE and the four-spin correlations near the Curie temperature ${T}_{\mathrm{C}}$. Such four-spin correlations do not contribute to the AHE, which relates to the three-spin correlations. Thus our theory shows an essential difference between the AHE and ISHE, providing an essential complement to Kondo's classic theory of the AHE in metals. We note the relation to skew-scattering mechanisms based on impurity scattering. Our theory can be compared to recent experimental results by Wei et al. [Nat. Commun. 3, 1058 (2012)] for the ISHE in ferromagnetic alloys.

Double metal-insulator transitions and low-temperature resistivity minimum in La&amp;lt;sub&amp;gt;1/3&amp;lt;/sub&amp;gt;Nd&amp;lt;sub&amp;gt;1/3&amp;lt;/sub&amp;gt;Ba&amp;lt;sub&amp;gt;1/3&amp;lt;/sub&amp;gt;MnO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; manganite

La 1/3 Nd 1/3 Ba 1/3 MnO 3 polycrystalline bulk samples in single phase form with colossal magnetoresistant effect were synthesized in air atmosphere by the conventional solid phase reaction process for ceramic, and their fundamental magnetoelectronic properties were discussed. X-ray diffraction pattern shows that the manganite substituted with two kinds of rare earth element has a cubic perovskite structure, instead of a rhombohedral. The temperature dependence curve of magnetization reveals the existence of spin ice due to the competition of ferromagnetism and antiferromagnetism in the manganite containing ions of Mn 3+ and Mn 4+ . The melting point of spin ice shifts toward low temperature with the increase of applied field. At H = 40 kOe, the phenomenon of spin freezing disappears. At room temperature, above the Currie temperature (~250 K), the hysterisis loop shows the behavior of parasitic ferromagnetism likely contributed by the fluctuation of lattice distortion. Double metal to insulator transitions, both strongly field dependent, were observed in the temperature dependence curve of resistivity. The resistivity peak near the Currie temperature is oriented from the intrinsic contribution of La 1/3 Nd 1/3 Ba 1/3 MnO 3 crystal, while the other peak at ~190 K sensitive to both applied field and sintering time is likely to be related to the tunneling of spin-polarized electrons at grain boundaries. An anomalous minimum of electrical resistivity at ~40 K reveals the existence of local magnetic disorders. By a simulation with the traditional least square method, we believe that the anomalous minimum fits the Kondo theory for diluted magnetic alloy.

The Kondo effect in ferromagnetic atomic contacts

Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk, electronic conduction in these materials takes place mainly through the s and p electrons, whereas the magnetic moments are mostly in the narrow d-electron bands, where they tend to align. This general picture may change at the nanoscale because electrons at the surfaces of materials experience interactions that differ from those in the bulk. Here we show direct evidence for such changes: electronic transport in atomic-scale contacts of pure ferromagnets (iron, cobalt and nickel), despite their strong bulk ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of local magnetic moments by the conduction electrons below a characteristic temperature. The Kondo effect creates a sharp resonance at the Fermi energy, affecting the electrical properties of the system; this appears as a Fano-Kondo resonance in the conductance characteristics as observed in other artificial nanostructures. The study of hundreds of contacts shows material-dependent log-normal distributions of the resonance width that arise naturally from Kondo theory. These resonances broaden and disappear with increasing temperature, also as in standard Kondo systems. Our observations, supported by calculations, imply that coordination changes can significantly modify magnetism at the nanoscale. Therefore, in addition to standard micromagnetic physics, strong electronic correlations along with atomic-scale geometry need to be considered when investigating the magnetic properties of magnetic nanostructures.

Tunable Fano‐Kondo effect in a quantum dot with an Aharonov‐Bohmring

We report experimental results and a theoretical model of the Fano-Kondo effect in a quantum dot embedded in an Aharonov-Bohm (AB) ring. A clear Fano-Kondo lineshape is observed in the coherent conductance as a function of the dot gate voltage. And the phase shift locking to π /2, which is a touchstone of the SU(2) Kondo theory is clearly observed in the middle of Kondo valleys. The variation of the lineshape versus the magnetic flux piercing the ring is similar to that expected for a simple double-slit system rather than that under “phase rigidity” for two-terminal quantum interference devices. We have extended the theoretical model of single-channel AB ring to that of a multi-channel case and succeeded to explain the above puzzling results. This demonstrates that useful information can be extracted even from transport through two terminal devices. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Critical point corrections for lattice systems

Lattice gas is considered in the framework of three-dimensional Ono–Kondo theory. It is shown that a nonlinear correction to the configurational energy results in significant improvement of phase behavior, especially near the critical point.

Theory of Multilayer Adsorption with Correct Critical Behavior

It is shown that an expansion of the configurational energy in powers of density allows correcting the critical behavior of Ono−Kondo theory for multilayer adsorption. The proposed approach results in a new equation relating densities in each layer with density in the bulk. This equation predicts spinodals and binodals which are in excellent agreement with the exact phase diagram for a two-dimensional system and with the phase diagram known from Monte Carlo simulations for a three-dimensional system.

