A. Rozen, J.M. Park, U. Zondiner, Y. Cao, D. Rodan-Legrain,  T. Taniguchi, K. Watanabe, Y. Oreg,  Ady Stern, E. Berg, P. Jarillo-Herrero and S. Ilani

arXiv:2009.01836 (2020).

In the 1950's, Pomeranchuk predicted that, counterintuitively, liquid 3He may solidify upon heating, due to a high excess spin entropy in the solid phase. Here, using both local and global electronic entropy and compressibility measurements, we show that an analogous effect occurs in magic angle twisted bilayer graphene. Near a filling of one electron per moir'e unit cell, we observe a dramatic increase in the electronic entropy to about 1kB per unit cell. This large excess entropy is quenched by an in-plane magnetic field, pointing to its magnetic origin. A sharp drop in the compressibility as a function of the electron density, associated with a reset of the Fermi level back to the vicinity of the Dirac point, marks a clear boundary between two phases. We map this jump as a function of electron density, temperature, and magnetic field. This reveals a phase diagram that is consistent with a Pomeranchuk-like temperature- and field-driven transition from a low-entropy electronic liquid to a high-entropy correlated state with nearly-free magnetic moments. The correlated state features an unusual combination of seemingly contradictory properties, some associated with itinerant electrons, such as the absence of a thermodynamic gap, metallicity, and a Dirac-like compressibility, and others associated with localized moments, such as a large entropy and its disappearance with magnetic field. Moreover, the energy scales characterizing these two sets of properties are very different: whereas the compressibility jump onsets at T~30K, the bandwidth of magnetic excitations is ~3K or smaller. The hybrid nature of the new correlated state and the large separation of energy scales have key implications for the physics of correlated states in twisted bilayer graphene.

U. Zondiner, A. Rozen, D. Rodan-Legrain, Y. Cao, R. Queiroz, T. Taniguchi, K. Watanabe, Y. Oreg, F. von Oppen, Ady Stern, E. Berg, P. Jarillo-Herrero and S. Ilani

Nature 582, 203-208 (2020).

Twisted bilayer graphene near the magic angle exhibits remarkably rich electron correlation physics, displaying insulating, magnetic, and superconducting phases. Here, using measurements of the local electronic compressibility, we reveal that these phases originate from a high-energy state with an unusual sequence of band populations. As carriers are added to the system, rather than filling all the four spin and valley flavors equally, we find that the population occurs through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moiré lattice. At each transition, a single spin/valley flavor takes all the carriers from its partially filled peers, "resetting" them back to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near the charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breakings.  The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state we reveal here, with its strongly broken electronic flavor symmetry and revived Dirac-like electronic character, is a key player in the physics of magic angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.

I.Khivrich and S. Ilani

Nature Comm. 11,2299 (2020)

Quantum sensing techniques have been successful in pushing the sensitivity limits in numerous fields, and hold great promise for scanning probes that study nano-scale devices and novel materials. However, forming a nano-scale qubit that is simple and robust enough to be placed on a scanning tip, and sensitive enough to detect various physical observables, is still a great challenge. Here we demonstrate a conceptually new qubit implementation in a carbon nanotube that achieves these requirements. In contrast to the prevailing semiconducting qubits that use electronic states in double quantum dots, our qubit utilizes the natural electronic wavefunctions in a single quantum dot. Using an ultraclean nanotube we construct a qubit from two wavefunctions with significantly different magnetic moments and spatial charge distributions, making it sensitive to both magnetic and electric fields. We use an array of gates to directly image these wavefunctions and demonstrate their localized moments. Owing to their different spatial structure, these wavefunctions also show radically different transport properties, giving us a simple transport-based qubit readout mechanism. Due to its narrow coherence-limited transition, the qubit demonstrates significantly better electric field detection sensitivity than a single electron transistor. Moreover, with the same qubit we demonstrate simultaneous probing of magnetic fields with DC sensitivity comparable to that of NV centers. Our technique has minimal requirements for device complexity, which can be implemented using a number of straightforward fabrication methods. These features make this atomic-like qubit a powerful new tool that enables a variety of new nanoscale imaging experiments.

J.A. Sulpizio†, L. Ella†, A. Rozen†, J. Birkbeck, D.J. Perello, D. Dutta, M. BenShalom, T. Taniguchi, K. Watanabe, T. Holder, R. Queiroz, A. Stern, T. Scaffidi, A.K. Geim, and S. Ilani , Nature 576, 75-79 (2019).

