Inverse Proximity Effect Research Articles

Measurement of spin polarization in [formula omitted]- In0.96Mn0.04As using Andreev reflection spectroscopy

We report on the transport behavior and spin polarization of carriers P for superconductor/ferromagnetic semiconductor (S-F) junctions. We fabricated Nb/ferromagnetic semiconductor p-In0.96Mn0.04As junctions with Hall voltage probes. Below ∼15 K, p- In0.96Mn0.04As becomes ferromagnetic and then the anomalous Hall effect is observed. Below the TC of the Nb electrodes (∼8.2 K), we observed a conductance reduction within the Nb superconducting energy gap voltage owing to the suppression of Andreev reflection by spin polarization in p- In0.96Mn0.04As. We have evaluated the degree of spin polarization in p- In0.96Mn0.04As experimentally by comparing the measured differential conductance with that obtained with the Strijkers model extended for spin-polarized Andreev reflection including the inverse proximity effect. Consequently, the P value extracted experimentally for p- In0.96Mn0.04As at 0.5 K was P=0.33∼0.795.

Planar S-(S/F)-S Josephson junctions induced by the inverse proximity effect

We present a utilization of the inverse proximity effect in superconductor-ferromagnet (S-F) bilayer to generate lateral Josephson junctions. The weak link is created by a Pd0.95Fe0.05 strip across a Nb bridge. Close to TC and in perpendicular magnetic field the junctions exhibit a modulation of the critical current IC(B) similar to a Fraunhofer interference pattern which proves the dc Josephson effect. The structure contains three weak links (two different areas) in series which result in the observation of two periods scalable with the areas penetrated by magnetic flux. Measurements of Shapiro steps prove the presence of the ac Josephson effect.

Polar Kerr effect as probe for time-reversal symmetry breaking in unconventional superconductors

The search for broken time-reversal symmetry (time-reversal symmetry breaking (TRSB)) in unconventional superconductors has intensified in the past few years as more systems have been predicted to possess such a state. Following our pioneering study of TRSB states in Sr2RuO4 using magneto-optic probes, we embarked on a systematic study of several other of these candidate systems. The primary instrument for our studies is the Sagnac magneto-optic interferometer that we recently developed. This instrument can measure magneto-optic Faraday or Kerr effects with an unprecedented sensitivity of 10 nrad at temperatures as low as 100 mK. In this paper, we review our recent studies of TRSB in several systems, emphasizing the study of the pseudogap state of high temperature superconductors and the inverse proximity effect in superconductor/ferromagnet proximity structures.

Spin-valve effect of spin-accumulation resistance in a double ferromagnet/superconductor junction

We have measured the transport properties of ferromagnet-superconductor nanostructures, where two superconducting aluminum (Al) electrodes are connected through two ferromagnetic iron (Fe) ellipsoids in parallel. We find that, below the superconducting critical temperature of Al, the resistance depends on the relative alignment of the ferromagnets' magnetization. This spin-valve effect is analyzed in terms of spin accumulation in the superconducting electrode submitted to inverse proximity effect.

Inverse Proximity Effect in Superconductor-Ferromagnet Bilayer Structures

Measurements of the polar Kerr effect using a zero-area-loop Sagnac magnetometer on Pb/Ni and Al/(Co-Pd) proximity-effect bilayers show unambiguous evidence for the "inverse proximity effect," in which the ferromagnet induces a finite magnetization in the superconducting layer. To avoid probing the magnetic effects in the ferromagnet, the superconducting layer was prepared much thicker than the light's optical-penetration depth. The sign and size of the effect, as well as its temperature dependence agree with recent predictions by Bergeret et al. [Phys. Rev. B 69, 174504 (2004)]10.1103/PhysRevB.69.174504.

Theory of superconducting and magnetic proximity effect in S/F structures with inhomogeneous magnetization textures and spin-active interfaces

We present a study of the proximity effect and the inverse proximity effect in a $\text{superconductor}\ensuremath{\mid}\text{ferromagnet}$ bilayer, taking into account several important factors which mostly have been ignored in the literature so far. These include spin-dependent interfacial phase shifts (spin-DIPS) and inhomogeneous textures of the magnetization in the ferromagnetic layer, both of which are expected to be present in real experimental samples. Our approach is numerical, allowing us to access the full proximity effect regime. In Sec. II of this work, we study the superconducting proximity effect and the resulting local density of states in an inhomogeneous ferromagnet with a nontrivial magnetic texture. Our two main results in Sec. II are a study of how Bloch and N\'eel domain walls affect the proximity-induced superconducting correlations and a study of the superconducting proximity effect in a conical ferromagnet. The latter topic should be relevant for the ferromagnet Ho, which was recently used in an experiment to demonstrate the possibility to generate and sustain long-range triplet superconducting correlations. In Sec. III of this work, we investigate the inverse proximity effect with emphasis on the induced magnetization in the superconducting region as a result of the ``leakage'' from the ferromagnetic region. It is shown that the presence of spin-DIPS modifies conclusions obtained previously in the literature with regard to the induced magnetization in the superconducting region. In particular, we find that the spin-DIPS can trigger an antiscreening effect of the magnetization, leading to an induced magnetization in the superconducting region with the same sign as in the proximity ferromagnet.

