Pick-up and assembling of chemically sensitive van der Waals heterostructures using dry cryogenic exfoliation.

Assembling atomic layers of van der Waals materials (vdW) combines the physics of two materials, offering opportunities for novel functional devices. Realization of this has been possible because of advancements in nanofabrication processes which often involve chemical processing of the materials under study; this can be detrimental to device performance. To address this issue, we have developed a modified micro-manipulator setup for cryogenic exfoliation, pick up, and transfer of vdW materials to assemble heterostructures. We use the glass transition of a polymer PDMS to cleave a flake into two, followed by its pick-up and drop to form pristine twisted junctions. To demonstrate the potential of the technique, we fabricated twisted heterostructure of Bi2Sr2CaCu2O8+x (BSCCO), a van der Waals high-temperature cuprate superconductor. We also employed this method to re-exfoliate NbSe2 and make twisted heterostructure. Transport measurements of the fabricated devices indicate the high quality of the artificial twisted interface. In addition, we extend this cryogenic exfoliation method for other vdW materials, offering an effective way of assembling heterostructures and twisted junctions with pristine interfaces.

High-temperature Josephson diode.

Many superconducting systems with broken time-reversal and inversion symmetry show a superconducting diode effect, a non-reciprocal phenomenon analogous to semiconducting p-n-junction diodes. While the superconducting diode effect lays the foundation for realizing ultralow dissipative circuits, Josephson-phenomena-based diode effect (JDE) can enable the realization of protected qubits. The superconducting diode effect and JDE reported thus far are at low temperatures (~4 K), limiting their applications. Here we demonstrate JDE persisting up to 77 K using an artificial Josephson junction of twisted layers of Bi2Sr2CaCu2O8+δ. JDE manifests as an asymmetry in the magnitude and distributions of switching currents, attaining the maximum at 45° twist. The asymmetry is induced by and tunable with a very small magnetic field applied perpendicular to the junction and arises due to interaction between Josephson and Abrikosov vortices. We report a large asymmetry of 60% at 20 K. Our results provide a path towards realizing superconducting Josephson circuits at liquid-nitrogen temperature.

Superconducting Cavity-Based Sensing of Band Gaps in 2D Materials.

The superconducting coplanar waveguide (SCPW) cavity plays an essential role in various areas like superconducting qubits, parametric amplifiers, radiation detectors, and studying magnon-photon and photon-phonon coupling. Despite its wide-ranging applications, the use of SCPW cavities to study various van der Waals 2D materials has been relatively unexplored. The resonant modes of the SCPW cavity exquisitely sense the dielectric environment. In this work, we measure the charge compressibility of bilayer graphene coupled to a half-wavelength SCPW cavity. Our approach provides a means to detect subtle changes in the capacitance of the bilayer graphene heterostructure, which depends on the compressibility of bilayer graphene, manifesting as shifts in the resonant frequency of the cavity. This method holds promise for exploring a wide class of van der Waals 2D materials, including transition metal dichalcogenides (TMDs) and their moiré, where DC transport measurement is challenging.

Perpendicular electric field drives Chern transitions and layer polarization changes in Hofstadter bands.

Moiré superlattices engineer band properties and enable observation of fractal energy spectra of Hofstadter butterfly. Recently, correlated-electron physics hosted by flat bands in small-angle moiré systems has been at the foreground. However, the implications of moiré band topology within the single-particle framework are little explored experimentally. An outstanding problem is understanding the effect of band topology on Hofstadter physics, which does not require electron correlations. Our work experimentally studies Chern state switching in the Hofstadter regime using twisted double bilayer graphene (TDBG), which offers electric field tunable topological bands, unlike twisted bilayer graphene. Here we show that the nontrivial topology reflects in the Hofstadter spectra, in particular, by displaying a cascade of Hofstadter gaps that switch their Chern numbers sequentially while varying the perpendicular electric field. Our experiments together with theoretical calculations suggest a crucial role of charge polarization changing concomitantly with topological transitions in this system. Layer polarization is likely to play an important role in the topological states in few-layer twisted systems. Moreover, our work establishes TDBG as a novel Hofstadter platform with nontrivial magnetoelectric coupling.

Quantum-noise-limited microwave amplification using a graphene Josephson junction.