Critical Point Corrections for Two- and Three-Dimensional Systems

Adsorption, the equilibrium between two-dimensional and three-dimensional fluids, is considered in the framework of Ono−Kondo lattice theory. A critical point correction to Ono−Kondo theory is proposed, and new approximations for binodals and spinodals are derived. These approximations describe the phase behavior near the critical point more accurately including two-dimensional and three-dimensional binodals. They also improve the prediction of adsorption behavior.

Adsorption of Amphiphilic Dimers at Surfaces

Adsorption of amphiphilic dimers is analyzed in the framework of density functional Ono–Kondo theory. There are three configurations for dimers absorbed at a surface: one parallel to the surface and two perpendicular to the surface (AB and BA, with A or B touching the surface, respectively). Densities of molecules in each configuration are calculated from density functional theory and compared to Monte Carlo simulation data. There is good agreement between theory and simulations. It is shown that the parallel configuration is preferred over the perpendicular configuration, except when there are very strong asymmetries in intermolecular forces. In most cases, the parallel configuration is even preferred over the combination of the two perpendicular configurations.

Non-Fermi-liquid behavior in R 1− xU xRu 2Si 2 (R=Th, Y and La; x⩽0.07)

Single-U-ion properties of heavy-electron compound URu 2Si 2 have been investigated by a series of diluting experiments on R 1− x U x Ru 2Si 2 (R=Th, Y, and La; x⩽0.07). A strong uniaxial magnetic anisotropy of local 5f moments is commonly observed throughout the dilutions. From a large enhancement of χ and C/ T as T→0, we suggest a non-Kramers, magnetic doublet Γ 5 (f 2) to be eminently suitable for the crystal-field lowest level, contrary to general views of low-lying singlets for the periodic system. We further demonstrate the absence of the conventional Kondo effect in all these dilutions. A strong suppression of 5f moments at low temperatures accompanies a non-trivial ln T decrease in ρ and a small change in the magnetic entropy. Specific-heat measurements in magnetic fields up to 12 T have revealed no sign of the residual entropy, eliminating the simple application of the two-channel Kondo theory to the present alloys. We also discuss a puzzling phase transition at 17.5 K of URu 2Si 2 in relation to the single-ion physics, providing our latest results of neutron-scattering experiments under high pressure.

Kondo effect in crossed Luttinger liquids

We present results for the Kondo effect in two crossed Luttinger liquids by using boundary conformal field theory. We predict two types of critical behaviors: either a two-channel Kondo fixed point with a nonuniversal Wilson ratio or a theory with an anomalous reponse that is identical to that found by Furusaki and Nagaosa for the Kondo effect in a single Luttinger liquid. Moreover, we discuss the relevance of perturbations like channel anisotropy in restoring a Fermi-liquid-like Kondo fixed point, and compare our results with the Kondo effect in a two-band Hubbard system modeled by a channel-dependent Luttinger Hamiltonian. The suppression of backscattering off the impurity produces a model similar to the four-channel Kondo theory. Consequences are discussed.

A lattice model for fluids with directional interactions

Here we analyze a lattice model for fluids with directional interactions in the framework of the Ono–Kondo theory. The free energy of the system is represented as an explicit function of the temperature and bulk density. It is shown that the model predicts both order–disorder and vapor–liquid phase transitions. This theory predicts a tricritical point where the vapor–liquid and order–disorder phase transitions both disappear. Also, it predicts retrograde condensation where the boundary of phase stability becomes a multivalued function of concentration. In addition, predictions of the theory are compared with Monte Carlo simulation data. It is shown that the partition function cannot be factored to predict separately the contributions of, for example, dispersion and hydrogen bonding interactions.

LOW-TEMPERATURE RESISTANCE MINIMUM AND ITS ORIGIN IN La2/3Ca1/3Mn1-xCuxO3(x=0.15) SYSTEM

The temperature dependence of resistance in the La2/3Ca1/3Mn1-xCuxO3(x=0 and 0.15) system at temperatures much lower than Curie temperature has been experimentally studied. It is found that a resistance minimum obviously appears in the zero-field R-T curve of the Cu-doped sample. Such a resistance minimum disappears when a magnetic field is applied to the sample. Based on Kondo theory, a quantitative comparison between the theory and the experimental data has been presented. It is shown that the experimental observation can be well understood within the framework of Kondo theory.

Adsorption Hysteresis in Porous Solids

Hysteresis has been observed in adsorption isotherms for a number of gas–solid systems and, generally, is attributed to adsorption in mesoporous materials with capillary condensation. This behavior is classified as Type IV or Type V in the IUPAC classification scheme. Here, lattice theory is used to predict adsorption behavior in pores. The Ono–Kondo theory is used with appropriate boundary conditions for fluid adsorption in infinite and semi-finite slit-like pores. It is shown that there can be phase transitions in the adsorbed phase which lead to hysteresis in kinetically controlled experiments. However, hysteresis in equilibrium behavior is exhibited only in pores of finite length. For finite-length pores, the interface geometry is predicted to be different during the processes of adsorption and desorption and this difference in interface shape leads to hysteresis. This simple molecular model is able to predict the change in the interface geometry without invoking the Kelvin equation or the macroscopic concept of surface tension.

