Spin-dependent Charge Transport Research Articles

Chiral Hybrid Perovskites (R-/S-CLPEA)4Bi2I10 with Enhanced Chirality and Spin–Orbit Coupling Splitting for Strong Nonlinear Optical Circular Dichroism and Spin Selectivity Effects

Chiral hybrid halide perovskites have attracted considerable attention in optoelectronics and spintronics owing to their unique optical, electric, and spin–orbit coupling (SOC) properties. In this article, with focus on the design scheme to increase the nonlinear optical (NLO) circular dichroism (CD) and spin selectivity effects by assembling chlorine-modified chiral R-/S-CLPEA [CLPEA = 1-(4-chlorophenyl)ethylamine] into bismuth-based perovskites, a pair of chiral hybrid perovskites (R-/S-CLPEA)4Bi2I10 were synthesized and characterized experimentally. A large NLO CD effect and strong spin-dependent charge transport were confirmed by the experimental measurements. The unique chirality-induced spin textures were illustrated in the SOC band structures from first-principles simulations. The combined theoretical and experimental results demonstrate that it is the synergic effect of large chirality of CLPEA and strong SOC effect of the Bi2I10 dimer that endows these perovskites with unique chiral spin textures, strong NLO CD effect, and remarkable spin-dependent charge transport properties, which are of importance for the functional design of chiral perovskites in nonlinear optics and spintronics.

Dynamical mean-field theory for spin-dependent electron transport in spin-valve devices

We present a combination of density functional theory and dynamical mean-field theory (DMFT) for computing the electron transmission through two-terminal nanoscale devices. The method is then applied to metallic junctions presenting alternating Cu and Co layers, which exhibit spin-dependent charge transport and the giant magnetoresistance (GMR) effect. The calculations show that the coherent transmission through the $3d$ states is greatly suppressed by electron correlations. This is mainly due to the finite lifetime induced by the electron-electron interaction and is directly related to the imaginary part of the computed many-body DMFT self-energy. At the Fermi energy, where in accordance with the Fermi-liquid behavior the imaginary part of the self-energy vanishes, the suppression of the transmission is entirely due to the shifts of the energy spectrum induced by electron correlations. Based on our results, we finally suggest that the GMR measured in Cu/Co heterostructures for electrons with energies about 1 eV above the Fermi energy is a manifestation of dynamical correlation effects.

Manipulating electron-spin polarization using cysteine-DNA chiral conjugates.

The chiral molecules are potential generators of high spin-filters due to their inherent inversion asymmetric helical electric field. We report a controlled spin-selective transmission of electrons through self-assembled monolayers of 15 base-paired double-stranded deoxyribonucleic acid functionalized with two enantiomeric cysteine molecules on gold explored through the quantum mechanical tunneling effect. We observed a controlled spin polarization of 33% with dextro-cysteine, whereas a mere 8% was observed with levo-cysteine molecules using these functionalizations at room temperature. The manipulation of electron's spin merely through such a small molecule could lead to significant advancement in the spin-dependent charge transport phenomena and related applications.

Room-Temperature Spin-Dependent Transport in Metalloporphyrin-Based Supramolecular Wires.

Here we present room‐temperature spin‐dependent charge transport measurements in single‐molecule junctions made of metalloporphyrin‐based supramolecular assemblies. They display large conductance switching for magnetoresistance in a single‐molecule junction. The magnetoresistance depends acutely on the probed electron pathway through the supramolecular wire: those involving the metal center showed marked magnetoresistance effects as opposed to those exclusively involving the porphyrin ring which present nearly complete absence of spin‐dependent charge transport. The molecular junction magnetoresistance is highly anisotropic, being observable when the magnetization of the ferromagnetic junction electrode is oriented along the main molecular junction axis, and almost suppressed when it is perpendicular. The key ingredients for the above effect to manifest are the electronic structure of the paramagnetic metalloporphyrin, and the spinterface created at the molecule–electrode contact.

Quantum Circuit Rules for Molecular Electronic Systems: Where Are We Headed Based on the Current Understanding of Quantum Interference, Thermoelectric, and Molecular Spintronics Phenomena?

In this minireview, we discuss important aspects of the various quantum phenomena (such as quantum interference, spin-dependent charge transport, and thermoelectric effects) relevant in single-molecule charge transport and list some of the basic circuit rules devised for different molecular systems. These quantum phenomena, in conjunction with the existing empirical circuit rules, can help in predicting some of the structure-property relationships in molecular circuits. However, a universal circuit law that predicts the charge transport properties of a molecular circuit has not been derived yet. Having such law(s) will help to design and build a complex molecular device leading to exciting unique applications that are not possible with the traditional silicon-based technologies. Based on the existing knowledge in the literature, here we open the discussion on the possible future research directions for deriving unified circuit law(s) to predict the charge transport in complex single-molecule circuits.

