No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Metallic conductance below T_ {c} inferred by quantum interference effects in layered La_ {1.2} Sr_ {1.8} Mn_ {2} O_ {7} single crystals
Abstract
The metallic nature of the low-temperature conductivity σ of the layered manganite La1.2Sr1.8Mn2O7 is not obviously consistent with the very low values of σ nor its slightly semiconducting behavior. We report low-temperature data in an applied field H which exhibit a positive √H correction to σ, for fields perpendicular to the Mn-O bilayers and also parallel to the layers, as well as for both current directions. This behavior is consistent with weak-localization effects in three-dimensional metals and with the theory of quantum interference, recently extended to highly anisotropic layered metals. In addition, a simple model is presented that adequately describes the positive magnetoconductance due to the suppression of spin waves.
... Aside from the metallic nature of trilayer AlN, we have also noticed unforeseen upturn at low temperatures in resistivity data. The observed upturn at low temperature as shown in Fig. 5(a), mainly emerges due to weak localization [54][55][56], spin-polarized tunneling via grain boundaries [57][58][59], electron-electron (e-e) scattering [60][61][62][63] or Kondo like scattering [22,[64][65][66][67], which is discussed below. ...
In this work for the first time, we are reporting the unusual observation of the Kondo effect with the coexistence of room temperature ferromagnetism in AlN/Al/AlN trilayer thin film. The grown film shows resistivity minimum at a temperature of ∼48 K, which shifts to the lower temperature on the application of magnetic fields. After considering various possibilities for an upturn in resistivity, we found that the Kondo scattering is responsible for upturn at low temperature. The simultaneous presence of ferromagnetism and Kondo scattering is explained by spatial variation of nitrogen vacancy defects from the film surface to the Al sandwich layer. Furthermore, magneto-transport properties of the film measured at different temperature exhibits both negative and positive components described by localized magnetic moment model for the spin scattering of carriers and two-band model, respectively. This work provides insight into the novel co-existence of ferromagnetism and Kondo effect in crystalline AlN.
... While Kondo effect dominates in intrinsically disordered samples 9,10 . Recently, QIEs in manganites have been intensively investigated to interpret the low-temperature resistivity minima 2,6,9,[11][12][13][14] . Generally, QIEs lead to correction to the resistivity from two different sources 5 : (i) electron-electron (e-e) interaction and subsequent modification of the density of states at the Fermi energy; (ii) weak localization (WL) effect arising from the self-interference of the wave pockets as they are backscattered coherently by the impurities or other defects. ...
Quantum interference effects (QIEs) dominate the appearance of low-temperature resistivity minimum in colossal magnetoresistance manganites. The T1/2 dependent resistivity under high magnetic field has been evidenced as electron-electron (e-e) interaction. However, the evidence of the other source of QIEs, weak localization (WL), still remains insufficient in manganites. Here we report on the direct experimental evidence of WL in QIEs observed in the single-crystal La0.7Sr0.3MnO3 (LSMO) ultrathin films deposited by laser molecular beam epitaxy. The sharp cusps around zero magnetic field in magnetoresistance measurements is unambiguously observed, which corresponds to the WL effect. This convincingly leads to the solid conclusion that the resistivity minima at low temperatures in single-crystal manganites are attributed to both the e-e interaction and the WL effect. Moreover, the temperature-dependent phase-coherence length corroborates the WL effect of LSMO ultrathin films is within a two-dimensional localization theory.
... There is no consensus in the literature for describing the low-temperature minimum in the resistivity of manganites such as La 1−x R x MnO 3 (R = Ag, Ba, Bi, Ca, Ce, Eu, Sm, Sr, K, etc.)252627282930313233. There are a few reports on the upturn resistivity behavior for the layered manganites La 2−2x R 1+2x Mn 2 O 7 (R = Ca, Sr)3435363738394041. However, scientific challenges for describing this behavior are still to be taken up as it is not well understood in these compounds. ...