Classification of Gibbs adsorption isotherms

The current IUPAC classification of gas–solid adsorption isotherms has two deficiencies: it is incomplete and it gives the incorrect impression that adsorption isotherms are always monotonically increasing functions of pressure. However, there are many isotherms that are not monotonic and show maxima. The reason for this deficiency is that the IUPAC classification is based conceptually on ideas about the `absolute' adsorption rather than on the Gibbs adsorption. It is shown here that supercritical gases show adsorption isotherms which are fundamentally different from those in the IUPAC classification. There are experimental data for hydrocarbons, inert gases, nitrogen, carbon oxide, carbon dioxide, nitrogen oxides, and others on microporous and macroporous adsorbents for supercritical temperatures showing nonmonotonic isotherms with maxima. It also is shown that the Ono–Kondo lattice model is able to predict all known types of adsorption behavior. By changing two energetic parameters (energies for adsorbate–adsorbate and adsorbate–adsorbent interactions), one can obtain smooth or stepped multilayer adsorption isotherms for macroporous adsorbents. It also is possible to describe adsorption on microporous adsorbents by imposing the appropriate boundary conditions. The Ono–Kondo model also predicts steps in isotherms similar to those observed in high resolution measurements for nitrogen and oxigen on zeolites. The monolayer version of the Ono–Kondo theory gives all known types of monolayer adsorption isotherms.

Intermediate valence to single ion Kondo behaviour in CePd 3B x alloys

Susceptibility, resistivity thermopower and magnetoresistivity data are presented for CePd 3B x alloys, where the boron enters the lattice interstitially. All the data are consistent with a smooth cross-over from intermediate valence in CePd 3 to trivalent local moment behaviour in CePd 3B 0.3. For x < 0.15 the alloys mirror the properties of CePd 3, but on reduced temperature scales. For 0.15 ⪕ x < 0.3 the alloys obey single impurity Kondo theory, while the x = 0.3 alloy is close to the localised limit.

Low temperature excitations in CeRu 2Si 2 − xGe x alloys

Alloys in the series CeRu 2Si 2 − x Ge x have been studied by inelastic neutron scattering. The high temperature quasielastic response is consistent with a smooth decrease in the 4f-conduction electron hybridisation with increasing x. The linewidths are strongly modulated with wavevector Q, indicating the presence of strong RKKY interactions. The single ion component of the dynamical response shows scaling with x and T in good agreement with standard Kondo theory. In the ordered state, for x ≈ 1, the longitudinal spin fluctuations are characterised by an inelastic peak. The strength of this inelastic response decreases with increasing x, as the exchange field wins over the Kondo screening. The neutron cross-sections are well described by a simple ansatz in which the screened state of the 4f electrons is represented by a pair of singlets separated by an energy of order k B T K.

Resistivity, magnetization, and specific heat of YbAgCu4 in high magnetic fields.

We report measurements of the resistivity \ensuremath{\rho}, Sommerfeld coefficient of specific heat \ensuremath{\gamma}, and magnetization M of polycrystalline ${\mathrm{YbAgCu}}_{4}$ in high magnetic fields 0\ensuremath{\le}B\ensuremath{\le}18 T [in the case of M(B) to 50 T]. A comparison of the temperature-dependent susceptibility \ensuremath{\chi}(T) as well as field-dependent Sommerfeld coefficient \ensuremath{\gamma} and magnetization to Kondo theory for a J=7/2 impurity shows that theory correctly predicts the functional dependence of these quantities on T and B, but the characteristic temperatures determined from the various measurements (120, 98, 77, and 63 K) differ by nearly a factor of 2, which is difficult to understand within the context of Kondo theory even when other possible contributions are considered. In addition the normalized (Wilson) ratio of \ensuremath{\chi} to \ensuremath{\gamma} is 1.00 at zero field (compared to 1.14 in Coqblin-Schrieffer theory) and decreases with increasing magnetic field. The magnetoresistance is positive at all temperatures, reaching a value \ensuremath{\Delta}\ensuremath{\rho}(B)/\ensuremath{\rho}(B=0)=0.6 at 25 mK and 18 T. The low-temperature magnetoresistivity \ensuremath{\Delta}\ensuremath{\rho}(B) varies as ${\mathit{B}}^{1.5}$. We argue that this is dominated by an ordinary impurity effect. Kohler's rule is clearly violated as the temperature is raised; the scattering rate appears to increase with field below 40 K and decrease with field above 40 K. This behavior is expected for an Anderson lattice when a pseudogap is present. At low temperature the resistivity increases as ${\mathit{AT}}^{2}$. The coefficient A (corrected for cyclotron-orbit effects) increases with field such that the ratio A(B)/\ensuremath{\gamma}(B${)}^{2}$ is a constant. Doping with Lu onto the Yb site, or with Ni onto the Cu site, changes the magnitude of the low-temperature resistivity in a manner consistent with the predictions of the theory of ligand-induced disorder in an Anderson lattice.