Spin memory of the topological material under strong disorder

Robustness to disorder is the defining property of any topological state. The ultimate disorder limits to topological protection are still unknown, although a number of theories predict that even in the amorphous state a quantized conductance might yet reemerge. Here we report that in strongly disordered thin films of the topological material Sb2Te3disorder-induced spin correlations dominate transport of charge—they engender a spin memory phenomenon, generated by the nonequilibrium charge currents controlled by localized spins. We directly detect a glassy yet robust disorder-induced magnetic signal in films free of extrinsic magnetic dopants, which becomes null in a lower-disorder crystalline state. This is where large isotropic negative magnetoresistance (MR)—a hallmark of spin memory—crosses over to positive MR, first with only one e2/h quantum conduction channel, in a weakly antilocalized diffusive transport regime with a 2D scaling characteristic of the topological state. A fresh perspective revealed by our findings is that spin memory effect sets a disorder threshold to the protected topological state. It also points to new possibilities of tuning spin-dependent charge transport by disorder engineering of topological materials.

Thermal Conductivity of Oxide Tunnel Barriers in Magnetic Tunnel Junctions Measured by Ultrafast Thermoreflectance and Magneto-Optic Kerr Effect Thermometry

Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be manipulated by a temperature gradient, which can be utilized for spintronic and spin caloritronic applications. Evaluation of the thermally induced phenomena requires knowledge of the temperature differences across the oxide tunnel barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to accurately measure thermal properties of an oxide tunnel barrier consisting of only a few atomic layers. In this work, we experimentally interrogate the temperature evolutions in Ru/oxide/FM/seed/MgO (oxide=MgO, MgAl2O4; FM=Co, CoFeB; seed=Pt, Ta) structures having perpendicular magnetic anisotropy using ultrafast thermometry. The Ru layer is optically thick and heated by ultrafast laser pulses; the subsequent temperature changes are monitored using thermoreflectance of Ru and magneto-optic Kerr effect (MOKE) of the FM layers. We independently measure the response times of Co and CoFeB magnetism using quadratic MOKE and obtain {\tau}em=0.2 ps for Co and 2 ps for CoFeB. These time scales are much shorter than the time scale of heat transport through the oxide tunnel barrier, which occurs at 10-3000 ps. We determine effective thermal conductivities of MgO and MgAl2O4 tunnel barriers in the range of 0.4-0.6 W m-1 K-1, comparable to an estimate of the series conductance of the Ru/oxide and oxide/FM interfaces and an order of magnitude smaller than the thermal conductivity of MgO thin films. We find that the electron-phonon thermal conductance near the tunnel barrier is only a factor of 5-12 larger than the thermal conductance of the oxide tunnel barrier. Therefore, the drop in the electronic temperature is approximately 20-30% larger than the drop in the phonon temperature across the tunnel barrier.

Two-step breakdown of a local ν=1 quantum Hall state

We report quantum Hall effect breakdown of a local filling factor v_local = 1 state formed in a bulk v_bulk = 2 system in an AlGaAs/GaAs heterostructure. When a finite source-drain bias is applied across the local system, the breakdown occurs in two steps. At low bias, quantized conductance through the v_local = 1 system breaks down due to inter-edge electron tunneling. At high bias, the incompressibility of the v_local = 1 system breaks down because the spin gap closes. The two steps are resolved by combining measurements of resistively detected nuclear magnetic resonance and shot noise, which allows one to evaluate electron spin polarization in the local system and spin-dependent charge transport through the system, respectively.

Spin-dependent charge transport through 2D chiral hybrid lead-iodide perovskites.

Chiral-induced spin selectivity (CISS) occurs when the chirality of the transporting medium selects one of the two spin ½ states to transport through the media while blocking the other. Monolayers of chiral organic molecules demonstrate CISS but are limited in their efficiency and utility by the requirement of a monolayer to preserve the spin selectivity. We demonstrate CISS in a system that integrates an inorganic framework with a chiral organic sublattice inducing chirality to the hybrid system. Using magnetic conductive-probe atomic force microscopy, we find that oriented chiral 2D-layered Pb-iodide organic/inorganic hybrid perovskite systems exhibit CISS. Electron transport through the perovskite films depends on the magnetization of the probe tip and the handedness of the chiral molecule. The films achieve a highest spin-polarization transport of up to 86%. Magnetoresistance studies in modified spin-valve devices having only one ferromagnet electrode confirm the occurrence of spin-dependent charge transport through the organic/inorganic layers.