Low-temperature transport and magneto-resistance properties were systemically studied for the bilayered polycrystalline LaSr\(_{2}\)Mn\(_{2}\)O\(_{7}\) manganite under an applied magnetic field from 0 to 9 T. The results obtained from the resistivity measurements between 70 and 130 K were fitted using the electron–electron (e–e) and electron–magnon (e–m) scattering models. The temperature dependence of resistivity showed an upturn behavior at low temperatures (\(\sim \)T \(<\) 45 K) under various applied magnetic fields. Best fits were obtained by using a variable-range hopping model. The magneto-resistance behavior observed at low temperatures supports the magnetic properties of the sample.
... As reported, the mean free path of electrons in the manganite in the high conductive state is the order of 1 nm in magnitude. 25 It may be even smaller at higher temperatures, where carrier scattering is strong. A diffusion distance of a few nanometers is plausible. ...
Rectifying and photoelectronic properties of the La <sub>0.67</sub> Ba <sub>0.33</sub> MnO <sub>3</sub>/ SrTiO <sub>3</sub>: Nb junctions with the film thickness from d=0.5 to 30 nm have been systematically studied. It is found that the electronic transport of the junction is dominated by quantum tunneling or thermoionic emission when film thickness is below or above 1 nm. The rectifying ratio and ideality factor, correspondingly, experience a sudden change as film thickness grows from 0.5 to 1 nm and a smooth variation with film thickness above 1 nm. The threshold film thickness for the establishment of a mature depletion layer is therefore 1 nm. The photoemission properties of the junctions also exhibit a strong dependence on film thickness. As experimentally shown, the photocurrent vanishes in the zero thickness limit, and grows rapidly with the increase in film thickness until d=6 nm , where a maximal photocurrent of ∼770 nA / mm <sup>2</sup> under the irradiance of the laser of 5 mW and 532 nm is obtained. After this maximum, an increase-to-decrease turning appears with further increasing film thickness. Taking into account the finite diffusion distance of the photocarriers and the strain-enhanced charge trapping in ultrathin film junctions, a theoretical description that well reproduces the experiment results can be obtained, which reveals the severe depression of finite diffusion distance of the extra carriers on photocurrent. The maximal diffusion distance thus obtained is ∼3.5 nm . Similar analyses have been performed for the La <sub>0.67</sub> Ca <sub>0.33</sub> MnO <sub>3</sub>/ SrTiO <sub>3</sub>: Nb junctions, and th-
-
e corresponding diffusion distance there is ∼1.5 nm .
Determining the fundamental mechanisms which are responsible for the observed anomalies in electrical transport is a key objective in condensed matter physics. Among these, quantum interference effects such as electron-electron (e-e) interaction and weak localization (WL) effects play an important role in determining the low temperature () electrical transport behaviour in disordered systems. Here, we provide experimental evidence for the observation quantum interference effects driven low temperature resistivity () minima in HoScSi and ErScSi. Temperature dependent magnetic and neutron diffraction (ND) studies clearly establishes that HoScSi undergoes a transition from paramagnetic (PM) to commensurate antiferromagnetic (AFM) state near _1Ho ~ 57 K, which decreases to 28 K (_1Er) in ErScSi. Furthermore, in HoScSi, incommensurate AFM state is observed below ~ 35 K. Additionally, cluster glass phases are also noted in both intermetallics at lower , which coexist with the antiferromagnetic state. These observed features have been attributed to presence of ferromagnetic (FM) and AFM exchange interactions. In addition to this, minima are noted near 20 K (_minHo) and 18 K (_minEr) in HoScSi and ErScSi, respectively. Systematic analysis of the magnetic field and dependence of indicate that the observed anomaly originates from disorder-enhanced e-e interaction and WL effect.