Hydrodynamics is a general description for the flow of a fluid, and is expected to hold even for fundamental particles such as electrons when inter-particle interactions dominate. While various aspects of electron hydrodynamics were revealed in recent experiments, the fundamental spatial structure of hydrodynamic electrons, the Poiseuille flow profile, has remained elusive. In this work we provide the first real-space imaging of Poiseuille flow of an electronic fluid, as well as visualization of its evolution from ballistic flow. Utilizing a scanning nanotube single electron transistor, we image the Hall voltage of electronic flow through channels of high-mobility graphene. We find that the profile of the Hall field across the channel is a key physical quantity for distinguishing ballistic from hydrodynamic flow. We image the transition from flat, ballistic field profiles at low temperature into parabolic field profiles at elevated temperatures, which is the hallmark of Poiseuille flow. The curvature of the imaged profiles is qualitatively reproduced by Boltzmann calculations, which allow us to create a ‘phase diagram’ that characterizes the electron flow regimes. Our results provide long-sought, direct confirmation of Poiseuille flow in the solid state, and enable a new approach for exploring the rich physics of interacting electrons in real space.

New&Views:  K. Ensslin, “Electrons in graphene go with the flow

Shapir, I.*, Hamo, A.*, Pecker, S., Moca, C. P., Legeza, Ö., Zarand, G., & Ilani, S.

Science 364, 870-875 (2019)

The quantum crystal of electrons, predicted more than eighty years ago by Eugene Wigner, is still one of the most elusive states of matter. Here, we present experiments that observe the one-dimensional Wigner crystal directly, by imaging its charge density in real-space. To measure this fragile state without perturbing it, we use a scanning probe platform that utilizes a pristine carbon nanotube as a scanning charge perturbation to image, with minimal invasiveness, the many-body electronic density within another nanotube. The obtained images, of few electrons confined in one-dimension, match those of strongly interacting crystals, with electrons ordered like pearls on a necklace. Comparison to theoretical modeling demonstrates the dominance of Coulomb interactions over kinetic energy and the weakness of exchange interactions. The quantum nature of the crystal emerges when we explore its tunneling through a potential barrier. Images of the density redistribution upon tunneling show that the tunneling involves the collective motion of multiple electrons. These experiments provide direct evidence of the formation of small Wigner crystals, and open the way for studying other fragile interacting states by imaging their many-body density in real space.

L. Ella, A. Rozen , J. Birkbeck, M. Ben-Shalom, D. Perello , J. Zultak , T. Taniguchi , K. Watanabe , A.K. Geim , S. Ilani , and J.A. Sulpizio*

Nature Nanotechnology 14, 480-487 (2019) 

Electron transport in nanoscale devices can often result in nontrivial spatial patterns of voltage and current that reflect a variety of physical phenomena, particularly in nonlocal transport regimes. While numerous techniques have been devised to image electron flows, the need remains for a nanoscale probe capable of simultaneously imaging current and voltage distributions with high sensitivity and minimal invasiveness, in magnetic field, across a broad range of temperatures, and beneath an insulating surface. Here we present such a technique for spatially mapping electron flows based on a nanotube single-electron transistor, which achieves high sensitivity for both voltage and current imaging. In a series of experiments using high-mobility graphene devices, we demonstrate the ability of our technique to visualize local aspects of intrinsically nonlocal transport, as in ballistic flows, which are not easily resolvable via existing methods. This technique should both aid in understanding the physics of two-dimensional electronic devices, as well as enable new classes of experiments that image electron flow through buried nanostructures in the quantum and interaction-dominated regimes.

Nanomechanical pump–probe measurements of insulating electronic states in a carbon nanotube

I. Khivrich, A. A. Clerk and S. Ilani

Nature Nanotechnology 14, 161-167 (2019)

Transport measurements have been an indispensable tool in studying conducting states of matter. However, there exists a large set of interesting states that are insulating, often due to electronic interactions or topology, and are difficult to probe via transport. Here, through an experiment on carbon nanotubes, we present a new approach capable of measuring insulating electronic states through their back action on nanomechanical motion. We use a mechanical pump–probe scheme, allowing the detection of shifts in both frequency and dissipation rate of mechanical vibrational modes, in an overall insulating system. As an example, we use this method to probe the non-conducting configurations of a double quantum dot, allowing us to observe the theoretically predicted signature of nanomechanical back action resulting from a coherently tunnelling electron. The technique opens a new way for measuring the internal electronic structure of a growing variety of insulating states in one- and two-dimensional systems.

Y. Efroni, S. Ilani, E. Berg

Physical review letters119.14 (2017): 147704.

Fractionalized excitations carrying half an electron charge were predicted many years ago to exist in conducting one-dimensional polymers, but so far have never been observed experimentally. Here we propose a rather simple experiment using a tensioned nanotube in a magnetic field that should finally enable the observation of these half-charge quasiparticles.

A. Hamo*, A. Benyamini*, I. Shapir*, I. Khivrich, J. Waissman, K. Kaasbjerg, Y. Oreg, F. von Oppen and S. Ilani

Nature 535, 395-400 (2016)


Can electrons be made to attract each other via their repulsion from other electrons?  In this work we demonstrate that this is in fact possible using a novel experimental platform consisting of pristine nanotube-based quantum devices and precision cryogenic manipulation.

