Temperature-dependence of the chirality-induced spin selectivity effect-Experiments and theory.

The temperature-dependence of the chirality-induced spin selectivity (CISS) effect can be used to discriminate between different theoretical proposals for the mechanism of the CISS effect. Here, we briefly review key experimental results and discuss the effect of temperature in different models for the CISS effect. We then focus on the recently suggested spinterface mechanism and describe the different possible effects temperature can have within this model. Finally, we analyze in detail recent experimental results presented in the work of Qian et al. [Nature 606, 902-908 (2022)] and demonstrate that, contrary to the original interpretation by the authors, these data actually indicate that the CISS effect increases with decreasing temperature. Finally, we show how the spinterface model can accurately reproduce these experimental results.

Emergence and Dynamical Stability of a Charge Time-Crystal in a Current-Carrying Quantum Dot Simulator.

Periodically driven open quantum systems that never thermalize exhibit a discrete time-crystal behavior, a nonequilibrium quantum phenomenon that has shown promise in quantum information processing applications. Measurements of time-crystallinity are currently limited to (magneto-) optical experiments in atom-cavity systems and spin-systems making it an indirect measurement. We theoretically show that time-crystallinity can be measured directly in the charge-current from a spin-less Hubbard ladder, which can be simulated on a quantum-dot array. We demonstrate that one can dynamically tune the system out and then back on a time-crystal phase, proving its robustness against external forcings. These findings motivate further theoretical and experimental efforts to simulate the time-crystal phenomena in current-carrying nanoscale systems.

Distinguishing Thermal from Nonthermal ("Hot") Carriers in Illuminated Molecular Junctions.

The search for the signature of nonthermal (so-called "hot") electrons in illuminated plasmonic nanostructures requires detailed understanding of the nonequilibrium electron distribution under illumination, as well as a careful design of the experimental system employed to distinguish nonthermal electrons from thermal ones. Here, we provide a theory for using plasmonic molecular junctions to achieve this goal. We show how nonthermal electrons can be measured directly and separately from the unavoidable thermal response and discuss the relevance of our theory to recent experiments.

Spinterface Origin for the Chirality-Induced Spin-Selectivity Effect.

When electrons are injected through a chiral molecule, the resulting current may become spin polarized. This effect, known as the chirality-induced spin-selectivity (CISS) effect, has been suggested to emerge due to the interplay between spin-orbit interactions and the chirality within the molecule. However, such explanations require unrealistically large values for the molecular spin-orbit interaction. Here, we present a theory for the CISS effect based on the interplay between spin-orbit interactions in the electrode, the chirality of the molecule (which induces a solenoid field), and spin-transfer torque at the molecule-electrode interface. Using a mean-field calculation with simple models for the molecular junction, we show that our phenomenological theory can qualitatively account for all key experimental observations, most importantly the magnitude of the CISS with realistic parameters. We also provide a set of predictions which can be readily tested experimentally.

Assistance of metal nanoparticles in photocatalysis - nothing more than a classical heat source.

In a recent paper, we derived a self-consistent theory of the steady-state electron distribution of a metal under continuous wave illumination which treats thermal and non-thermal effects on the same footing. Here, we re-derive the main analytical results of that study from very simple arguments, and draw a series of conclusions which contradict claims made in previous studies of the steady-state distribution. In particular, we show that the faster chemical reactions reported in many previous papers are extremely unlikely to originate from high energy non-thermal electrons. Instead, the faster reactions very likely originate from a purely thermal effect.

Size Matters: Cocatalyst Size Effect on Charge Transfer and Photocatalytic Activity.

Hybrid semiconductor-metallic nanostructures play an important role in a wide range of applications and are key components in photocatalysis. Here we reveal that the nature of a nanojunction formed between a semiconductor nanorod and metal nanoparticle is sensitive to the size of the metal component. This is reflected in the activity toward hydrogen production, emission quantum yields, and the efficiency of charge separation which is determined by transient absorption spectroscopy. A set of Ni decorated CdSe@CdS nanorods with different tip size were examined, and an optimal metal domain size of 5.2 nm was obtained. Remarkably, charge separation time constants were found to be nonvariant with metal tip size. It is proposed that electron transfer mechanism encompasses two consecutive but separate processes: slow charge migration along the rod toward the interface, followed by fast interface crossing of the electron from the semiconductor into the metal phase. The first migration step dominates the time constant for the charge separation process and is not affected by the metal size. The efficiency of charge separation on the other hand was found to be sensitive to metal size. It is suggested that Coulomb blockade charging energy and a size-dependent Schottky barrier contribute to the metal size effect on charge transfer probability across the semiconductor-metal nanojunction. These two opposing trends result in an optimal metal size domain for the cocatalyst. This work is expected to benefit a broad range of applications utilizing semiconductor-metal nanocomposites.