Effect of Oxidative Damage on Charge and Spin Transport in DNA.

A Hall device was used for measuring spin polarization on electrons that are either reorganized within the molecules or transmitted through the self-assembled monolayers of DNA adsorbed on the device surface. We were able to observe spin-dependent charge polarization and charge transport through double-stranded DNA of various lengths and through double-stranded DNA containing oxidative damage. We found enhancement in the spin-dependent transport through oxidatively damaged DNA. This phenomenon can be rationalized either by assuming that the damaged DNA is characterized by a higher barrier for conduction or by charge transfer through the DNA being conducted through at least two channels, one involves the bases and is highly conductive but less spin selective, while the other pathway is mainly through the ribophosphate backbone and it is the minor one in terms of charge transmission efficiency, but it is highly spin selective.

Enhanced and Facet-specific Electrocatalytic Properties of Ag/Bi2Fe4O9 Composite Nanoparticles.

Ag/Bi2Fe4O9 nanoparticles (BFO NPs) have been synthesized using a two-step approach involving glycine combustion and visible light irradiation. Their structures were characterized in detail using X-ray diffraction, transmission electron microscope, scanning electron microscopy, and scanning transmission electron microscopy techniques. Their electrocatalytic properties were studied through enzymatic glucose detection with an amperometric biosensor. The Ag deposited on selective crystal facets of BFO NPs significantly enhanced their electrocatalytic activity. To gain insights into the origin of the enhanced electrocatalytic activities, we have carried out studies of Ag+ reduction and Mn2+ oxidation reaction at the {200} and {001} facets, respectively. The results suggest effective charge separation on the BFO NP surfaces, which is likely responsible for the enhanced electrocatalytic properties. Furthermore, enhanced ferromagnetism was observed after the Ag deposition on BFO NPs, which may be related to the improved electrocatalytic properties through spin-dependent charge transport. The facet-specific electrocatalytic properties are highly interesting and desired for chemical reactions. This study demonstrates that Ag/BFO NPs are potentially useful for electrocatalytic applications including biosensing and chemical synthesis with high product selectivity.

Enhanced spin-dependent charge transport of Co-(Al-fluoride) granular nanocomposite by co-separate sputtering

Spin-dependent charge transport behavior involving the recently discovered tunnel-type magneto-dielectric (TMD) and magnetoresistance (TMR) effects was studied in Co-(Al-fluoride) granular nanocomposites. By setting a changeable partition height (t = 1–4 cm) on a substrate holder in a conventional co-sputtering (CS) deposition system, we developed a co-separate sputtering (CSS) method to fabricate Co-(Al-F) granular nanocomposites. XPS analysis shows that the Al content remains balanced between the Al metal and Al-F compounds by controlling t. This phenomenon can be attributed to the magnetron plasma interference from the two target sources. Fittings between TMR and normalized magnetization suggest that the CSS films with clear granular structures may have high spin polarization. Compared with the CS samples (t = 0 cm), the CSS films with t = 4 cm show enhanced charge transport properties with a maximum TMD ratio (0.5%) and TMR ratio (7.2%) under a magnetic field of H = 10 kOe. This study demonstrates that the Al-F tunnel barrier between Co granules plays an essential role in controlling the charge transport behavior and will be of significance for applications in field sensors and impedance-tunable devices with large magnetic-field response.

Analyzing Spin Selectivity in DNA-Mediated Charge Transfer via Fluorescence Microscopy.

Understanding spin-selective interactions between electrons and chiral molecules is critical to elucidating the significance of electron spin in biological processes and to assessing the potential of chiral assemblies for organic spintronics applications. Here, we use fluorescence microscopy to visualize the effects of spin-dependent charge transport in self-assembled monolayers of double-stranded DNA on ferromagnetic substrates. Patterned DNA arrays provide background regions for every measurement to enable quantification of substrate magnetization-dependent fluorescence due to the chiral-induced spin selectivity effect. Fluorescence quenching of photoexcited dye molecules bound within DNA duplexes is dependent upon the rate of charge separation/recombination upon photoexcitation and the efficiency of DNA-mediated charge transfer to the surface. The latter process is modulated using an external magnetic field to switch the magnetization orientation of the underlying ferromagnetic substrates. We discuss our results in the context of the current literature on the chiral-induced spin selectivity effect across various systems.