This chapter presents an overview of the electronic structure and the Fermi Surface of the colossal magnetoresistive (CMR) oxides, with an emphasis on recent angle resolved photoemission results of the bilayer manganite La2–2x
Sr1+2x
Mn2O7. These experiments have now advanced to the stage that the low energy excitations very near the Fermi level can be directly measured. The key transport parameters and energy scales can be directly determined from the data. One of the critical outcomes of these and other experiments is the observation and study of a pseudogap: a suppression of spectral weight near the Fermi energy. This effect is shown to be crucial in understanding the very high resistivities as well as the CMR effect. We argue that this pseudogap is likely to be an unusual type of fluctuating charge density wave (CDW) gap, in which the variations in charge density cooperate with the lattice distortions (e.g. Jahn-Teller distortions) as well as with the spin degrees of freedom (double exchange). An other important factor consistent with this data appears to be nanoscale electronic phase separation.
From the magnetic Compton-profile (MCP) measurement, we have directly differentiated for the first time the populations in two eg-type orbitals ( x2-y2 and 3z2-r2) in a manganite. The experimental MCP’s along the [001] direction for La2-2xSr1+2xMn2O7 at x = 0.35 and 0.42 are fitted by the theoretical profiles obtained from the (MnO6)8- ab initio calculations. The calculation confirms that the MCP clearly detects the oxygen hybridization in the eg orbitals. The eg state is dominated by the x2-y2-type orbital with almost constant population, while the population in the 3z2-r2-type orbital decreases with increasing the hole concentration x.
The transport properties of manganite thin films characterized by a weak-localization transition have been studied. Detailed voltage noise measurements show a specific 1/f noise spectrum below the transition temperature. A theoretical interpretation in terms of universal conductance fluctuations explains the nature of the unconventional electric noise, suggesting a direct connection between the weak-localization phenomenon and universal conductance fluctuations. The universal nature of the mechanism allows its detection in different systems under the weak-localization regime.
The low-temperature (< 60 K) transport properties of as-grown La0.7Sr0.3MnO3 ultrathin films, deposited by the molecular beam epitaxy, have been investigated as a function of the sample thickness (from 40 to 3.5 nm) and in the presence of an external magnetic field. With decreasing thickness, a clear low-temperature resistivity minimum slightly affected by the application of the magnetic field has been observed, and its presence has been possibly interpreted in terms of quantum interference effects. As a function of the thickness, a crossover from a three-dimensional (3D) to a two-dimensional (2D) behavior of the system takes place below 20 nm. A re-entrant 3D behavior is induced in ultrathin films by the application of large (>20 kOe) magnetic fields. Negative values of the magnetoresistance have been observed in all of the investigated samples for all of the measured magnetic fields.
The resistivity of epitaxial La0.7Ca0.3MnO3 films deposited on SrTiO3 shows an upturn at low temperatures that is enhanced by the application of a strong magnetic field. This can be understood within the theory of quantum interference effects taking both the interaction and the weak localization correction into account.
Tunneling data on La1.28Sr1.72Mn2O7 crystals confirm Coulomb interaction effects through the √E dependence of the density of states. Importantly, the data and analysis at high energy E show a pileup of states: most of the states removed from near the Fermi level are found between ∼40 and 130 meV, from which we infer the possibility of universal behavior. The agreement of our tunneling data with recent photoemission results further confirms our analysis.
Positive magnetoresistance (MR) in La0.7Pb0.3MnO3 single crystals was observed between temperatures 4.2 and 50 K, in addition to the negative colossal magnetoresistance (CMR) at higher temperatures. The positive MR increases with temperature lowered and reaches 20% at T=4.2 K in a magnetic field of H=8 T. From temperature and field dependences of the resistivity, it follows that the positive MR at low temperatures arise from quantum interference effects (QIE). The QIE-induced MR observed in the CMR manganites indicates that the Coulomb interaction and disorder may play an important role in their transport properties.
Conductivity measurements suggest that a complex magnetic structure exists in a layered manganite La2−2xSr1+2xMn2O7 (x=0.58) that was thought to be a generic A-type antiferromagnet at low temperatures. In the ab planes, we observe metallic conductivity due to double exchange, and the low-temperature data strongly imply an unexpected in-plane spin canting. Fits to a rigorous model determine the super- and double-exchange parameters, can explain the excess c-axis conductivity by spin waves, and correctly predict the measured susceptibility. Magnetization data also reveal very weak double-exchange coupling along the c axis that supports significant in-plane orbital polarization.
