News and posts

Jeanie Lau

In a flat band system, the charge carriers’ energy-momentum relation is very weakly dispersive. The resultant large density of states and the dominance of Coulomb potential energy relative to the kinetic energy often favor the formation of strongly correlated electron states, such as ferromagnetism, nematicity, antiferromagnetism, superconductivity, and charge density waves. The advent of two-dimensional (2D) materials and their heterostructures has ushered in a new era for exploring, tuning and engineering of flat band system. Here I will present our results on transport measurements of high quality few-layer 2D material devices, including intrinsic magnetism and helical edge states in few-layer graphene, and observation of both superconductivity and the Mott-like insulating state in a tBLG device with a twist angle of ~0.93°.

Jacobo Santamaria

The Josephson effect results from the coupling of two superconductors across a non- superconducting spacer to yield a quantum coherent state. In ferromagnets, singlet (opposite- spin) Cooper pairs decay over very short distances, and thus Josephson coupling requires a nanometric spacer. This is unless equal-spin triplet pairs are generated which, theoretically, can couple superconductors across much longer distances. Despite many experimental hints of triplet superconductivity, long range triplet Josephson effects have remained elusive. In this talk I will discuss a micron-range Josephson coupling across the half-metallic ferromagnet
La0.7Sr0.3MnO3 combined with the high-temperature superconductor YBa2Cu3O7 in planar junctions. These display the Josephson physics’ hallmarks: critical current oscillations due to flux quantization and quantum phase locking under microwave excitation. The marriage of high- temperature quantum coherent transport and full spin polarization brings unique opportunities for the practical realization of superconducting spintronics, and enables novel strategies for quantum computing.

Mario Cuoco

In this talk I will present different routes to generate and manipulate topological phases due to the interplay between superconductivity and magnetism. The search for new variants of semimetals (SMs) recently highlighted the interplay of Dirac fermions physics and magnetism. Indeed, antiferromagnetic (AFM) SMs can be obtained where both time and inversion are broken while their combination is kept [1,2] or due to chiral- [2] and time-symmetry [2,3] combined with non-symmorphic transformations [2]. Here, we discuss materials, i.e. transition metal oxide systems, that can exhibit AFM-SM phase due to orbitally directional double- exchange effects [4, 2]. In this context, the impact of s-wave spin-singlet pairing on AFM-SMs with Dirac points or nodal loops at the Fermi level [5] is generally shown to convert the semimetal into various types of nodal topological superconductors. The changeover from fully gapped to gapless phases is dictated by symmetry properties of the AFM-superconducting state that set out the occurrence of a large variety of electronic topological transitions [4].
Finally, I will focus on various quantum platforms marked by spin-singlet or spin-triplet pairing interfaced with non-trivial magnetic patterns and discuss the nature of the emerging topological phases [6,7,8]. The coexistence of ferromagnetism or antiferromagnetism with spin-triplet superconductivity is also analysed and discussed with respect to relevant materials cases.

[1] P. Tang, Q. Zhou, G. Xu, and S.-C. Zhang, Nat. Phys. 12, 1100 (2016).
[2] W. Brzezicki and M. Cuoco, Phys. Rev. B 95, 155108 (2017).
[3] S. M. Young and B. J. Wieder, Phys. Rev. Lett. 118, 186401 (2017).
[4] W. Brzezicki, C. Noce, A. Romano, and M. Cuoco, Phys. Rev. Lett. 114, 247002 (2015).
[5] W. Brzezicki and M. Cuoco, Phys. Rev. B 97, 064513 (2018).
[6] M. T. Mercaldo, M. Cuoco, P. Kotetes, Phys. Rev. B 94, 140503(R) (2016).
[7] A. Romano, P. Gentile, C. Noce, I. Vekhter, M. Cuoco, Phys. Rev. Lett. 110, 267002 (2013). 8. P. Kotetes, M. T. Mercaldo, M. Cuoco, Phys. Rev. Lett. 123, 126802 (2019).
[9] M. T. Mercaldo, P. Kotetes, M. Cuoco, Phys. Rev. B 100, 104519 (2019).

Paola Gentile

The most recent advances in nanotechnology have demonstrated the possibility to create flexible semiconductor nanomaterials which are bent into curved, deformable objects ranging from semiconductor nanotubes, to nanohelices, etc. The consequences of the nanowire bending on the electronic quantum properties have been demonstrated to become of particular importance in systems with structure inversion asymmetry, where the interplay between nanoscale deformations and Rashba spin-orbit coupling (RSOC) [1] allows an all-geometrical and electrical control of electronic spin textures and spin transport properties [2,3], including the possibility to induce topological nontrivial phases [4-6]. In the presence of superconducting pairing, inversion symmetry breaking (ISB) makes neither spin nor parity good quantum numbers anymore. The ensuing mixing of even spin-singlet and odd spin- triplet channels leads to a series of novel features, from unconventional surface states to topological phases. Within this framework, we have explored the impact that nanoscale geometry has on superconducting properties of low-dimensional materials, showing that the interplay between RSOC and shape deformations can lead to novel paths for a geometric manipulation of the superconducting state, both for spin-singlet and spin-triplet quantum configurations [7], then significantly affecting the Josephson effect of weak links between Rashba coupled straight superconducting nanowires with geometric misalignment [8] as well as between nanowires of topological superconductors with non-trivial geometric curvature [9].