Morphology effects on spin-dependent transport and recombination in polyfluorene thin films

We have studied the role of spin-dependent processes on conductivity in polyfluorene (PFO) thin films by preforming continuous wave (cw) electrically detected magnetic resonance (EDMR) spectroscopy at temperatures between 10 K and room temperature using microwave frequencies between about 1 GHz and 20 GHz, as well as pulsed EDMR at the $X$ band (10 GHz). Variable frequency EDMR allows us to establish the role of spin-orbit coupling in spin-dependent processes whereas pulsed EDMR allows for the observation of coherent spin motion effects. We used PFO for this study in order to allow for the investigation of the effects of microscopic morphological ordering since this material can adopt two distinct intrachain morphologies: an amorphous (glassy) phase, in which monomer units are twisted with respect to each other, and an ordered (\ensuremath{\beta}) phase, where all monomers lie within one plane. In thin films of organic light-emitting diodes, the appearance of a particular phase can be controlled by deposition parameters and solvent vapor annealing, and is verified by electroluminescence spectroscopy. Under bipolar charge-carrier injection conditions, we conducted multifrequency cw EDMR, electrically detected Rabi spin-beat experiments, and Hahn echo and inversion-recovery measurements. Coherent echo spectroscopy reveals electrically detected electron-spin-echo envelope modulation due to the coupling of the carrier spins to nearby nuclear spins. Our results demonstrate that, while conformational disorder can influence the observed EDMR signals, including the sign of the current changes on resonance as well as the magnitudes of local hyperfine fields and charge-carrier spin-orbit interactions, it does not qualitatively affect the nature of spin-dependent transitions in this material. In both morphologies, we observe the presence of at least two different spin-dependent recombination processes. At room temperature and 10 K, polaron-pair recombination through weakly spin-spin coupled intermediate charge-carrier pair states is dominant, while at low temperatures, additional signatures of spin-dependent charge transport through the interaction of polarons with triplet excitons are seen in the half-field resonance of a triplet spin-1 species. This additional contribution arises since triplet lifetimes are increased at lower temperatures. We tentatively conclude that spectral broadening induced by hyperfine coupling is slightly weaker in the more ordered \ensuremath{\beta}-phase than in the glassy phase since protons are more evenly spaced, whereas broadening effects due to spin-orbit coupling, which impacts the distribution of $g$-factors, appear to be somewhat more significant in the \ensuremath{\beta}-phase.

Spin-Dependent Transport through Chiral Molecules Studied by Spin-Dependent Electrochemistry

ConspectusMolecular spintronics (spin + electronics), which aims to exploit both the spin degree of freedom and the electron charge in molecular devices, has recently received massive attention. Our recent experiments on molecular spintronics employ chiral molecules which have the unexpected property of acting as spin filters, by way of an effect we call “chiral-induced spin selectivity” (CISS). In this Account, we discuss new types of spin-dependent electrochemistry measurements and their use to probe the spin-dependent charge transport properties of nonmagnetic chiral conductive polymers and biomolecules, such as oligopeptides, L/D cysteine, cytochrome c, bacteriorhodopsin (bR), and oligopeptide-CdSe nanoparticles (NPs) hybrid structures. Spin-dependent electrochemical measurements were carried out by employing ferromagnetic electrodes modified with chiral molecules used as the working electrode. Redox probes were used either in solution or when directly attached to the ferromagnetic electrodes. During the electrochemical measurements, the ferromagnetic electrode was magnetized either with its magnetic moment pointing “UP” or “DOWN” using a permanent magnet (H = 0.5 T), placed underneath the chemically modified ferromagnetic electrodes. The spin polarization of the current was found to be in the range of 5–30%, even in the case of small chiral molecules. Chiral films of the l- and d-cysteine tethered with a redox-active dye, toludin blue O, show spin polarizarion that depends on the chirality. Because the nickel electrodes are susceptible to corrosion, we explored the effect of coating them with a thin gold overlayer. The effect of the gold layer on the spin polarization of the electrons ejected from the electrode was investigated. In addition, the role of the structure of the protein on the spin selective transport was also studied as a function of bias voltage and the effect of protein denaturation was revealed. In addition to “dark” measurements, we also describe photoelectrochemical measurements in which light is used to affect the spin selective electron transport through the chiral molecules. We describe how the excitation of a chromophore (such as CdSe nanoparticles), which is attached to a chiral working electrode, can flip the preferred spin orientation of the photocurrent, when measured under the identical conditions. Thus, chirality-induced spin polarization, when combined with light and magnetic field effects, opens new avenues for the study of the spin transport properties of chiral molecules and biomolecules and for creating new types of spintronic devices in which light and molecular chirality provide new functions and properties.