The temperature T and magnetic-field H dependences of anisotropic in-plane ρab and out-of-plane ρc resistivities are investigated in single crystals of the bilayer manganite La1.2Sr1.8Mn2O7. Below the Curie transition temperature Tc=125 K, ρab and ρc display almost the same temperature dependence with an up-turn around 50 K. In the metallic regime (50 K <~T<~ 110 K), both ρab(T) and ρc(T) follow a T9/2 dependence, consistent with the two-magnon scattering. We found that the value of the proportionality coefficient Babfit and the ratio of the exchange interaction Jab/Jc, obtained by fitting the data, are in excellent agreement with the calculated Bab based on the two-magnon model and Jab/Jc deduced from neutron scattering, respectively. This provides further support for this scattering mechanism. At even lower T, in the nonmetallic regime (T<50 K), both the in-plane σab and out-of-plane σc conductivities obey a T1/2 dependence, consistent with weak-localization effects. Hence this demonstrates the three-dimensional metallic nature of the bilayer manganite La1.2Sr1.8Mn2O7 at T<Tc.
We have grown c-axis oriented epitaxial thin films of the bilayered perovskite La2-2xSr1+2xMn2O7 (x=0.3,0.4) by laser molecular beam epitaxy on NdGaO3 and SrTiO3 substrates. X-ray diffraction and high-resolution transmission electron microscopy (TEM) revealed excellent epitaxial quality and phase purity of the films. However, a high density of stacking faults could also be detected by TEM. A comparison of magnetotransport measurements within the ab plane and along the c-axis direction showed an intrinsic c-axis tunneling magnetoresistance effect associated with nonlinear current-voltage characteristics for the x=0.3 compound. In addition to the colossal magnetoresistance effect around the Curie temperature TC, at temperatures below about 40 K an additional high-field magnetoresistance was found probably due to a strain and disorder induced re-entrant spin glass state in both the x=0.3 and 0.4 compound. Our experiments show that the coherency strain in the high-quality epitaxial films results in pronounced differences in the magnetotransport behavior as compared to single crystals.
We report on transport measurements on micron-sized mesa structures fabricated on single crystals of the layered manganite La1.4Sr1.6Mn2O7, probing the magnetoresistance of a single magnetic domain for current transport perpendicular to the layers. In magnetic fields applied perpendicular to the layers switching of the magnetoresistance was observed in fields well below 0.4 T, while for parallel fields it decreased continuously. We interpret these observations within the framework of spin-polarized tunneling between adjacent MnO2 bilayers forming an intrinsic stack of spin valves. © 2002 American Institute of Physics.
We have synthesized (110)-oriented epitaxial thin films of the bilayer (n = 2) manganite, La1.2Sr1.8Mn2O7, with the metallic/ferromagnetic a-b planes lying perpendicular to the substrate surface and the c-axis aligned in the plane of the film. X-ray diffraction and transmission electron microscopy confirm the alignment of the a-b planes along the [10] substrate direction. The films consist primarily of the n = 2 phase with a minor component of the n = 1 (La,Sr)2MnO4 and n = ∞ (La,Sr)MnO3 phases. A resistivity maximum coincides with a ferromagnet/paramagnet transition at a reduced Tc ∼ 90 K (versus 120 K for bulk), indicative of the effects of epitaxial strain. The films display similar anisotropic properties to their bulk counterpart with the magnetically easy direction confined to the a-b planes and 20–200 times lower resistivity for current flowing along the a-b planes compared to the c-axis.
Temperature dependence of resistivity of double layered manganite La2−2xCa1+2xMn2O7 (‘x’ = 0.1, 0.2, 0.3 and 0.4) has been investigated in the temperature range of 4.2–50 K. An upturn in the resistivity was observed for all the compositions. The increase of resistivity with the decrease in temperature was found to be due to combined effect of weak localization and electron–electron scattering. The observed minimum in the resistivity is attributed to a competition between weak localization, electron–electron scattering and electron–phonon scattering.
