Triangular magnetic structures have gained considerable interest due to their rich magnetic behavior and structural simplicity. These structures contain the motif of a triangle as the main structural feature, leading to geometric frustration and implicitly to degenerate magnetic ground states. Most of the previous work on triangular lattice structures was performed on simple transition metal halides or oxides. Therefore, it presents an interesting challenge for materials scientists to synthesize new class of materials that preserve the quasi-two dimensionality of the structures. This talk will feature two class of materials (1) triangular materials synthesized using high-pressure hydrothermal method (2) AREQ2 (A = Alkali metal, RE= rare earth, Q = O, S, Se) triangular magnetic materials. 

First, I will focus on synthesis, magnetism and use of neutron diffraction to characterize the magnetic phase diagram of several classes of hydrothermally synthesized oxy-anions based transition metal compound series (EOyx-, E = As, Mo, Se). These linking groups can lead to an enormous array of new structure types with great potential for exploring and characterizing new emergent phenomena. Here, I will focus the role of vanadate building blocks (VO43-) in magnetically interesting transition metal layered materials. The vanadates display a rich diversity of structural behavior including multiple bridging modes such as corner and edge sharing. In addition, the presence of vacant d-orbitals in the bridging center can have a significant effect on the magnetic coupling behavior. As the first example, SrM(VO4)(OH) (Mn, Co, Ni) possess on-dimensional magnetic lattice with totally different magnetic properties depending on the transition metal cation even though they crystallizes in same space group. Further, Na2BaM(VO4)2 (M = Mn2+, Fe2+, Co2+) series all have similar chemical structures and are members of the glaserite family, but each one displays dramatically different magnetic behavior between room temperature and 2 K. Another interesting system for discussion is the mixed vanadate carbonate material A2M3(VO4)2(CO3) where A = K, Rb and M = Mn2+, Co2+. The chemical structure is quite complex and has two unique layers, one built of corner sharing vanadates and one with the trigonal planar carbonates. The material also has a complex magnetic behavior and undergoes three magnetic phase transitions between 300-2 K. 

In the second part of my talk, I will focus on the synthesis of ARESe2 single crystal growth, magnetic properties and elastic and inelastic neutron scattering. These crystals crystallize in either trigonal (R-3m) or hexagonal (P63/mmc) crystal systems and associate with an ideal triangular RE3+ layers. The magnetic properties and heat capacity of these compounds were characterized down to 0.4 K. The Yb-compounds exhibit a broader peak in heat capacity ~10 K suggesting short range ordering. 

DNA is a new generation material for molecular data storage, with a potential storage capacity several orders of magnitude greater than current methods. Data stored in DNA can be encoded (written) and decoded (read) using sequencing technologies. Advantages of DNA data storage are (i) high data density, (ii) high stability, (iii) ease of copy, and (iv) low energy. Current methods rely on slow, expensive, complicated synthesis of long DNA to write, followed by costly, high error rate (10%) reads. We overcome these challenges with a low-cost, enzyme-free, mix-and-detect method for high fidelity DNA data reading, writing, and rewriting using nanopore technology and universal rewritable blank medium DNA without the need for nucleic acid synthesis. This technology has applications not only in DNA data storage, but also in DNA barcoding for high throughput screening of nucleic acid secondary structure and drug/ligand binding.

Claudio Mazzoli will present some opportunities of scientific investigation by micro-spectro-scattering in the soft X-ray regime of Resonant X-ray Scattering, as implemented and developed at CSX (the Coherent Soft X-ray scattering beamline of NSLS-II, BNL). The peculiarity of such an integrated approach in terms of space, time, energy scales, and correlations, allows revealing unique properties of materials from the microscopic point of view, and shining new light on a variety of interesting cases. Electronic orderings, inhomogeneities, self-organization, collective dynamics and interplay of degrees of freedom will be presented, together with some future ideas.

Metallic systems with magnetic ions embedded which have been prepared to undergo a second-order phase transition at zero Kelvin, namely the quantum critical systems, historically appear to fall into two distinct categories: (chemically) heavily-doped systems in which the unusual properties can be attributed to a disorder-induced distribution of Kondo shielding temperatures and (nearly) stoichiometric systems where the departures from Fermi-liquid theory have been attributed to intrinsic instabilities. We show that this historic distinction between doped and stoichiometric systems should be left to history: we find that magnetic clusters associated with a distribution of Kondo shielding temperatures found in heavily-doped quantum critical Ce(Fe0.755Ru0.245)2Ge2 are also present in CeRu2Si2, a stoichiometric system close to a quantum critical point. In both the doped and stoichiometric system, the response of these clusters that emerge upon cooling dominates the macroscopic response of these systems. This implies that the dominant physics that drives heavily doped systems, namely spontaneous formation of magnetic clusters, should also play a leading role in the response of homogeneous systems. This represents a notable departure of how the physics that governs quantum critical points has been described in the literature and it might even point the way towards a magnetic pairing mechanism in high Tc superconductors.

Convolutional neural network (CNN) and graph convolutional network (GCN) has gained huge success in various tasks, from image classification, video processing to speech recognition and natural language understanding. The success stems from both the well-designed neural network architecture and the increasing computing power in recent hardware. Many attempts have been made to extend these frameworks to biological problems, with varying success. In this talk, I will present the applications of using both CNN and GCN models to predict Mg2+/small molecule binding sites/modes in RNA molecules. These approaches exploit the information of the local binding environment and predict the most probable distribution of the Mg2+ sites or ligand binding modes. Further comparisons between our methods and various types of methods validate the machine-learning approaches.  