[1] P. Gentile., M. Cuoco, C. Ortix, SPIN, Vol. 3, No. 2, 1340002 (2013).
[2] Z.-J. Ying, P. Gentile, C. Ortix, M. Cuoco, Phys. Rev. B 94, 081406(R) (2016).
[3] G. Francica, P. Gentile, M. Cuoco, EPL 127, 30001 (2019).
[4] P. Gentile, M. Cuoco, C. Ortix, Phys. Rev. Lett. 115, 256801 (2015).
[5] P. G., V. Benvenuto, C. Ortix, C. Noce, M. Cuoco, Condens. Matter 4 (1), 25 (2019).
[6] S. Pandey, N. Scopigno, P. Gentile, M. Cuoco, C. Ortix, Phys. Rev. B 97, 241103(R) (2018).
[7] Z.-J. Ying, M. Cuoco, C. Ortix, P. Gentile, Physical Review B 96, 100506(R) (2017).
[8] Z.-J. Ying, M. Cuoco, P. Gentile, C. Ortix, 2017 16th International Superconductive Electronics Conference (ISEC), IEEE Xplore (2018).
[9] G. Francica, M. Cuoco, P. Gentile, Phys. Rev. B 101, 094504 (2020).

Dimitri Efetov

When twisted close to a magic relative orientation angle near 1 degree, bilayer graphene has flat moire superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics, notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moire band filling factors nu = 0, +(-) 1, +(-) 2, +(-) 3, and reveals new superconductivity regions below critical temperatures as high as 3 K close to - 2 filling. In addition we find novel orbital magnetic states with non-zero Chern numbers. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moire flat bands, including near charge neutrality. We further will discuss recent experiments including screened interactions, fragile topology and the first applications of this amazing new materials platform.

Shane Cybart

In Feynman’s infamous 1959 lecture entitled, "There's Plenty of Room at the Bottom " he inspired and foreshadowed the emergence of nanoengineering. He suggested that finely focused electron, and ion beams would aid our eyes and hands to precisely engineer structures at the atomic level. Currently, electron beam lithography systems and gallium focused ion beams are ubiquitous in nanotechnology and can routinely be used to create structures of the order of tens of nanometers. However, the ability to scale to the sub-10 nm has been a technological challenge until the development of gas field ion sources (GFIS) over the past decade. The GFIS source, utilizes a single crystal tungsten wire sharpened to just 3 atoms. Helium gas is field ionized by one of these atoms, creating a helium ion beam with diameter of only 0.25nm! This instrument is emerging as an important tool for sub-10nm structuring of materials. Helium ion beams have significant advantages. Helium is small and chemically inert which allows it to be used for direct modification of materials properties without etching away material or employing resists.
My research group has been utilizing GFIS for direct patterning of ceramic high-temperature superconducting materials for quantum electronics. The helium ion beam induces nanoscale disorder from irradiation into the crystalline structure which converts the electrical properties of the material from superconductor to insulator. Insulating feature sizes of less than 2nm have been successfully demonstrated and many unique novel devices have been realized. Much of this success is due to the irradiation sensitivity of electrical transport in high temperature superconductors. This sensitivity results from loosely bound oxygen atoms (~1-8ev) in the crystal lattice that are easily displaced into interstitial or anti-site defects. I will describe details of the GFIS materials modification process and highlight applications in quantum sensing, and high frequency detection.

Jagadeesh S. Moodera

Surfaces and interfaces play a pivotally defining role for many of the topologically driven nontrivial quantum phenomena. A good example is the prediction of Majorana zero modes (MZMs or the Majorana pair) to occur in a topological superconductor (TSC) – viz., superconducting surface state of gold. [1] A fermion in a TSC can separate in space into two parts known as MZMs. Thus, Majorana pair are Fermionic states, each of which is an antiparticle of itself, and are required to always appear in pair together with its partner. According to the theoretical proposal of Potter and Lee, [1] under the right conditions, a superconducting gold nanowire with (111) crystalline surface with its large Rashba spin-orbit (S-O) splitting could host the Majorana pair. Utilizing the interplay between superconductivity, S-O coupling and Zeeman field we laid the foundation to realize MZM. [2] We have experimentally optimized a novel stable heterostructures, to achieve all these three interactions, to directly observe the MZM pair using a low temperature with high vector field scanning tunneling microscope, by probing the ferromagnetic EuS island over the gold surface. [3] With this two-dimensional stable metal platform, by means of the Shockley surface state (SS) of (111)-gold (Au) with induced superconductivity, we can envision a novel approach to building non-local qubits that are intrinsically fault-tolerant. In this talk I will be presenting our path towards the observation of MZMs.
Supported by John Templeton Foundation Grants No. 39944 and 60148, ONR Grants N00014-16-1-2657, N00014-20-1-2306 and NSF DMR 1700137