Josephson junctions (JJs) and their tunable properties, including their nonlinearities, play an important role in superconducting qubits and amplifiers. JJs together with the circuit quantum electrodynamics architecture form many key components of quantum information processing1. In quantum circuits, low-noise amplification of feeble microwave signals is essential, and Josephson parametric amplifiers (JPAs)2 are the widely used devices. The existing JPAs are based on Al-AlOx-Al tunnel junctions realized in a superconducting quantum interference device geometry, where magnetic flux is the knob for tuning the frequency. Recent experimental realizations of two-dimensional (2D) van der Waals JJs3-5 provide an opportunity to implement various circuit quantum electrodynamics devices6-8 with the added advantage of tuning the junction properties and the operating point using a gate potential. While other components of a possible 2D van der Waals circuit quantum electrodynamics architecture have been demonstrated, a quantum-noise-limited amplifier, an essential component, has not been realized, to the best of our knowledge. Here we implement a quantum-noise-limited JPA using a graphene JJ, that has a linear resonance gate tunability of 3.5 GHz. We report 24 dB amplification with 10 MHz bandwidth and -130 dBm saturation power, a performance on par with the best single-junction JPAs2,9. Importantly, our gate-tunable JPA works in the quantum-limited noise regime, which makes it an attractive option for highly sensitive signal processing. Our work has implications for novel bolometers; the low heat capacity of graphene together with JJ nonlinearity can result in an extremely sensitive microwave bolometer embedded inside a quantum-noise-limited amplifier. In general, this work will open up the exploration of scalable device architectures of 2D van der Waals materials by integrating a sensor with the quantum amplifier.

Nanowire bolometer using a 2D high-temperature superconductor.

Superconducting nanowires are very important due to their applications ranging from quantum technology to astronomy. In this work, we implement a non-invasive process to fabricate nanowires of high-Tcsuperconductor Bi2Sr2CaCu2O8+δ. We demonstrate that our nanowires can be used as bolometers in the visible range with very high responsivity of 9.7 × 103V W-1. Interestingly, in a long (30µm) nanowire of 9 nm thickness and 700 nm width, we observe bias current-dependent localized spots of maximum photovoltage. Moreover, the scalability of the bolometer responsivity with the normal state resistance of the nanowire could allow further performance improvement by increasing the nanowire length in a meander geometry. We observe phase slip events in nanowires with small cross-sections (12 nm thick, 300 nm wide, and 3µm long) at low temperatures. Our study presents a scalable method for realizing sensitive bolometers working near the liquid-nitrogen temperature.

Dynamics of Interfacial Bubble Controls Adhesion Mechanics in Van der Waals Heterostructure.

Two-dimensional van der Waals heterostructures (vdWH) can result in novel functionality that crucially depends on interfacial structure and disorder. Bubbles at the vdWH interface can modify the interfacial structure. We probe the dynamics of a bubble at the interface of a graphene-hBN vdWH by using it as the drumhead of a NEMS device because nanomechanical devices are exquisite sensors. For drums with different interfacial bubbles, we measure the evolution of the resonant frequency and spatial mode shape as a function of electrostatic pulling. We show that the hysteretic detachment of layers of vdWH is triggered by the growth of large bubbles. The bubble growth takes place due to the concentration of stress resembling the initiation of fracture. The small bubbles at the heterostructure interface do not result in delamination as they are smaller than a critical fracture length. We provide insight into frictional dynamics and interfacial fracture of vdWH.

Superconducting Vortex-Charge Measurement Using Cavity Electromechanics.

As the magnetic field penetrates the surface of a superconductor, it results in the formation of flux vortices. It has been predicted that the flux vortices will have a charged vortex core and create a dipolelike electric field. Such a charge trapping in vortices is particularly enhanced in high-Tc superconductors (HTS). Here, we integrate a mechanical resonator made of a thin flake of HTS Bi2Sr2CaCu2O8+δ into a microwave circuit to realize a cavity-electromechanical device. Due to the exquisite sensitivity of cavity-based devices to the external forces, we directly detect the charges in the flux vortices by measuring the electromechanical response of the mechanical resonator. Our measurements reveal the strength of surface electric dipole moment due to a single vortex core to be approximately 30 |e|aB, equivalent to a vortex charge per CuO2 layer of 3.7 × 10-2|e|, where aB is the Bohr radius and e is the electronic charge.

Observation of Standing Spin Waves in a van der Waals Magnetic Material.

Spin waves are studied for data storage, communication, and logic circuits in the field of spintronics based on their potential to substitute electrons. The recent discovery of magnetism in 2D systems such as monolayer CrI3 and Cr2 Ge2 Te6 has led to a renewed interest in such applications of magnetism in the 2D limit. Here, direct evidence of standing spin waves is presented along with the uniform precessional resonance modes in the van der Waals magnetic material, CrCl3 . This is the first direct observation of standing spin-wave modes, set up along a thickness of 20 mm, in a van der Waals material. Standing spin waves are detected in the vicinity of both branches, optical and acoustic, of the antiferromagnetic resonance. Magnon-magnon coupling and softening of resonance modes with temperature enable extraction of interlayer exchange field as a function of temperature.

