Professors Jain and Sochnikov received NSF research grant entitled “New Quantum Elastocaloric Demagnetization Refrigeration for the Millikelvin Range”. A major focus of their research will be the cooling of quantum chips. For this purpose, their teams will study ‘spin liquids’, which can be harnessed to achieve millikelvin temperatures without magnetic fields. At such low temperatures, […]
Associate Professor of Physics Menka Jain and the Institute of Materials Science is co-organizing a workshop-28th International Workshop on Oxide Electronics (IWOE) in Maine next month. The IWOE series has become an important venue to discuss recent advances and emerging trends in this developing field. The aim of the workshop is to provide an interdisciplinary […]
UConn’s collaboration with the Department of Defense Air Force Research Laboratory (AFRL) is launching a new project. It is titled Multiscale Modeling and Characterization of Metamaterials, Functional Ceramics and Photonics. This is a $4.7 M contract with $1M for Physics. The project’s goal is to explore and advance the understanding of electronic, photonic, magnetic, and […]
Most superconductors only work when they’re super cold. Chemists and metallurgists have experimented with different combinations of elements for years, trying to get superconductors that work at temperatures close to room temperature. Sochnikov and his students are thinking about it differently. What if mechanical changes such as squeezing or stretching could make a material a superconductor?
Physicists used to think that superconductivity – electricity flowing without resistance or loss – was an all or nothing phenomenon. But new evidence suggests that it’s not so clear cut, at least in copper oxide superconductors. “If we understood why copper oxide is a superconductor at such high temperatures, we might be able to synthesize a better one”, says UConn physicist Ilya Sochnikov. Sochnikov and his colleagues at Rice University, Brookhaven National Lab and Yale recently figured out part of that puzzle, and they report their results in the latest issue of Nature.
PhD student Lukasz Kuna and undergraduate Hope Whitelock participate in a symposium “Mesoscopic phenomena in ceramic materials” arranged by Materials Scientist Serge Nakhmanson at the “Electronic and Advanced Materials Conference” in Orlando, Florida. Four UConn students including Tulsi Patel, Krishna Chaitanya Pitike, Lukasz Kuna and Hope Whitlock showcased their research.
Whoever said rules were made to be broken wasn’t a physicist. When something doesn’t act the way you think it should, either the rules are wrong, or there’s new physics to be discovered. Which is exactly what UConn’s Connor Occhialini ’18 (CLAS), an honors student majoring in physics and math, found when he began researching scandium fluoride. Most materials swell as they heat up. Scandium fluoride must be doing something else, reasoned Occhialini. […]
John Mangeri’s Award Lands Him in Argonne National Laboratory John Mangeri (left) with his SCGSR-award host Dr. Olle Heinonen (right) in front of the Chemistry building (bldg. 200) at Argonne National Laboratory. (Photo credit to Dr. Andrea Jokisaari) By Katherine Eastman John Mangeri, a Ph.D. candidate in Dr. Serge Nakhmanson’s “Complex Materials by Computational Design” […]
Dr. Sochnikov is a recipient of Montana Instruments Cold Science Exploration Awards Lab Startup Grant. Dr. Ilya Sochnikov has just started new scanning SQUID microscopy lab at the University of Connecticut. Ilya Sochnikov’s research focuses on nanoscale quantum phenomena in new materials. An emergence of a new phenomenon or a phase transition occurs when interactions […]
October 6, 2015 – Kim Krieger – UConn Communications Jason Hancock, Assistant Professor in Physics, with graduate students, Erin Curry and Sahan Handunkanda, have been investigating a substance that shrinks when it warms. Most materials swell when they warm, and shrink when they cool. But UConn physicist Jason Hancock has been investigating a substance that responds in reverse: […]
March 26, 2015 – Tim Miller Researchers have made an experimental breakthrough in explaining a rare property of an exotic magnetic material, potentially opening a path to a host of new technologies. From information storage to magnetic refrigeration, many of tomorrow’s most promising innovations rely on sophisticated magnetic materials, and this discovery opens the door to harnessing […]
Electronic pseudogap from fluctuations in low dimensional materials
Most metal-to-insulator and metal-to-superconductor transitions are so dramatic that certain symmetries are also concurrently broken, and an energy gap opens in lockstep with this process. But there are electronic systems that develop energy gaps without any broken symmetry, most notably the “pseudogap” in cuprate superconductors. In this talk, I will show two examples of electronic pseudogap in unexpected places: the heavily hole-doped cuprates [1], and an excitonic insulator candidate Ta2NiSe5 [2]. The former is supposedly a good metal where mean-field BCS is thought to apply, and the latter is a structural symmetry-breaking system with strong electron-phonon coupling. Via angle-resolved photoemission spectroscopy and x-ray scattering, we show the electronic gap to persist well above the transition temperature in both systems. With insights from controlled numerical calculations, we show that fluctuation is an important factor when describing the properties of low dimensional material systems. Finally, I will discuss a few new directions in the study of fluctuations.
