Session MM04: V: General Physics III

Signatures of ferromagnetic order and spin-triplet pairing state in two-dimensional niobium diselenide

The co-existence of ferromagnetism and superconductivity becomes possible through unconventional superconducting pairing state. Though ferromagnetic superconductivity has been reported in heavy fermions, ZrZn₂, and rare-earth and hydroxide compounds, the nature of Cooper pairs for coupling the two competing orders is still under debate. Here, we investigate the role of disorder on atomically thin niobium diselenide (NbSe₂) intercalated with dilute cobalt (Co) atoms and show that such systems spontaneously display ferromagnetism below the superconducting transition temperature (T_C). We elucidate the origin of the superconductivity-triggered ferromagnetism by characterizing the tunnelling magnetoresistance (TMR) via vertical magnetic tunnel junctions (Co/BN/Co-doped NbSe₂), which shows a bistable state below T_C and deep inside the superconducting gap, suggesting a Ruderman–Kittel–Kasuya–Yosida (RKKY)-induced ferromagnetism mediated by Cooper pairs. We then performed non-local spin valve measurements to directly resolve the pairing state in the undoped NbSe₂ channel at 50 mK, which shows unambiguous Hanle precession signals with estimated spin diffusion length up to micrometre, indicating the presence of intrinsic spin-triplet state in NbSe₂ and consequently, the crucial role of triplet Cooper pairs for generating a ferromagnetic order in superconducting condensate. Our discovery opens the door for engineered ferromagnetic superconductors in 2D form.   

Motivating semiclassical gravity: a classical-quantum approximation for bipartite quantum systems

We derive a "classical-quantum" approximation scheme for a broad class of bipartite quantum systems from fully quantum dynamics. In this approximation, one subsystem evolves via classical equations of motion with quantum corrections and the other subsystem evolves quantum mechanically with equations of motion informed by the evolving classical degrees of freedom. Using perturbation theory, we derive an estimate for the growth rate of entanglement of the subsystems and deduce a "scrambling time" - the time required for the subsystems to become significantly entangled from an initial product state. We argue that a necessary condition for the validity of the classical-quantum approximation is consistency of initial data with the generalized Bohr correspondence principle. We illustrate the general formalism by numerically studying the fully quantum, fully classical, and classical-quantum dynamics of a system of two oscillators with nonlinear coupling. This system exhibits parametric resonance, and we show that quantum effects quench parametric resonance at late times. Lastly, we present a curious late-time scaling relation between the average value of the von Neumann entanglement of the interacting oscillator system and its total energy: S ∼ 2/3(ln E).   

Influence of temperature on the light emission response in electrically polarized graphene oxide films

The influence of temperature on the light emission response in electrically polarized graphene oxide (GO) samples, is presented here. GO films were synthesized by employing the double thermal decomposition (DTD) method. Compositional, vibrational, morphological, and electrical properties were studied. Results revealed that as the temperature changes, a blueshift in the emission spectrum was identified, as expected for a semiconductor material. The light emission mechanism was described mainly by thermal emission and temperature dependence was studied. The results suggest that GO films are an attractive material to develop advanced electronics light emitting devices.   

Can the public’s trust in science - and scientists - be restored?

The title of this talk quotes the title of an interview conducted by the University of Rochester  (UR) News Center posted on-line on July 20, 2022 [1]. Ironically, that was two months after the UR Vice President for Research determined, after conducting an inquiry,  that there was nothing wrong with data presented in a 2020 UR paper claiming the discovery of room temperature superconductivity [2], contrary to what had been reported in refereed journals [3] and private communications to UR. The paper in question [2] was retracted by the journal two months later, and an investigation is currently under way [4]. Universities, scientific journals, and funding agencies play an important role in contributing to the public’s trust in science. Trust is enhanced when these institutions enforce supporting data availability provisions they have on their books, and take seriously reports of evidence of scientific misconduct. I will discuss several examples in the last 3 years where institutions, journals and funding agencies acted in ways that were deleterious to the public’s trust in science and scientists, in connection with claims of high temperature superconductivity in hydrides under pressure.

 

[1] University of Rochester News Center, July 20, 2022.

[2] E. Snider et al, Nature 586, 373–377 (2020).

[3] J. E. Hirsch and D. van der Marel, Matter Radiat. Extremes 7, 048401 (2022).