Bulk valley transport and Berry curvature spreading at the edge of flat bands.

2D materials based superlattices have emerged as a promising platform to modulate band structure and its symmetries. In particular, moiré periodicity in twisted graphene systems produces flat Chern bands. The recent observation of anomalous Hall effect (AHE) and orbital magnetism in twisted bilayer graphene has been associated with spontaneous symmetry breaking of such Chern bands. However, the valley Hall state as a precursor of AHE state, when time-reversal symmetry is still protected, has not been observed. Our work probes this precursor state using the valley Hall effect. We show that broken inversion symmetry in twisted double bilayer graphene (TDBG) facilitates the generation of bulk valley current by reporting experimental evidence of nonlocal transport in a nearly flat band system. Despite the spread of Berry curvature hotspots and reduced quasiparticle velocities of the carriers in these flat bands, we observe large nonlocal voltage several micrometers away from the charge current path - this persists when the Fermi energy lies inside a gap with large Berry curvature. The high sensitivity of the nonlocal voltage to gate tunable carrier density and gap modulating perpendicular electric field makes TDBG an attractive platform for valley-twistronics based on flat bands.

On-Demand Local Modification of High-Tc Superconductivity in Few Unit-Cell Thick Bi2 Sr2 CaCu2 O8+δ.

High-temperature superconductors (HTSs) are important for potential applications and for understanding the origin of strong correlations. Bi2 Sr2 CaCu2 O8+δ (BSCCO), a van der Waals material, offers a platform to probe the physics down to a unit-cell. Guiding the flow of electrons by patterning 2DEGS and oxide heterostructures has brought new functionality and access to new science. Similarly, modifying superconductivity in HTS locally, on a small length scale, is of immense interest for superconducting electronics. A route to modify superconductivity locally by depositing metal on the surface is reported here by transport studies on few unit-cell thick BSCCO. Deposition of chromium (Cr) on the surface over a selected area of BSCCO results in insulating behavior of the underlying region. Cr locally depletes oxygen in CuO2 planes and disrupts the superconductivity in the layers below. This technique of modifying superconductivity is suitable for making sub-micrometer superconducting wires and more complex superconducting devices.

Facile deterministic cutting of 2D materials for twistronics using a tapered fibre scalpel.

We present a quick and reliable method to cut 2D materials for creating 2D twisted heterostructures and devices. We demonstrate the effectiveness of using a tapered fibre scalpel for cutting graphene. Electrical transport measurements show evidence of the desired twist between the graphene layers fabricated using our technique. Statistics of the number of successfully twisted stacks made using our method is compared with h-BN assisted tear-and-stack method. Also, our method can be used for twisted stack fabrication of materials that are few nanometers thick. Finally, we demonstrate the versatility of the tapered fibre scalpel for other shaping related applications for sensitive 2D materials.

Nontrivial quantum oscillation geometric phase shift in a trivial band.

Quantum oscillations provide a notable visualization of the Fermi surface of metals, including associated geometrical phases such as Berry's phase, that play a central role in topological quantum materials. Here we report the existence of a new quantum oscillation phase shift in a multiband system. In particular, we study the ABA-trilayer graphene, the band structure of which is composed of a weakly gapped linear Dirac band, nested within a quadratic band. We observe that Shubnikov-de Haas (SdH) oscillations of the quadratic band are shifted by a phase that sharply departs from the expected 2π Berry's phase and is inherited from the nontrivial Berry's phase of the linear band. We find this arises due to an unusual filling enforced constraint between the quadratic band and linear band Fermi surfaces. Our work indicates how additional bands can be exploited to tease out the effect of often subtle quantum mechanical geometric phases.

Landau Level Diagram and the Continuous Rotational Symmetry Breaking in Trilayer Graphene.

The sequence of the zeroth Landau levels (LLs) between filling factors ν=-6 to 6 in ABA-stacked trilayer graphene (TLG) is unknown because it depends sensitively on the nonuniform charge distribution on the three layers of ABA-stacked TLG. Using the sensitivity of quantum Hall data on the electric field and magnetic field, in an ultraclean ABA-stacked TLG sample, we quantitatively estimate the nonuniformity of the electric field and determine the sequence of the zeroth LLs. We also observe anticrossings between some LLs differing by 3 in LL index, which result from the breaking of the continuous rotational to C_{3} symmetry by the trigonal warping.