[1] Phys Rev X 11, 031068 (2021); Nat Mater 22, 671 (2023)
Dr. Shafique Adam, Washington University in St. Louis
A narrow magic window for ultraflat bands and emergent heavy fermions near the magic angle in twisted bilayer graphene
The notion of a single “magic angle” in twisted bilayer graphene has evolved into a fascinating array of magic angles and ranges each describing different facets of the material’s behavior. While the original continuum model predicted a nominal magic angle, its simplicity ignored the intricate interplay of different physical phenomena. For example, lattice relaxation [1] near the magic angle shifts its value upward, only to be counteracted by pseudomagnetic fields. Including a symmetry allowed relaxation parameter changes this magic angle to a magic range. Yet another magic angle emerges from the coupling to phonons when the Fermi velocity equals the phonon sound velocity. Building upon this rich tapestry of magical effects, we will discuss our recent work on the convergence of lattice relaxation and Hartree interaction near the magic angle [2]. We unveil a previously unreported Lifshitz transition to a Fermi surface topology that supports a “heavy fermion” pocket and an ultraflat band pinned to the Fermi energy. Analytical and numerical insights shed light on the narrow “magic angle range” where the “heavy fermion” is stable and make predictions for its experimental observation. We believe that the bands presented here are accurate at high temperature and provide a good starting point to understand the myriad of complex behavior observed in this system.
[1] “Analytical Model for Atomic Relaxation in Twisted Moiré Materials” by MMA Ezzi, GN Pallewela, C De Beule, EJ Mele, and S Adam, arXiv:2401.00498 (2024)
[2] “A self-consistent Hartree theory for lattice-relaxed magic-angle twisted bilayer graphene” by MMA Ezzi, L Peng, Z Liu, JHZ Chao, GN Pallewela, D Foo, and S Adam arXiv:2404.17638 (2024)
Structured Light and Induced Vorticity in Superconductors
Questions of controlling the quantum states of matter via light have been at the forefront of research on driven phases. We demonstrate the effects of imprinted vorticity on superconducting coherent states using structured light. Within the framework of the generalized time-dependent Ginzburg-Landau equation, we show the induction of coherent vortex pairs moving in phase with electromagnetic wave oscillation. The structured light, generated by a Laguerre-Gaussian beam, provides light sources with various quantum properties, such as spin angular momentum and orbital angular momentum. This state of light is also well known as an optical vortex, characterized by a twisted phase front. In the current work, we investigate the optically induced dynamics of superconducting coherent states using both normal light sources and optical vortices. These results uncover rich hydrodynamics of superconducting states and suggest new optical applications for imprinting quantum states on superconducting materials.
Bi-polaron superconductivity in the low density limit
It has been assumed for decades that high values of Tc from the electron-phonon coupling are impossible. At weak-to-intermediate coupling strength this result follows from the Migdal-Eliashberg theory, while at strong coupling, when bipolarons form, the transition temperatures are low because of the exponential effective mass enhancement. However, the latter conclusion was based on numerical solutions of the Holstein model. I will discuss a different model with coupling based on the displacement modulated hopping of electrons and argue that much larger values of the bipolaron Tc can be achieved in this setup. Non-locality of the problem gives rise to small-size, yet relatively light bipolarons, which can be studied by an exact sign-problem-free quantum Monte Carlo approach even in the presence of strong Hubbard and Coulomb potentials. We find that Tc in this model generically and significantly exceeds typical upper bounds based on Migdal-Eliashberg theory or superfluidity of Holstein bipolarons, and, thus, offers a route towards the design of high-Tc superconductors via functional material engineering. Finally, there are indications for even better prospects in systems with non-linear electron-phonon coupling.