Spin-polarized carrier injection effect in ferromagnetic semiconductor/diffusive semiconductor/superconductor junctions

We study the transport properties of a p-InMnAs/n-InAs/Nb junction where a p-InMnAs can be regarded as a spin injector. Differential conductance of the n-InAs channel is measured as a function of injection current from p-InMnAs or from Nb at 20 mK. A conductance minimum appears at zero-bias voltage with no current injection. As the injection current from p-InMnAs increases, the minimum gradually disappears. This conductance behaviour is very different from that of the injection case from Nb. We also calculate the conductance in the n-InAs channel by taking account of the exchange field in the InAs channel that is induced by InMnAs ferromagnet. The difference between the conductance behaviours on injection current direction can be explained by the inverse proximity effect that the exchange field is also induced in the superconducting electrode.

Spin screening of magnetic moments and inverse proximity effect in ferromagnet/superconductor nanostructures

It is shown that the short-range oscillating spin polarization of conduction electrons around a magnetic moment embedded in a superconducting film is screened by a long-range antiferromagnetic term, which has its origin in Cooper pairing. Using these results, an exchange model of the proximity effect in ferromagnet/superconductor (F/S) nanostructures is proposed. The mutual accommodation of inhomogeneous superconducting and magnetic order parameters in the F/S nanostructures is studied within the framework of this model. It is shown that F/S nanostructures of the first type allow only the presence of homogeneous ferromagnetic ordering in the F layers that coexists with superconductivity in the S layers if the exchange fields h are lower than a critical field hc. It is found that in the F/S nanostructures of the second type for exchange fields h between hc1 and hc2, where hc1(2) is the lower (upper) critical exchange field, a nonuniform cryptoferromagnetic modulation is induced in the spin structure of the F films. The conditions for the coexistence of inhomogeneous magnetism and superconductivity in the F/S nanostructures EuO∕Al, EuO∕V, EuS∕Al, and LaCaMnO∕YBaCuO are explored. The theory is used to interpret the experimental data.

Inverse proximity effect in superconductor-ferromagnet structures: From the ballistic to the diffusive limit

The inverse proximity effect, i.e. the induction of a magnetic moment in the superconductor in superconductor-ferromagnet (S/F) junctions is studied theoretically. We present a microscopic approach which combines a model Hamiltonian with elements of the well established quasiclassical theory. With its help we study systems with arbitrary degree of disorder, interface transparency and thickness of the layers. In the diffusive limit we recover the result of previous works: the direction of the induced magnetization $M$ is opposite to the one of the F layer. However, we show that in the ballistic case the sign of $M$ may be positive or negative depending on the quality of the interface and thickness of the layers. {We show that, regardless of its sign, the penetration length of the magnetic moment into the superconductor is of the order of the superconductor coherence length, which demonstrates that the effect has a superconducting origin.}

Induced ferromagnetism due to superconductivity in superconductor-ferromagnet structures

We consider a superconductor-ferromagnet (S/F) structure and assume that above the superconducting transition temperature $T_{c}$ the magnetic moment exists only in F. {In a simple model of the ferromagnet (the exchange field is of the ferromagnetic type for all energies)}we show by an explicit calculation that below $T_{c}$ the magnetic moment may penetrate the superconductor. {In this model} its direction in S is opposite {to the magnetization of free electrons} in the ferromagnet. The magnetization spreads over a large distance which is of the order of the superconducting coherence length $\xi_{S}$ and can much exceed the ferromagnet film thickness. At the same time the magnetic moment in the ferromagnet is reduced. This inverse proximity effect may explain the reduction in magnetization observed in recent experiments and may lead to a strong interaction between the ferromagnetic layers in F/S/F structures.

Quantum transport through carbon nanotubes: Proximity-induced and intrinsic superconductivity

We report low temperature transport measurements on suspended single walled carbon nanotubes (both individual tubes and ropes). The technique we have developed, where tubes are soldered on low resistive metallic contacts across a slit, enables a good characterization of the samples by transmission electron microscopy. It is possible to obtain individual tubes with a room temperature resistance smaller than 40 kOhms, which remain metallic down to very low temperatures. When the contact pads are superconducting, nanotubes exhibit proximity induced superconductivity with surprisingly large values of supercurrent. We have also recently observed intrinsic superconductivity in ropes of single walled carbon nanotubes connected to normal contacts, when the distance between the normal electrodes is large enough, since otherwise superconductivity is destroyed by (inverse) proximity effect. These experiments indicate the presence of attractive interactions in carbon nanotubes which overcome Coulomb repulsive interactions at low temperature, and enables investigation of superconductivity in a 1D limit never explored before.