[4] Edwin Cartlidge, “Why a blockbuster superconductivity claim met a wall of scepticism”, Nature News, September 6, 2023.

    

STEM60+ - STEM Education for Older Adults

This presentation discusses the outcome of the first STEM60+ workshop held in September 2023, which was motivated by the belief that STEM education for older adults has great societal and personal benefits, yet present offerings are minimal. Experts and decision-makers from different disciplines met to develop strategies to increase STEM education efforts for mature adults with the goal of a more highly educated older population. Reflecting the diversity of the workshop, the presentation will provide social, health, and educational perspectives on the issue and address potential future pathways.   

X-RAY FLUORESCENCE IMAGING OF METAL ACCUMULATIONS IN BRAIN: A LOOK INTO DISEASED BRAIN

Disruption in homeostasis of metals present inside the brain are implicated in the etiologies of several neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease. Hence, acquiring knowledge on spatial distribution and concentration of these metals within the brain paves a way to better understand the mechanisms of these diseases. The techniques that have been utilized for decades such as imaging followed by immunohistochemical staining of tissue, auto-metallography, mass spectroscopy either alter the concentration of metals from tissue fixation or lack spatial resolution 

Here we use synchrotron X-ray fluorescence spectroscopy on unfixed tissue sections to measure concentrations of multiple metals of interest both at tissue and cellular level within the brains.Our results indicate that mercury (Hg), a highly toxic heavy metal is localized within the choroid plexus and lateral cerebral ventricular wall of our animal model (wildtype small Indian mongoose). We were further able to locate that astrocytes preferentially accumulate this metal, this metal colocalizes almost to a full extent with selenium, indicating a Se-based detoxification mechanism of Hg. We were also able to localize manganese (Mn) to purkinje cellular layer in the cerebellum of SLC39A14 (a manganese transporter gene) - knocked out model of mice thereby reporting that in addition to the previously reported periventricular zone, cells from cerebellum play a major role in the etiology of Parkinson's disease.   

Transient analysis of energy harvesting from piezoelectric ceramic transducers under impulse mechanical excitation

Self-powered flexible and wearable sensors that conform to the human body are the futuristic trend in providing point-of-care health services. Body movements and resulting mechanical vibrations serve as one of the most reliable sources to power such a sensor. To this end, piezoelectric transducers provide efficient energy harvesting from mechanical vibrations. As such, this study is designed to investigate the transient response of energy harvesting from piezoelectric ceramic transducers for impulse excitation. A metal ball of 151 g falling from a height of 50 cm exerted 0.472 Ns of impulse excitation onto circular diaphragm piezoelectric transducers of different diameters. The generated voltage across a shunt load was measured using a benchtop digital voltmeter. An average of 7 V of energy was generated from a 50 mm diameter piezo transducer. Transient analysis was carried out with both shunt capacitors and resistors. The generated output voltage was critically damped within ~ 1100 ms for the shunt capacitor and over-damped within ~150 ms for the shunt resistor. Moreover, connecting multiple piezo transducers in parallel resulted in a higher output voltage of 14 V than the series connection of piezo transducers. The variation is mainly attributed to random changes in the polarity of the generated voltages. The results suggest that full wave rectification with a shunt capacitor would enhance the electrical energy extraction from impulse mechanical excitation.   

Can increasing the size and flexibility of a molecule reduce decoherence?

Coherent superposition of electronic states, created by ionizing a molecule, can initiate ultrafast dynamics of the electron density. Correlation between nuclear and electron motions, however, typically dissipates the electronic coherence in only a few femtoseconds, especially in larger and more flexible molecules. We, therefore, use ab initio semiclassical dynamics to study decoherence in a sequence of organic molecules of increasing size and find, surprisingly, that extending the carbon skeleton in propynal analogs slows down decoherence and extends the duration of charge migration. To elucidate this observation, we decompose the overall decoherence into contributions from individual vibrational modes and show that: (1) The initial decay of electronic coherence is caused by high- and intermediate-frequency vibrations via momentum separation of nuclear wavepackets evolving on different electronic surfaces. (2) At later times, the coherence disappears completely due to the increasing position separation in the ow-frequency modes. (3) In agreement with another study, we observe that only normal modes preserving the molecule's symmetry induce decoherence. All together, we justify the enhanced charge migration by a combination of increased hole-mixing and the disappearance of decoherence contributions from specific vibrational modes: CO stretching in butynal and H rockings in pentynal.