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Quantum materials with diversity of experimental techniques

 NQR Study of Spin Dynamics (Karen Sauer)

General and fundamental information necessary for spintronic and neuromorphic device engineering can be obtained by an incredibly powerful but underutilized technique: zero-field nuclear magnetic resonance (NMR). The nuclei in the quantum material provide a microscopic probe of the local magnetic and electric fields, through the magnetic Zeeman interaction and the electric quadrupole interaction. The intrinsic electromagnetic fields create energy splitting, as well as energy level broadening, which can be observed through a type of radio-frequency spectroscopy known as NMR. These fields are dominated by the electron distribution and polarization, and are quite sensitive to phase changes, structural polymorphism, defects, and strain. NMR, therefore, is poised to complement efforts to probe the structural polymorphism and spintronic potential of MoTe2-WTe2 alloys.

For example, relaxation time (time to return to thermal equilibrium after perturbation) is affected by electron-electron interaction and has a strong dependence on the magnetic susceptibilities of the conduction electrons. Such T1 measurements therefore provide an excellent probe of the spin dynamics of the system. Traditionally, NMR is a bulk technique, but recent theoretical and experimental work show that it can be extended to nanocrystals as well as 2D materials. Using NMR, we propose to study the phase transitions and spin dynamics of MoTe2-WTe2 alloys; other material of interest to the QMC include chromium iodide (CrI3) and strontium titanate (SrTiO3). This work will be guided by DFT calculations of the local electric field gradient, a necessary step to identify the radio frequencies associated with each material.


Solid State Quantum Memory in Rare-Earth Ions Ensemble Trapped in Crystals (Ming Tian)

Quantum memory that faithfully stores and retrieves quantum states is in high demand for many applications. The requirements for a practical quantum memory can be summarized as having high efficiency and fidelity, long storage time, high bandwidth/bit rate, and multimode capacity. The most viable setting for quantum networking and long-distance quantum communication is to use photons as the flying information carrier and atoms as the stationary storage medium. Therefore, realization of quantum memory relies on faithfully mapping photonic states onto atomic states and reversing the process at a later time.

Quantum memory based on atomic frequency comb (AFC) have shown promise for superior performance, such as memory efficiency up to 100 percent, high conditional fidelity not affected by decoherence, long storage time (up to seconds), a large time-bandwidth product for high bit rates up to gigabit/s, multimode capacity, large range of operational wavelengths, and being completely solid state with relatively simple setup using off-the-shelf components.

An AFC is formed by arranging an ensemble of atoms according to their optical resonant frequency in a comb pattern (shown in Figure 1a). A photon represented by a wave packet interacts with the AFC mapping the quantum state of the flying qubit on to the atomic state of the ensemble. Collective atomic state can be converted into a photon in a later time. AFC protocol relies on two steps. First is to create the comb shape through optical pumping. The second is the storage and retrieval through a photon echo process. The main challenge of AFC-based quantum memory is to improve the memory efficiency and fidelity; both depend on the spectral distribution of the atomic ensemble across the AFC. We have worked on experiment and theory to create an AFC and study how its characteristics affect the efficiency and fidelity. Figure 2 shows an example of the efficiency changes with the amplitude and the background in the AFC’s spectrum caused by the population relaxation among the three atomic energy levels of thulium ions.


Structural Transitions and Spintronics in 2D Materials (Patrick Vora)

While our machine learning algorithms hunt for the next generation of emergent quantum materials, we will hone our multi-vector experimental approach by studying MoTe2–WTe2 alloys. These materials are composed of two-dimensional (2D) layers stacked upon each other and are a subset of transition metal dichalcogenides. A wide range of unconventional and computationally promising quantum phenomena are realizable in these alloys. At the heart of this versatility is the intrinsic structural polymorphism of the MoTe2-WTe2 alloy system, which we illustrate in the Figure here.

Structural polymorphism is a behavior where a material may crystallize in more than one configuration depending on the environmental conditions. In MoTe2-WTe2 alloys, three such phases are accessible, each with its own unique electronic, optical, and magnetic properties. The first identified configuration was a layered hexagonal crystal structure referred to as 2H and was found to host a semiconducting electronic state with robust spintronic properties that can be controlled optically and electrically. By alloying MoTe2 with WTe2, two additional structural phases are accessible.

The first is 1T’, a distorted version of 2H that is semimetallic and hosts robust spin states. This 1T’ phase is promising for neuromorphic computation as heat, charge, and strain induced switching between the 1T’ and 2H phases are possible. The second phase is very similar to the 1T’ phase, indeed isostructural at the monolayer level, but involves a different alignment of layers in the crystal structure. This shift in layer alignment breaks inversion symmetry and results in a topologically protected band structure as well as topologically protected spin states. Referred to as the Td phase, it is one of the most promising for realizing computational platforms based on topologically non-trivial physics.

Clearly, the versatility of MoTe2-WTe2 alloys is promising for making progress on numerous computational fronts. The material itself is also quite fresh to the community, with publications on the phase diagram only appearing this year by Patrick Vora’s group. Below we delineate how we will use the powerful capabilities of our collaborative skill set to understand this complex alloy system under a range of conditions. The result of this five-year effort will be a complete understanding of the electronic, optical, and magnetic properties of the MoTe2-WTe2 alloy systems as a function of layer number, as well as their incorporation into novel device heterostructures.

   > Structural, Electronic, and Topological Phase Transitions

While it is possible to trigger structural phase changes between the 2H and 1T’ phases in MoTe2, alloying with WTe2 potentially reduces the energy barrier between these phases and thereby also reduces the power required to trigger a phase change. The phase diagram provides insight into what temperature and W composition are needed to achieve either the 2H, 1T’, or Td phases, but what is missing for the practical realization of devices is an understanding of how to trigger these transitions dynamically. Resistivity and optical studies of these alloys under various stimuli (strain, temperature, magnetic field) can provide substantial insight into the kinetics and energetics that drive phase transitions between 2H and 1T’/Td phases.

Using our combined expertise in characterization (Karen Sauer, Ming Tian, Patrick Vora), strain and nanophotonic engineering (Pilgyu Kang, Patrick Vora), and electronic device engineering (Dimitrios Ioannou, Qiliang Li, Pilgyu Kang), we will explore dynamic phase transitions between the different crystal phases of MoTe2–WTe2 alloys. We will implement a combined approach to investigate the transitions through these various crystallographic phases.

We have already undertaken optical studies in our collaboration to explore the topological phase transition known to occur in MoTe2 from the 1T’ phase to the Td phase upon warming above 250 K. We will further explore this transition using temperature-dependent magnetoresistance studies to understand how the topological transition is affected by magnetic fields. These studies will be carried out as a function of alloy composition and flake thickness. Understanding the topological transition will provide vital information needed for the future planning of topologically based computing technologies.

   > Electronic, Optical, and Mechanical Control of Spin-Valley Properties

Additional potential for spintronic manipulation exists in the form of a linked valleytronic-spintronic interface. This capability is inherent to TMDs, where the combination of broken inversion symmetry in monolayer TMDs along with the broken sublattice symmetry leads to extremely strong optical transitions that are spin polarized and valley polarized. This provides optical and electronic methods for the initialization and manipulation of the spin-valley states in 2D materials. We will explore the spin-valley properties of the MoTe2-WTe2 alloys and how they can be dynamically controlled using optical, electronic, and mechanical techniques. The alloy composition dependence is thought to be potentially valuable as disorder can in some ways suppress spin-valley depolarization, and therefore make the quantum spin/valley states more robust.

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