Proximity effect in ultrathin Pb/Ag multilayers within the Cooper limit

We report on transport and tunneling measurements performed on ultrathin Pb/Ag (strong-coupled superconductor/normal metal) multilayers evaporated by quench condensation. The critical temperature and energy gap of the heterostructures oscillate with addition of each layer, demonstrating the validity of the Cooper limit model in the case of multilayers. We observe excellent agreement with a simple theory for samples with layer thickness larger than $30\mathrm{\AA{}}.$ Samples with single layers thinner than $30\mathrm{\AA{}}$ deviate from the Cooper limit theory. We suggest that this is due to the ``inverse proximity effect'' where the normal-metal electrons improve screening in the superconducting ultrathin layer and thus enhance the critical temperature.

Inverse proximity effect in a strongly correlated electron system.

An anomalous superconducting proximity effect between a strongly correlated electron system and a normal metal is demonstrated. The model system is a 2D ultrathin superconducting quench-condensed Pb film. Such a highly disordered film has a reduced transition temperature (T(c) = 1.7 K) due to the strong e(-)-e(-) interaction. Instead of weakening the superconductivity, an overlayer of Ag on Pb induces an increase of both the T(c) and the gap. The restoration of the electron screening brought about by the quasiparticles from the normal metal can explain this striking inverse proximity effect.

30 nm resolution zero proximity lithography on high-Z substrates

In order to fabricate masks for x-ray lithography, there is growing interest in subtractive patterning of high atomic mass (high-Z) materials such as tungsten or gold. Favorable writing speeds and sub-50 nm resolution without proximity effects combine to make heavy ion focused ion beam lithography an ideal candidate for this area of nanofabrication. Using a 50 keV Ga+ beam with an 8 nm spot diameter, we have exposed a variety of proximity effect test patterns in 60 nm thick PMMA on 0.5 μm thick tungsten films. The results indicate that 30-nm resolution or better is possible at line/space pitches as small as 80 nm. The test patterns show no apparent proximity effects at these dimensions. An anomalous ‘‘inverse proximity effect’’ was observed, and was determined to be an artifact of the scanning electron microscope technique used to observe the PMMA resist.

The redshift distribution of Ly-alpha clouds and the proximity effect

The results of a systematic analysis of the redshift distribution of Lyα forest clouds are presented. We form a Lyα line sample from 38 QSO spectra which exhibit neither broad absorption line systems nor the "associated" Z_abs_ ~ Z_em_ metal line systems. The absorption line sample includes only Lyα lines which occur between Lyβ and Lyα emission, which have rest frame equivalent width greater than 0.36 A, and which are not associated with any known metal line systems. The sample contains 950 lines, the largest by far in this kind of study. We confirm that there is overwhelming evidence for the evolution in number density of Lyα forest lines: for the functional form dN/dz is proportional to (1 + z)<SUP>gamma</SUP>^, we find γ = 2.37 +/- 0.26. Using two mutually independent samples we confirm the existence of an inverse effect or proximity effect in that the Lyα line number density tends to be systematically lower near the QSO emission redshifts than elsewhere. We find no positive evidence that the proximity effect is correlated significantly with the QSO luminosities, contrary to previous results. Explanations are sought to reconcile this paradox. We also find that the proximity effect is uncorrelated with the radio properties of QSOs. We show that the data are entirely consistent with a model in which the inverse effect is due to the enhanced ionization of the Lyα clouds by the nearby QSOs. Assuming the model is correct, the background mean intensity, J_v_, is found to be log J_v_ ~ 21.0, where J_v_ is the Lyman limit intensity in unit ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^. This value of J_v_ is consistent with a previous result from a similar study (BDO). When the proximity effect is taken into consideration, the redshift index is found to be γ = 2.75 +/- 0.29. This value should be considered as a more realistic estimate for the redshift evolution of Lyα clouds than the previously quoted value of γ = 2.37. In addition, we consider two other alternative explanations for the inverse effect, i.e., (1) the Lyα clouds may not be big enough to cover the entire broad emission line regions, or (2) the functional form, (1 + z)<SUP>gamma</SUP>^, may be inadequate to describe the redshift distribution of the Lyα clouds, thus causing the inverse effect. We find no evidence in the data, however, that support either of them. Further tests are suggested.