[1] A. C. Potter & P. A. Lee PRL 105, 227003 (2010); PRB 85, 094516 (2012)
[2] Peng Wei, Sujit Manna, Marius Eich, Patrick Lee and J. S. Moodera, Phys. Rev. Lett. 122, 247002 (2019)
[3] Sujit Manna, Peng Wei, Yingming Xie, Kam Tuen Law, Patrick A. Lee and Jagadeesh S. Moodera, Proc. Natl. Acad. Sci. 117 (16) 8775-8782 (Apr. 21, 2020)

Peter Liljeroth

There has been a surge of interest in designer materials that would realize electronic responses not found in naturally occurring materials. For example, it is not clear if topological superconductivity [1], which is a key ingredient in topological quantum computing, exist in any single material. These limitations can be overcome in designer van der Waals (vdW) heterostructures, where the desired physics emerges from the engineered interactions between the different components.
Molecular-beam epitaxy (MBE) growth allows the construction of vertical heterostructures with clean and high-quality interfaces [2]. We use MBE to grow islands of ferromagnetic CrBr3 [3] on a superconducting NbSe2 substrate. This combines out of plane ferromagnetism with Rashba spin-orbit interactions and s-wave superconductivity and allows us to realizate topological superconductivity in a van der Waals heterostructure [4]. We characterize the resulting one-dimensional edge modes using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The use of vdW heterostructures with uniform and high-quality interfaces is promising for future device structures and further control of topological superconductivity through external stimuli (e.g. electrostatic gating).

[1] M. Sato, and Y. Ando, Topological superconductors: a review. Rep. Prog. Phys. 80, 076501 (2017)
[2] S. Kezilebieke, M. N. Huda, P. Dreher, I. Manninen, Y. Zhou, J. Sainio, R. Mansell, M.M. Ugeda, S. van Dijken, H.-P. Komsa, P. Liljeroth, Electronic and Magnetic Characterization of Epitaxial VSe2 Monolayers on Superconducting NbSe2, Commun. Phys. 3, 116 (2020)
[3] W. Chen, Z. Sun, Z. Wang, L. Gu, X. Xu, S. Wu, C. Gao, Direct observation of van der Waals stacking–dependent interlayer magnetism. Science 366, 983 (2019)
[4] S. Kezilebieke, M. N. Huda, V. Vaňo, M. Aapro, S.C. Ganguli, O.J. Silveira, S. Głodzik, A.S. Foster, T. Ojanen, P. Liljeroth, Topological superconductivity in a designer ferromagnet-superconductor van der Waals heterostructure, arXiv:2002.02141 (2020)

Manfred Sigrist

Chiral superconductors are two-fold degenerate and domains of opposite chirality can form, separated by domain walls. While there are experimental indications for domain formation in some unconventional superconductors assumed to realize chiral Cooper pairing, there has not been any unambiguous proof for their existence. In this talk we consider the impact domain walls in chiral superconductors can have on the critical currents. For this purpose we consider various domain wall orientations for both chiral p-wave and chiral d-wave superconductors. We demonstrate that selection rules and crystalline anisotropy play an essential role in connecting the two chiral domains coherently through the domain wall. In particular, we illustrate the case of a domain wall parallel to the basal plane for a chiral p-wave superconductor. The possibility to realize half-flux vortices in these domain walls will be analyzed and used as a tool to estimate the critical currents. The possibility of experimental verication will be discussed.

Eli Zeldov

The emergence of flat bands and of strongly correlated and superconducting phases in twisted bilayer graphene crucially depends on the interlayer twist angle upon approaching the magic angle. Utilizing a scanning nanoSQUID-on-tip, we attain tomographic imaging of the Landau levels and derive nanoscale high precision maps of the twist-angle disorder in high quality hBN encapsulated devices, which reveal substantial twist-angle gradients and a network of jumps [1]. We show that the twist-angle gradients generate large gate tunable in-plane electric fields, unscreened even in the metallic regions, which drastically alter the quantum Hall state by forming edge channels in the bulk of the samples. The correlated states are found to be particularly fragile with respect to twist-angle disorder. We establish the twist-angle disorder as a fundamentally new kind of disorder, which alters the local band structure and may significantly affect the correlated and superconducting states.

[1] A. Uri, S. Grover, Y. Cao, J. A. Crosse, K. Bagani, D. Rodan-Legrain, Y. Myasoedov, K. Watanabe, T. Taniguchi, P. Moon, M. Koshino, P. Jarillo-Herrero, and E. Zeldov, Nature 581, 47 (2020)