Photoconductance from Exciton Binding in Molecular Junctions.

We report on a theoretical analysis and experimental verification of a mechanism for photoconductance, the change in conductance upon illumination, in symmetric single-molecule junctions. We demonstrate that photoconductance at resonant illumination arises due to the Coulomb interaction between the electrons and holes in the molecular bridge, so-called exciton-binding. Using a scanning tunneling microscopy break junction technique, we measure the conductance histograms of perylene tetracarboxylic diimide (PTCDI) molecules attached to Au-electrodes, in the dark and under illumination, and show a significant and reversible change in conductance, as expected from the theory. Finally, we show how our description of the photoconductance leads to a simple design principle for enhancing the performance of molecular switches.

How Kondo-holes create intense nanoscale heavy-fermion hybridization disorder.

Replacing a magnetic atom by a spinless atom in a heavy-fermion compound generates a quantum state often referred to as a "Kondo-hole". No experimental imaging has been achieved of the atomic-scale electronic structure of a Kondo-hole, or of their destructive impact [Lawrence JM, et al. (1996) Phys Rev B 53:12559-12562] [Bauer ED, et al. (2011) Proc Natl Acad Sci. 108:6857-6861] on the hybridization process between conduction and localized electrons which generates the heavy-fermion state. Here we report visualization of the electronic structure at Kondo-holes created by substituting spinless thorium atoms for magnetic uranium atoms in the heavy-fermion system URu(2)Si(2). At each thorium atom, an electronic bound state is observed. Moreover, surrounding each thorium atom we find the unusual modulations of hybridization strength recently predicted to occur at Kondo-holes [Figgins J, Morr DK (2011) Phys Rev Lett 107:066401]. Then, by introducing the "hybridization gapmap" technique to heavy-fermion studies, we discover intense nanoscale heterogeneity of hybridization due to a combination of the randomness of Kondo-hole sites and the long-range nature of the hybridization oscillations. These observations provide direct insight into both the microscopic processes of heavy-fermion forming hybridization and the macroscopic effects of Kondo-hole doping.

Persistence of Correlations in Neurotransmitter Transport through the Synaptic Cleft.

The "quantum brain" proposal can revolutionize our understanding of cognition if proven valid. The core of the most common "quantum brain" mechanism is the appearance of correlated neuron triggering induced by quantum correlations between ions. In this work, we examine the preservation of the correlations created in the pre-synaptic neurons through the transfer of neurotransmitters across the synaptic cleft, a critical ingredient for the validity of the "quantum brain" hypothesis. We simulated the transport of two neurotransmitters at two different clefts, with the only assumption that they start simultaneously, and determined the difference in their first passage times. We show that in physiological conditions, the correlations are persistent even if the parameters of the two neurons are different.

Tunable thermal switching via DNA-based nano-devices.

DNA has a well-defined structural transition--the denaturation of its double-stranded form into two single strands--that strongly affects its thermal transport properties. We show that, according to a widely implemented model for DNA denaturation, one can engineer DNA 'heattronic' devices that have a rapidly increasing thermal conductance over a narrow temperature range across the denaturation transition (∼350 K). The origin of this rapid increase of conductance, or 'switching', is the softening of the lattice and suppression of nonlinear effects as the temperature crosses the transition temperature and DNA denatures. Most importantly, we demonstrate that DNA nano-junctions have a broad range of thermal tunability by varying the sequence and length, and exploiting the underlying nonlinear behavior. We discuss the role of disorder in the base sequence, as well as the relation to genomic DNA. These results set the basis for developing thermal devices out of materials with nonlinear structural dynamics, as well as understanding the underlying mechanisms of DNA denaturation.

Nature of the superconductor-insulator transition in disordered superconductors.