Discrimination between spin-dependent charge transport and spin-dependent recombination in π-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

It has been corroborated in the past that Pauli blockade mechanisms of weakly spin-coupled charge carrier pairs undergoing transitions into doubly occupied singlet states are among the most dominant spin-selection rules influencing room-temperature magnetoresistance and luminescence of organic semiconductors. It has remained unclear though whether these pairs consist of equal or opposite charges, and thus, whether the spin-dependent transitions are recombination or charge transport processes. Here, the authors report on simultaneous transient measurements of electric current and optical emission changes in organic light emitting diodes due to pulsed magnetic resonance induced charge carrier spin manipulation. When a room-temperature steady-state current is changed magnetic resonantly, the electroluminescence changes show identical dynamic and magnetic resonant signatures, indicating that both observables are controlled by the same spin-dependent electronic process. However, a quantitative analysis of the optical emission change reveals that its magnitude significantly exceeds what is expected from the magnitude of the electrical current change, implying that the optical emission change occurs not only because of the device current change but also due to a change of the radiative recombination rate, thereby showing that the observed spin-dependent process is due to recombination, not transport.

Magnetic field cycling effect on the non-linear current-voltage characteristics and magnetic field induced negative differential resistance in α-Fe1.64Ga0.36O3 oxide

We have studied current-voltage (I-V) characteristics of α-Fe1.64Ga0.36O3, a typical canted ferromagnetic semiconductor. The sample showed a transformation of the I-V curves from linear to non-linear character with the increase of bias voltage. The I-V curves showed irreversible features with hysteresis loop and bi-stable electronic states for up and down modes of voltage sweep. We report positive magnetoresistance and magnetic field induced negative differential resistance as the first time observed phenomena in metal doped hematite system. The magnitudes of critical voltage at which I-V curve showed peak and corresponding peak current are affected by magnetic field cycling. The shift of the peak voltage with magnetic field showed a step-wise jump between two discrete voltage levels with least gap (ΔVP) 0.345(± 0.001) V. The magnetic spin dependent electronic charge transport in this new class of magnetic semiconductor opens a wide scope for tuning large electroresistance (∼500-700%), magnetoresistance (70-135 %) and charge-spin dependent conductivity under suitable control of electric and magnetic fields. The electric and magnetic field controlled charge-spin transport is interesting for applications of the magnetic materials in spintronics, e.g., magnetic sensor, memory devices and digital switching.

Fundamental physics behind high-efficiency organo-metal halide perovskite solar cells

Polarization and spin-dependent excited states and charge transport.

Self-Assembled Ti Quantum Wire on Zigzag Graphene Nanoribbons with One Edge Saturated

Using first-principles calculations, we study on the energetics, kinetics, and electronic properties of transition metal atoms Ti, Mn, and Au on zigzag graphene nanoribbons (ZGNRs) with one edge saturated by two H atoms C(2H), while the other one by one H atom C(H). Because of the larger magnetic bearded edge states on the C(2H) edge, all these three adatoms prefer adsorbing on the C(2H) edge, but Ti binds stronger to ZGNRs on the C(2H) edge than both Mn and Au atoms. The saturated C(2H) edge not only dramatically enhances the Ti mobility from the center to C(2H) edge but also weakens the diffusion isotropy; thus, Ti quantum wire is formed readily along the C(2H) edge. Along Ti atomic wire there is a spin-up conducting channel contributed by pure Ti 3d state, indicating spin-dependent charge transport properties. This may open new avenues in fabricating metal quantum wires.

Spin Transport in Be Edge-Doped Graphene Nanoribbon

We report an atomistic simulation of spin dependent charge transport in zigzag graphene nanoribbons with 4 zigzag chains doped by a Beryllium atom on one edge. The spin dependent density functional theory with norm-conserving atomic basis set is employed to describe the system and the current versus voltage behavior is calculated by the nonequilibrium Green's function method for quantum transport. The Be impurity atom suppresses the local magnetization near the edge and the transmitted charge current becomes spin polarized accordingly. Both spin-up and spin-down transmission spectra are modified significantly but in different ways. Distinguished from the previous doping results of other impurity elements, here we observe negative differential resistance for only one of the spins in the nonlinear transport regime below bias 1.5 V. Molecular projected Hamiltonian energy spectrum near the impurity shows that the impurity removes the energy degeneracy of spin in perfect ribbon. The current versus voltage shows semiconductor behavior with fluctuating spin polarization of amplitude up to 37%.