Abrupt p-n junction using ionic gating at zero-bias in bilayer graphene.

Graphene is a promising candidate for optoelectronic applications. In this report, a double gated bilayer graphene FET has been made using a combination of electrostatic and electrolytic gating in order to form an abrupt p-n junction. The presence of two Dirac peaks in the gating curve of the fabricated device confirms the formation of a p-n junction. At low temperatures, when the electrolyte is frozen intentionally, the photovoltage exhibits a six-fold pattern indicative of the hot electron induced photothermoelectric effect that has also been seen in graphene p-n junctions made using metallic gates. We have observed that the photovoltage increases with decreasing temperature indicating a dominant role of supercollision scattering. Our technique can also be extended to other 2D materials and to finer features that will lead to p-n junctions which span a large area, like a superlattice, that can generate a larger photoresponse. Our work creating abrupt p-n junctions is distinct from previous works that use a source-drain bias voltage with a single ionic gate creating a spatially graded p-n junction.

Growth of high-quality Bi2Sr2 CaCu2O8+δ whiskers and electrical properties of resulting exfoliated flakes.

In this work, we demonstrate a simple technique to grow high-quality whiskers of Bi2 Sr2 CaCu2 O8+δ - a high T c superconductor. Structural analysis shows the single-crystalline nature of the grown whiskers. To probe electrical properties, we exfoliate these whiskers into thin flakes (~50 nm thick) using the scotch-tape technique and develop a process to realize good electrical contacts. We observe a superconducting critical temperature, T c , of 86 K. We map the evolution of the critical current as a function of temperature. With 2-D materials emerging as an exciting platform to study low-dimensional physics, our work paves the way for future studies on two-dimensional high-T c superconductivity.

Strong electronic interaction and multiple quantum Hall ferromagnetic phases in trilayer graphene.

Quantum Hall effect provides a simple way to study the competition between single particle physics and electronic interaction. However, electronic interaction becomes important only in very clean graphene samples and so far the trilayer graphene experiments are understood within non-interacting electron picture. Here, we report evidence of strong electronic interactions and quantum Hall ferromagnetism seen in Bernal-stacked trilayer graphene. Due to high mobility ∼500,000 cm2 V-1 s-1 in our device compared to previous studies, we find all symmetry broken states and that Landau-level gaps are enhanced by interactions; an aspect explained by our self-consistent Hartree-Fock calculations. Moreover, we observe hysteresis as a function of filling factor and spikes in the longitudinal resistance which, together, signal the formation of quantum Hall ferromagnetic states at low magnetic field.

Dynamical strong coupling and parametric amplification of mechanical modes of graphene drums.

Mechanical resonators are ubiquitous in modern information technology. With the possibility of coupling them to electromagnetic and plasmonic modes, they hold promise as the key building blocks in future quantum information technology. Graphene-based resonators are of interest for technological applications due to their high resonant frequencies, multiple mechanical modes and low mass. The tension-mediated nonlinear coupling between various modes of the resonator can be excited in a controllable manner. Here we engineer a graphene resonator with large frequency tunability at low temperatures, resulting in a large intermodal coupling strength. We observe the emergence of new eigenmodes and amplification of the coupled modes using red and blue parametric excitation, respectively. We demonstrate that the dynamical intermodal coupling is tunable. A cooperativity of 60 between two resonant modes of ∼100 MHz is achieved in the strong coupling regime. The ability to dynamically control the coupling between the high-frequency eigenmodes of a mechanical system opens up the possibility of quantum mechanical experiments at low temperatures.

Nanoscale Electromechanics To Measure Thermal Conductivity, Expansion, and Interfacial Losses.

We study the effect of localized Joule heating on the mechanical properties of doubly clamped nanowires under tensile stress. Local heating results in systematic variation of the resonant frequency; these frequency changes result from thermal stresses that depend on temperature dependent thermal conductivity and expansion coefficient. The change in sign of the linear expansion coefficient of InAs is reflected in the resonant response of the system near a bath temperature of 20 K. Using finite element simulations to model the experimentally observed frequency shifts, we show that the thermal conductivity of a nanowire can be approximated in the 10-60 K temperature range by the empirical form κ = bT W/mK, where the value of b for a nanowire was found to be b = 0.035 W/mK(2), significantly lower than bulk values. Also, local heating allows us to independently vary the temperature of the nanowire relative to the clamping points pinned to the bath temperature. We suggest a loss mechanism (dissipation ~10(-4)-10(-5)) originating from the interfacial clamping losses between the metal and the semiconductor nanostructure.