The interplay of superconductivity and disorder has intrigued scientists for several decades. Disorder is expected to enhance the electrical resistance of a system, whereas superconductivity is associated with a zero-resistance state. Although superconductivity has been predicted to persist even in the presence of disorder, experiments performed on thin films have demonstrated a transition from a superconducting to an insulating state with increasing disorder or magnetic field. The nature of this transition is still under debate, and the subject has become even more relevant with the realization that high-transition-temperature (high-T(c)) superconductors are intrinsically disordered. Here we present numerical simulations of the superconductor-insulator transition in two-dimensional disordered superconductors, starting from a microscopic description that includes thermal phase fluctuations. We demonstrate explicitly that disorder leads to the formation of islands where the superconducting order is high. For weak disorder, or high electron density, increasing the magnetic field results in the eventual vanishing of the amplitude of the superconducting order parameter, thereby forming an insulating state. On the other hand, at lower electron densities or higher disorder, increasing the magnetic field suppresses the correlations between the phases of the superconducting order parameter in different islands, giving rise to a different type of superconductor-insulator transition. One of the important predictions of this work is that in the regime of high disorder, there are still superconducting islands in the sample, even on the insulating side of the transition. This result, which is consistent with experiments, explains the recently observed huge magneto-resistance peak in disordered thin films and may be relevant to the observation of 'the pseudogap phenomenon' in underdoped high-T(c) superconductors.

The effect of fluctuations, thermal and otherwise, on the temperature dependence of thermopower in aromatic chain single-molecule junctions.

We report a theoretical study of the thermopower of single-molecule junctions with focus on phenyl-based molecular junctions. In contrast to prior studies, thermal fluctuations of the torsional angle between the phenyl rings and variations in the position of the molecular level alignment with respect to the electrode Fermi energy were taken into account. Full thermopower histograms were obtained, and their dependence on the magnitude of the fluctuations was studied. We found that at large molecular orbital variations, the thermopower becomes strongly dependent on the torsion angle and can even change sign. This results in a marked effect of fluctuations on the thermopower distribution, yielding an average thermopower at high temperatures that differs (smaller or larger) from the fluctuation-free value, depending on the strength of fluctuations. We therefore conclude that fluctuations should be taken into account both when extracting single-molecule parameters, such as the molecular level-Fermi level offset, and in predictions of the thermopower of molecular junctions.

Dynamical coupling and negative differential resistance from interactions across the molecule-electrode interface in molecular junctions.

Negative differential resistance - a decrease in current with increasing bias voltage - is a counter-intuitive effect that is observed in various molecular junctions. Here, we present a novel mechanism that may be responsible for such an effect, based on strong Coulomb interaction between electrons in the molecule and electrons on the atoms closest to the molecule. The Coulomb interaction induces electron-hole binding across the molecule-electrode interface, resulting in a renormalized and enhanced molecule-electrode coupling. Using a self-consistent non-equilibrium Green's function approach, we show that the effective coupling is non-monotonic in bias voltage, leading to negative differential resistance. The model is in accord with recent experimental observations that showed a correlation between the negative differential resistance and the coupling strength. We provide detailed suggestions for experimental tests which may help to shed light on the origin of the negative differential resistance. Finally, we demonstrate that the interface Coulomb interaction affects not only the I-V curves but also the thermoelectric properties of molecular junctions.

Signature of Quantum Coherence in the Exciton Energy Pathways of the LH2 Photosynthetic Complex.

Unraveling the energy transfer pathways in photosynthetic complexes is an important step toward understanding their structure-function interplay. Here, we use an open quantum systems approach to investigate energy transfer within the LH2 photosynthetic apparatus and its dependence on environmental conditions. We find that energy transfer pathways strongly depend on the environment-induced dephasing time. A comparison between the computational results and experiments performed on similar systems demonstrates that quantum coherences are present in these systems under physiological conditions and have an important role in shaping the energy transfer pathways. Moreover, our calculations indicate that relatively simple spectroscopy experiments can be used to detect traces of quantum coherence. Finally, our results suggest that quantum coherence may play a role in photosynthesis, but not in enhancing the efficiency as was previously suggested.

Real-time monitoring of reaction stereochemistry through single-molecule observations of chirality-induced spin selectivity.

Stereochemistry has an essential role in organic synthesis, biological catalysis and physical processes. In situ chirality identification and asymmetric synthesis are non-trivial tasks, especially for single-molecule systems. However, going beyond the chiral characterization of a large number of molecules (which inevitably leads to ensemble averaging) is crucial for elucidating the different properties induced by the chiral nature of the molecules. Here we report direct monitoring of chirality variations during a Michael addition followed by proton transfer and keto-enol tautomerism in a single molecule. Taking advantage of the chirality-induced spin selectivity effect, continuous current measurements through a single-molecule junction revealed in situ chirality variations during the reaction. Chirality identification at a high sensitivity level provides a promising tool for the study of symmetry-breaking reactions and sheds light on the origin of the chirality-induced spin selectivity effect itself.

Spinterface chirality-induced spin selectivity effect in bio-molecules.

