Extensive Theoretical Efforts Research Articles

Magnon-mediated qubit coupling determined via dissipation measurements.

Controlled interaction between localized and delocalized solid-state spin systems offers a compelling platform for on-chip quantum information processing with quantum spintronics. Hybrid quantum systems (HQSs) of localized nitrogen-vacancy (NV) centers in diamond and delocalized magnon modes in ferrimagnets-systems with naturally commensurate energies-have recently attracted significant attention, especially for interconnecting isolated spin qubits at length-scales far beyond those set by the dipolar coupling. However, despite extensive theoretical efforts, there is a lack of experimental characterization of the magnon-mediated interaction between NV centers, which is necessary to develop such hybrid quantum architectures. Here, we experimentally determine the magnon-mediated NV-NV coupling from the magnon-induced self-energy of NV centers. Our results are quantitatively consistent with a model in which the NV center is coupled to magnons by dipolar interactions. This work provides a versatile tool to characterize HQSs in the absence of strong coupling, informing future efforts to engineer entangled solid-state systems.

Raman spectra investigation on the pressure-induced phase transition in titanium nitride (TiN)

Titanium nitride (TiN) exhibits extremely high wear resistance and corrosion resistance, leading to a broad range of applications. Despite the extensive theoretical efforts on the structural stability of TiN under pressure, a few experimental observations of the pressure-induced phase transition through x-ray diffraction are still in debate. Here, the Raman spectra of TiN under pressure are studied in a diamond anvil cell up to 36 GPa. The discontinuity in the pressure dependent phonon frequencies suggests a phase transition at around 5 GPa with the bonding between Ti and N hardly changing. The phase transition is ascribed to an isostructural transition or a vacancy filling mechanism.

Development of an analytical simulation framework for angle-resolved photoemission spectra

Angle-resolved photoemission spectroscopy (ARPES) is probably the most important experimental technique for the investigation of the electronic structure of solids. With respect to the interpretation of measured complex photoemission spectra, extensive theoretical efforts are often involved, as one needs a calculation of the band structure, a mapping of the band structure to the observed complex photoemission spectrum, and the inclusion of many-body effects. Typical methods that include the mentioned necessary steps for the interpretation are usually theoretically costly (such as time-dependent density functional theory). In contrast to purely numerical methods, analytical physical theories grant an easier approach in the implementation and, in general, allow for a more direct insight into the involved physical processes. Here, we show that combining a Keldysh Green's function formulation of the photoelectric current, together with a tight-binding parametrization of band structures, gives a powerful tool set for the simulation of ARPES spectra. Our approach is fast, capable of including many-body effects, and easy to be implemented and, therefore, from the viewpoint of an experimentalist, a valuable tool for the interpretation of photoemission spectra.

Evolution of Cooperation with Peer Punishment under Prospect Theory

Social dilemmas are among the most puzzling issues in the biological and social sciences. Extensive theoretical efforts have been made in various realms such as economics, biology, mathematics, and even physics to figure out solution mechanisms to the dilemma in recent decades. Although punishment is thought to be a key mechanism, evolutionary game theory has revealed that the simplest form of punishment called peer punishment is useless to solve the dilemma, since peer punishment itself is costly. In the literature, more complex types of punishment, such as pool punishment or institutional punishment, have been exploited as effective mechanisms. So far, mechanisms that enable peer punishment to function as a solution to the social dilemma remain unclear. In this paper, we propose a theoretical way for peer punishment to work as a solution mechanism for the dilemma by incorporating prospect theory into evolutionary game theory. Prospect theory models human beings as agents that estimate small probabilities and loss of profit as greater than they actually are; thus, those agents feel that punishments are more frequent and harsher than they really are. We show that this kind of cognitive distortion makes players decide to cooperate to avoid being punished and that the cooperative state achieved by this mechanism is globally stable as well as evolutionarily stable in a wide range of parameter values.

A Microscopic Model of the Electrochemical Vibrational Stark Effect: Understanding VSF Spectroscopy of (bi)Sulfate on Pt(111)

Much work over the past several decades has demonstrated that the vibrational frequencies of adsorbates on metal surfaces in ultra-high vacuum (UHV) shift with changing surface coverage. Extensive theoretical efforts have shown that this shift can be quantitatively understood as the result of dipole-dipole interaction. A similar phenomenon is known from spectro-electrochemistry (the so-called electrochemical Vibrational Stark Effect (VSE)): the vibrational frequency of the adsorbate is observed to change as a function of applied external potential. Typically this potential-dependent change in adsorbate frequency is assumed to be linear (or piece-wise linear) with applied bias and thus changes in slope are assigned to changes in adsorbate structure or chemical reactions. Here we measure the change in spectral response of the (bi)sulfate anion adsorbed on a Pt(111) working electrode with changes in applied bias and quantitatively interpret the results using a microscopic model of the VSE that explicitly accounts for the effect of dipole-dipole interaction. Using this model we show that changes in external bias that cause a linear change in molecular polarizability can induce a nonlinear change in the center frequency and intensity of the spectral response of the adsorbed (bi)sulfate: the nonlinearity of the VSE depends both on the local field at potential of zero charge and also the relative change induced by external bias. These results imply that essentially all spectroelectrochemical studies that have attributed chemical significance to changes in slope of spectral parameters with applied bias should be revisited, as potential dependent dipole-dipole interaction can induce such changes independent of any structural or chemical change.

Influence of the mesogenic shape on the molecular dynamics and phase-biaxiality of liquid crystal main-chain polymers

Ever since the first theoretical phase diagram that was published by Straley in 1974, extensive theoretical efforts have been made on predicting the molecular shape and properties of mesogens that can form a biaxial-nematic phase. The theories suggest that mesogens, whose form is board-like rather than rod-like, are more likely to form biaxial phases. In this study we show that the onset of phase-biaxiality for nematic main-chain polymers can indeed, without significantly changing the glass transition temperature, be shifted to considerably higher temperatures by laterally broadening the mesogenic moieties. The investigations were performed by means of 2H-NMR spectroscopy on directly deuterated polymers and yield results that differ considerably from what could hitherto be observed in spin-probe measurements on side-chain polymers with similarly broadened mesogens.