We have grown (110)-oriented thin films of the bilayer manganite La <sub>1.2</sub> Sr <sub>1.8</sub> Mn <sub>2</sub> O <sub>7</sub> [ n=2 phase of the Ruddlesden-Popper (RP) family] by pulsed laser deposition. The structural, magnetic, and magnetotransport properties of these films depend strongly on the growth conditions as well as the film thickness. Optimal growth conditions of 900 ° C and 100 mTorr O <sub>2</sub> balance the oxygenation of the n=2 phase without causing its decomposition into the n=1 and ∞ phases of the RP family. With increasing film thickness, the film evolves from a mixture of the n=1 , 2, and ∞ phases to a film composed primarily of the n=2 phase. The thicker films possess bulklike properties with a ferromagnetic/paramagnetic transition that coincides with a resistivity maximum at a reduced T<sub>c</sub>∼90 K (vs 120 K in bulk) and an anisotropy ratio, ρ<sub>c</sub>/ρ<sub>ab</sub>∼20–200 over the temperature range 5–380 K .
Transmission of information using the spin of the electron as well as its charge requires a high degree of spin polarization at surfaces. However, at surfaces this degree of polarization can be quenched by competing interactions. Using a combination of surface-sensitive X-ray and tunnelling probes, we show for the quasi-two-dimensional bilayer manganites that only the outermost Mn-O bilayer is affected: it is a 1-nm-thick insulator that exhibits no long-range ferromagnetic order, whereas the next bilayer displays the full spin polarization of the bulk. Such an abrupt localization of the surface effects is due to the two-dimensional nature of the layered manganite, and the loss of ferromagnetism is attributed to weakened double exchange in the reconstructed surface bilayer and a resultant antiferromagnetic phase. The creation of a well-defined surface insulator atop a fully spin-polarized bulk demonstrates the ability of two of the most demanding components of an ideal magnetic tunnel junction to self-assemble naturally.
Conductivity data for La(2-2x)Sr(1+2xMn2O7 (x = 0.6) show a first-order transition from an orbital- or charge-ordered insulator to a metal as the temperature falls below approximately 160 K. The change in conductivity is 100 times larger than that seen previously in any single-phase manganite in zero field. The metallic low-temperature state is similar to x = 0.58, but x = 0.58 shows no evidence of orbital or charge order. This result supports a conclusion that strongly coupled magnetic-conductive transitions are first order.
Angle-resolved photoemission spectroscopy data for the bilayer manganite La1.2Sr1.8Mn2O7 show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La1.2Sr1.8Mn2O7 can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction. Comment: 4 pages, 3 figures, in press
Neutron scattering investigations of the paramagnetic correlations in the layered manganite La{sub 1.2}Sr {sub 1.8}MnâOâ , which exhibits colossal magnetoresistance above the Curie transition at T{sub C}=112 K , show that spins in neighboring layers within each bilayer are strongly canted at an average angle that is dependent on both the magnetic field and the temperature, as predicted by de Gennes. The in-plane correlation length does not diverge at T{sub C} , although the magnetic Bragg intensity obeys critical scaling below T{sub C} , with the same temperature dependence as the zero-field electrical conductance. {copyright} {ital 1998} {ital The American Physical Society}
MANGANESE oxides with the cubic perovskite structure (typified by LaMnO3) have stimulated considerable interest because of their magnetoresistive properties1–9; they exhibit extremely large changes in electrical resistance in response to applied magnetic fields, a property that is of technological relevance for the development of magnetic memory and switching devices. But for such applications to be viable, great improvements will be needed in both the sensitivity and temperature dependence of the magnetoresistive response. One approach under consideration for optimizing these properties is chemical substitution10. Here we demonstrate an alternative strategy, in which we synthesize layered variants of the cubic perovskite parent compounds that have a controlled number of MnO2 sheets per unit cell. This strategy is structurally analogous to that employed for the systematic exploration of the high-transition-temperature copper oxide superconductors11. We find that the magneto-resistive properties of these materials depend sensitively on the dimensionality of the manganese oxide lattice. Although the properties of our materials are still far from optimal, further exploration of this series of layered perovskites may prove fruitful.