The chirality-induced spin selectivity (CISS) effect, namely the dependence of current through a chiral molecule on spin of the electron, was discovered over two decades ago, and has been suggested for various spin- and chirality-related applications. Yet, quite surprisingly, its physical origin remains elusive, and no theoretical description can quantitatively describe it. Here, we propose a theory for the CISS effect in bio-molecular junctions, based on the interplay between spin-orbit coupling in the electrodes, molecular chirality and spin-transfer torque across the electrode-molecule interface. This theory leads to the first ever quantitative analysis of experimental data, and provides insights into the origin of the CISS effect. The theory presented here can be used to analyze past experiments and to design new experiments, which may lead to deeper understanding of what is considered one of the outstanding problems in molecular electronics and nano-scale transport.

Photothermal nonlinearity in plasmon-assisted photocatalysis.

Understanding the intricate relationship between illumination and temperature in metallic nano-particles is crucial for elucidating the role of illumination in various physical processes which rely on plasmonic enhancement but are also sensitive to temperature. Recent studies have shown that the temperature rise in optically thick ensembles of metal nanoparticles under intense illumination is dominated by the thermal conductivity of the host, rather than by the optical properties of the metal or the host. Here, we show that the temperature dependence of the thermal conductivity of the host dominates the nonlinear photothermal response of these systems. In particular, this dependence typically causes the temperature rise to become strongly sublinear, reaching even several tens of percent. We then show that this effect can explain experimental observations in several recent plasmon-assisted photocatalysis experiments. Under certain conditions, we show that thermal emission may also contribute to photothermal nonlinearity. This shows that any claim for the dominance of non-thermal electrons in plasmon-assisted photocatalysis must account first for this photothermal nonlinear mechanism.

Hybridization wave as the "hidden order" in URu2Si2.

A phenomenological model for the "hidden order" transition in the heavy-Fermion material URu(2)Si(2) is introduced. The hidden order is identified as an incommensurate, momentum-carrying hybridization between the light hole band and the heavy electron band. This modulated hybridization appears after a Fano hybridization at higher temperatures takes place. We focus on the hybridization wave as the order parameter in URu(2)Si(2) and possibly other materials with similar band structures. The model is qualitatively consistent with numerous experimental results obtained from, e.g., neutron scattering and scanning tunneling microscopy. Specifically, we find a gaplike feature in the density of states and the appearance of features at an incommensurate vector Q(*)∼0.6π/a(0). Finally, the model allows us to make various predictions which are amenable to current experiments.

Theory of "Hot" Photoluminescence from Drude Metals.

We provide a complete quantitative theory for light emission from Drude metals under continuous wave illumination, based on our recently derived steady-state nonequilibrium electron distribution. We show that the electronic contribution to the emission exhibits a dependence on the emission frequency which is very similar to the energy dependence of the nonequilibrium distribution, and characterize different scenarios determining the measurable emission line shape. This enables the identification of experimentally relevant situations, where the emission lineshapes deviate significantly from predictions based on the standard theory (namely, on the photonic density of states), and enables the differentiation between cases where the emission scales with the metal object surface or with its volume. We also provide an analytic description (which is absent from the literature) of the (polynomial) dependence of the metal emission on the electric field, its dependence on the pump laser frequency, and its nontrivial exponential dependence on the electron temperature, both for the Stokes and anti-Stokes regimes. Our results imply that the emission does not originate from either Fermion statistics (due to e-e interactions), and even though one could have expected the emission to follow boson statistics due to involvement of photons (as in Planck's Black Body emission), it turns out that it deviates from that form as well. Finally, we resolve the arguments associated with the effects of electron and lattice temperatures on the emission, and which of them can be extracted from the anti-Stokes emission.

Do photosynthetic complexes use quantum coherence to increase their efficiency? Probably not.

Answering the titular question has become a central motivation in the field of quantum biology, ever since the idea was raised following a series of experiments demonstrating wave-like behavior in photosynthetic complexes. Here, we report a direct evaluation of the effect of quantum coherence on the efficiency of three natural complexes. An open quantum systems approach allows us to simultaneously identify their level of "quantumness" and efficiency, under natural physiological conditions. We show that these systems reside in a mixed quantum-classical regime, characterized by dephasing-assisted transport. Yet, we find that the change in efficiency at this regime is minute at best, implying that the presence of quantum coherence does not play a substantial role in enhancing efficiency. However, in this regime, efficiency is independent of any structural parameters, suggesting that evolution may have driven natural complexes to their parameter regime to "design" their structure for other uses.