Circuit complexity and functionality: a thermodynamic perspective
We explore a link between complexity and physics for circuits of given functionality. Taking advantage of the connection between circuit counting problems and the derivation of ensembles in statistical mechanics, we tie the entropy of circuits of a given functionality and fixed number of gates to circuit complexity. We use thermodynamic relations to connect the quantity analogous to the equilibrium temperature to the exponent describing the exponential growth of the number of distinct functionalities as a function of complexity. This connection is intimately related to the finite compressibility of typical circuits. Finally, we use the thermodynamic approach to formulate a framework for the obfuscation of programs of arbitrary length – an important problem in cryptography – as thermalization through recursive mixing of neighboring sections of a circuit, which can viewed as the mixing of two containers with “gases of gates”. This recursive process equilibrates the average complexity and leads to the saturation of the circuit entropy, while preserving functionality of the overall circuit. The thermodynamic arguments hinge on ergodicity in the space of circuits which we conjecture is limited to disconnected ergodic sectors due to fragmentation. The notion of fragmentation has important implications for the problem of circuit obfuscation as it implies that there are circuits with same size and functionality that cannot be connected via local moves. Furthermore, we argue that fragmentation is unavoidable unless the complexity classes NP and coNP coincide.
Tunable moire sublattices in twisted square homobilayers: exploiting fundamental principles for new technologies
Stacking and twisting atomically thin bilayers at small angles produces an approximate periodic pattern, due to the overlap of the crystal layers. These devices, dubbed “moire” bilayers, exhibit a high degree of tunability and variability: through choice of twist angle, constituent layers, and gating. To date, a number of such devices have been built which have demonstrated a plethora of novel phases, including non-trivial topology and Mott physics. Despite this explosion in moire research, moire bilayers have been almost exclusively formed from layers with triangular/hexagonal crystal geometry, and where the valence bands are centered on the Gamma or K/K’ high symmetry points. Here we theoretically demonstrate that moire devices formed from square bilayers can be used to simulate the ground state of the Hubbard model, but where the ratio of the nearest-neighbor (t) and next-to-nearest neighbor (t’) tunneling can be tuned between zero and infinity, in situ via an electric field. If experimentally realized, such a device would be the first of its kind, and would open a pathway toward the testing of a number of proposed exotic phases, such as a spin-liquid and d+id superconductivity. Most importantly, the square Hubbard model is a quintessential model for high-Tc in cuprates, where numerics has demonstrated the absence of superconductivity when t’=0.
The Superconducting Diode Effect And Spontaneous Symmetry Breaking In Multi-Layer Graphene
The superconducting diode effect, defined as nonreciprocity in the critical supercurrent, provides a unique window into the nature of the superconducting phase. It has been argued that a zero-field diode effect in the superconducting transport requires inversion and time-reversal symmetries to be simultaneously broken. Along this vein, the zero-field superconducting diode effect in multi-layer graphene provides direct evidence of the microscopic coexistence between superconductivity and time-reversal symmetry breaking. In this talk, I will discuss our recent efforts that utilize the angle-resolved measurement of transport nonreciprocity to directly probe the nature of spontaneous symmetry breaking in the normal phase. By investigating the interplay between transport nonreciprocity, ferromagnetism, and superconductivity, our findings suggest that the exchange-driven instability in the momentum space plays a key role in the zero-field superconducting diode effect.
Novel Strongly Correlated Phases in Stacked TMD Bilayers
Two-dimensional transition metal dichalcogenides (TMDs) have emerged as an exciting platform to stack and twist bilayers to engineer strongly correlated quantum phases. Here we present a theory to describe the recent realization of a heavy fermion state in stacked MoTe2/WSe2 bilayers. An extension of this theory that allows for the formation of unconventional superconductivity through repulsive nearest neighbor interactions will be used to show how to realize the p-wave BEC to BCS transition.