While ferromagnets have been known and exploited for millennia, antiferromagnets were only discovered in the 1930s. The elusive nature indicates antiferromagnets’ unique properties: At large scale, due to the absence of global magnetization, antiferromagnets may appear to behave like any non-magnetic material; At the microscopic level, however, the opposite alignment of spins forms a rich internal structure. In topological antiferromagnets, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. We study this exciting possibility in an antiferromagnetic Axion insulator, even-layered MnBi2Te4 flakes. We report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions.

Reference:

A. Gao, et al.  “Layer Hall effect in a 2D topological axion antiferromagnet.” Nature 595, 521 (2021).

In one spatial dimension, enhanced thermal and quantum fluctuations should preclude the existence of any long range ordered superfluid phase of matter.  Instead, the quantum liquid should be described at low energies by an emergent hydrodynamic framework known as Tomonaga-Luttinger liquid theory.  In this talk I will present details on some complimentary experimental and theoretical searches for this behavior in helium-4 including: (1) pressure driven superflow through nanopores, and (2) the excitation spectrum of a confined superfluid inside nano-engineered porous silica-based structures. For flow experiments, we have devised a framework that is able to quantitatively describe dissipation at the nanoscale leading to predictions for the critical velocity borne out by recent superflow measurements in nanopores.  In confined porous media, with radii reduced via pre-plating with rare gases, I will discuss ab initio simulations of phase and density correlations inside the pore that are in agreement with recent neutron scattering measurements.   Taken together, these results indicate significant progress towards the experimental observation of a truly one-dimensional quantum liquid.

This work was supported by the NSF through grants DMR-1809027 and DMR-1808440.  

Materials manipulation via ion or laser beams can achieve precisely tuned atomic geometries that are necessary, e.g. to engineer interactions between defects in quantum materials and for fabricating novel electronic devices with nanoscale dimensions. In addition, such beams are also used to characterize and probe materials properties by means of electronic and optical excitations. I will discuss recent quantum- mechanical first-principles predictions for electron dynamics and the subsequent ionic motion that follows after an excitation of the electronic system. Using real-time time-dependent density functional theory we simulated the underlying ultrafast time scales of electron dynamics in semiconductors and metals. Examples include long-lived electronic excitations in proton, electron, and laser irradiated bulk semiconductors that facilitate diffusion of point defects, such as oxygen vacancies in MgO. We compare such bulk simulations to aluminum surfaces under irradiation, for which we quantify electron emission, charge capture, and pre-equilibrium effects that are unique to thin films or two-dimensional materials. Limitations and possible extensions of the theoretical description will be included in the discussion.

Spintronics-based technology, which uses spins to represent and propagate information, holds promise to realize devices that surpass the current CMOS transistor technology in power, density and speed. For example, magnetic random-access memory (MRAM) based on magnetic tunnel junctions were identified as promising non-volatile memory but its use has been limited. A second generation MRAM-based on spin transfer torque has reduced currents. However, next generation MRAM based on pure spin currents may provide even more energy efficiency. My research is focused on developing power-efficient ways to generate, propagate and manipulate spins via pure spin currents. In order to develop such pure spin current technologies, the development of new materials such as topological insulators must come hand in hand with the development of new devices. In this talk, I will discuss (i) low damping ferromagnetic insulating thin films for achieving efficient spin current generation in spintronic devices, (ii) spin current generation in these films and large spin-charge interconversion in neighboring layers, (iii) spin interactions in ferromagnetic insulator/topological insulator heterostructures. Together these results lay the foundation for new energy-efficient pure spin current-based electronics.

Reference:

1. Li, P. et al. Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator. Nano Lett. 21, 1, 84 (2021).

2. Li, P. et al. Switching magnetization utilizing topological surface state. Science Advances 5, eaaw3415 (2019).

In this talk, I will discuss rich topological behavior in two related models – the Majorana wire and a Su-Schrieffer-Heeger ladder- in the presence of potential energy landscapes. An introduction of the two models and of techniques that directly provide information on edge-state properties will form the starting point for obtaining topological phase diagrams in these models. In the case of these systems subject to a quasiperiodic potential, a beautiful topological phase diagram emerges mimicking Hofstadter’s butterfly patterns. In the case of disordered potential landscapes, Anderson localization physics informs the behavior of the disordered topological phase diagram.  Finally, I will discuss the possible implementation of this physics in a variety of experimental systems, including solid state, cold atomic and electro-mechanical settings. 

Amorphous oxide semiconductors (AOS)—ternary or quaternary oxides of post-transition metals—have attracted a lot of attention due to high carrier mobility which is an order of magnitude larger than that of amorphous silicon (a-Si:H). Unlike Si-based semiconductors, AOS exhibit optical, electrical, thermal, and mechanical properties that are comparable or even superior to those possessed by their crystalline counterparts. However, the properties of AOS are extremely sensitive to deposition conditions, oxygen stoichiometry, and metal composition, rendering the available research data inconsistent or hard to reproduce, thus, hampering further progress. Moreover, owing to the weak metal-oxygen bonding as well as many degrees of freedom in disordered materials, defects in AOS have the structural, thermal, and electronic characteristics that differ fundamentally from those in the crystalline transparent conducting oxides.

To navigate the large parameter space for AOS materials, computationally-intensive ab-initio Molecular Dynamics simulations followed by comprehensive structural analysis and accurate Density-Functional calculations, are performed for several AOS families. Integrated with systematic experimental measurements, the results provide microscopic understanding of complex relationships between the morphology, carrier generation, and electron transport across the crystalline-amorphous transition and help derive versatile design principles for next-generation transparent amorphous semiconductors with a combination of properties not achievable in Si-based architectures.