For the c-axis conductivity, σc, of the
colossal-magnetoresistive layered manganite
La1.4Sr1.6Mn2O7 below
Tc experimental evidence supports the double-exchange (DE)
selection rule, i.e., σc~cos2(η/2),
where η is the angle between Mn spins in neighboring bilayers. Below
Tc, the data also determine the interbilayer magnetic
exchange constant, which is reasonable with respect to recent spin-wave
data, and the magnetocrystalline anisotropy. For c-axis fields, a mixed
AF and spin-flop state (similar to the intermediate state of type-I
superconductors) is a rigorous prediction of the data and modeling.
Zener has suggested a type of interaction between the spins of magnetic ions which he named "double exchange." This occurs indirectly by means of spin coupling to mobile electrons which travel from one ion to the next. We have calculated this interaction for a pair of ions with general spin S and with general transfer integral, b, and internal exchange integral J. One result is that while the states of large total spin have both the highest and lowest energies, their average energy is the same as for the states of low total spin. This should be applicable in the high-temperature expansion of the susceptibility, and if it is, indicates that the high-temperature Curie-Weiss constant theta should be zero, and 1chi vs T a curved line. This is surprising in view of the fact that the manganites, in which double exchange has been presumed to be the interaction mechanism, obey a fairly good Curie-Weiss law. The results can be approximated rather well by a simple semiclassical model in which the spins of the ion cores are treated classically. This model is capable of rather easy extension to the problem of the whole crystal, but the resulting mathematical problem is not easily solved except in special circumstances, e.g., periodic disturbances (spin waves).
Recently, Jonker and Van Santen have found an empirical correlation between electrical conduction and ferromagnetism in certain compounds of manganese with perovskite structure. This observed correlation is herein interpreted in terms of those principles governing the interaction of the d-shells of the transition metals which were enunciated in the first paper of this series. Both electrical conduction and ferromagnetic coupling in these compounds are found to arise from a double exchange process, and a quantitative relation is developed between electrical conductivity and the ferromagnetic Curie temperature.
Temperature-dependent neutron diffraction studies on melt-grown crystals of the layered compound La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} reveal a dramatic structural response to the onset of ferromagnetism and the coincident insulator-metal transition. Unlike the related manganite perovskites, whose Jahn-Teller distorted octahedra become more regular at temperatures below the insulator-metal transition, the MnO{sub 6} octahedra of this layered material are more severely distorted when the charge is itinerant than when it is localized. {copyright} {ital 1997} {ital The American Physical Society}
In connection with two groups of experimental data, a calculation is performed of quantum interference corrections in a quasi-two-dimensional metal to conductivity as a function of temperature and magnetic field. For the temperature dependence a cross-over between a two-dimensional (2D) (higher temperatures) and 3D (lower temperatures) behavior is obtained. In the 3D limit the relative correction is isotropic. In the 2D limit it is extremely anisotropic having a logarithmic dependence in the ab plane, and a power law for the c conductivity. The interaction correction to the density of states is calculated as a function of temperature. A criterion for the crossover from 2D to 3D behavior differs strongly from the interference correction, and makes the 2D limit unlikely. In the 3D limit this correction is proportional to √T. Comparison between corrections of different origins favors the interference logarithmic correction for the in-plane conductivity, in agreement with experiment. The c-axis correction can be of either origin, and is nonlogarithmic, as observed in some experiments. The dependence of the relative correction on magnetic field in the 3D limit is always proportional to √H, in agreement with experiment, and the coefficient depends only on the orientation of the magnetic field.
Results are reported for the conductance anisotropies in single crystals of the layered colossal magnetoresistive manganites. Well below the magnetic ordering temperatures, the zero-field anisotropy increases from ∼230 for La1.24Sr1.76Mn2O7 to >10 000 for La1.4Sr1.6Mn2O7, consistent with a c-axis antiferromagnetic ground state in the latter, while the spin-independent conductivity anisotropy only increases from ∼230 to ∼300. Significant c-axis ferromagnetic fluctuations are seen in La1.4Sr1.6Mn2O7, but no evidence is found for the recently reported two-dimensional magnetic ordering transition at higher temperatures. Direct evidence suggests that spin-orbit coupling is weaker than any other possible sources of orbital ordering.
Low-temperature magnetotransport properties have been investigated for a single crystal of La1.3Sr1.7Mn2O7 with a bilayered structure to reveal the ground-state nature of the quasi-two-dimensional ferromagnetic state. The specific heat measured in a magnetic field indicates a finite density of state of electrons at the Fermi level with seemingly least mass renormalization, which are subject to the weak-localization effect as evidenced by the T1/2 dependence of the conductivity. The highly anisotropic magnetoresistance at low temperatures with respect to the field direction arises from the spin-valve effect due to the ferromagnetic-domain rotation as well as from the field destruction of the localization effect, proving a quasi-two-dimensional diffusive metal as the ground state.
This paper reviews the progress made in the last several years in understanding the properties of disordered electronic systems. Even in the metallic limit, serious deviations from the Boltzmann transport theory and Fermi-liquid theory have been predicted and observed experimentally. There are two important ingredients in this new understanding: the concept of Anderson localization and the effects of interaction between electrons in a disordered medium. This paper emphasizes the theoretical aspect, even though some of the relevant experiments are also examined. The bulk of the paper focuses on the metallic side, but the authors also discuss the metal-to-insulator transition and comment on problems associated with the insulator.
Conditions for Anderson localization are derived for three cases: (1) anisotropic three-dimensional metal, (2) quasi-two-dimensional metal, and (3) quasi-one-dimensional metal. For all these cases the conductivity at [ital T]=0 as well as the interference correction are calculated. The simplest models are used. From the estimate [Delta][sigma]/[sigma][similar to]1, localization conditions are obtained. It is shown that localization takes place in all three cases but in cases (2) and (3) the critical value of the random potential is essentially reduced if the overlap integrals are small. In a two-dimensional metal this refers to the conductivity along the planes whereas for the conductivity perpendicular to the planes the three-dimensional condition applies, i.e., contrary to common wisdom localization in this direction is more difficult to reach than along the planes.
A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance
of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77
kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device
applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are
discussed.
12 Due to the strong Hund's rule coupling of the conduction electron spin to the Mn ions, it is unlikely that the spin splitting effects or spin relaxation arising from spin-orbit interactions will affect the results in the nearly saturated FM state at low temperatures
- Khmel 'nitskii
- Zh
- Eksp
- K Li E Teoran
- And J F Gray
- Mitchell Physical Review
Khmel'nitskii, Zh. Eksp. Teor. Fiz 81, 768 ͑1981͒ ͓Sov. Phys. JETP 54, 411 ͑1981͔͒. 11 P. A. Lee and T. V. Ramakrishnan, Rev. Mod. Phys. 57, 287 ͑1985͒. 12 Due to the strong Hund's rule coupling of the conduction electron spin to the Mn ions, it is unlikely that the spin splitting effects or spin relaxation arising from spin-orbit interactions will affect the results in the nearly saturated FM state at low temperatures. 13 R. Osborn, S. Rosenkranz, D. N. Argyiou, L. Vasiliu-Doloc, J. W. Lynn, S. K. Sinha, J. F. Mitchell, K. E. Gray, and S. D. Bader, Phys. Rev. Lett. 81, 3964 ͑1998͒. 14 A. F. Ioffe and A. R. Regel, Prog. Semicond. 4, 237 ͑1960͒. 15 A. A. Abrikosov, Phys. Rev. B 50, 1415 ͑1994͒. QING'AN LI, K. E. GRAY, AND J. F. MITCHELL PHYSICAL REVIEW B 63 024417 024417-6