New&Views:  T. Kontos, “Attractive Electrons from Nanoengineering

See also: ScienceNews,,

Supplementary information

YZ Chen, Felix Trier, T Wijnands, RJ Green, N Gauquelin, R Egoavil, Dennis Valbjørn Christensen, G Koster, M Huijben, N Bovet, S Macke, F He, R Sutarto, Niels Hessel Andersen, GEDK Prawiroatmodjo, TS Jespersen, JA Sulpizio, M Honig, Søren Linderoth, S Ilani, Jo Verbeeck, G Van Tendeloo, Guus Rijnders, GA Sawatzky, Nini Pryds

Nature Materials 14, 801-806 (2015)

Supplementary information

J. A. Sulpizio, S. Ilani, P. Irvin and J. Levy
Annual Review of Materials Research 44, 117-149 (2014)

Recent advances in complex oxide heterostructures, interfaces formed between two different transition-metal oxides, have heralded a new era of materials and physics research. We review the exciting developments in the physics of these systems, with a focus on the nanoscale probes employed to unravel their complex behavior.

J. Ruhman, Arjun Joshua, S. Ilani and E. Altman

Phys. Rev. B 90, 125123 (2014)

The prototypical oxide interface, LAO/STO, is composed of two insulating, nonmagnetic oxides, yet experiments have shown it plays host to a range of magnetic phenomena, including ferromagnetic domains and anisotropic magnetotransport.  We present a theory to unify these various magnetic phases which relies on the coupling between itinerant electrons and localized magnetic moments originating from interfacial charge traps.

A. Benyamini*, A. Hamo*, S. Viola Kusminskiy, F. von Oppen and S. Ilani
Nature Physics 10, 151-156 (2014)

Despite tremendous advances in controlling electrons and phonons in engineered nanosystems, control over their coupling is still widely lacking. Here we demonstrate the ability to fully tailor electron-phonon interactions in a new class of nanotube-based mechanical resonators by creating gate-defined quantum dots along the length of the vibrating nanotube.

See also:

Supplementary information

M. Honig*, J. A. Sulpizio*, J. Drori, A. Joshua, E Zeldov and S. Ilani
Nature Materials 12, 1085-1086 (2013)

Through electrostatic imaging using a novel nanotube-based scanning single electron transistor, we unravel the microscopic origin for the anomalous piezoelectricity in strontium titanate (STO), the substrate used universally in the creation of complex oxide interfaces.  We further demonstrate how tetragonal domains in STO give rise to a striped potential landscape that can markedly influence the flow of electrons through complex oxide interfaces.

New&Views:  “Oxide interfaces: Streaks of conduction”

See also:

Supplementary information

S. Pecker, F. Kuemmeth, A Secchi, M. Rontani, D. C. Ralph, P. L. McEuen and S. Ilani
Nature Physics 9, 576-581 (2013)

In the presence of strong interactions, two electrons confined to a string are predicted to form a Wigner molecule and localize at the end of the string. We create such an electronic state within an ultraclean carbon nanotube, and use tunneling spectroscopy to explore its energy spectrum and quantum symmetry.

See also:

Supplementary information

J. Waissman*, M. Honig*, S. Pecker*, A. Benyamini*, A. Hamo* and S. Ilani
Nature Nanotechnology 8, 569-574 (2013)

The creation of disorder-free and locally-tunable condensed matter systems has remained an outstanding challenge, limiting the ability to design quantum Hamiltonians for electrons.  In this work we establish a new technique for creating such tunable, pristine electron systems in carbon nanotubes suspended over complex electronic circuits, enabling a new era of experiments in quantum electronics and nanomechanics.

News&Views: Z. Zhong, “Pristine quantum devices on demand”.

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Supplementary information

Arjun Joshua, J. Ruhman, S. Pecker, E. Altman, and S. Ilani

PNAS 110 no. 24, 9633-9638 (2013)

The LAO/STO interface hosts a novel system consisting of localized magnetic moments and extended 2D electrons.  Using anisotropic magnetotransport and anomalous Hall Effect measurements, we uncover an intrinsic, tunable coupling between the itinerant electrons and localized moments in this interface, resulting in a polarized electronic phase at high densities.

Supplementary information

Arjun Joshua, S. Pecker, J. Ruhman, E. Altman, and S. Ilani

Nature Communications 3, 1129, 1-7 (2012)

While the LAO/STO interface has fascinated researchers with its rich variety of phenomena, including ferromagnetism and superconductivity, the question of the key physical ingredients underlying its emergent properties has remained open.   We reveal here through magnetotransport that a universal Lifshitz transition between d-electron orbitals of different symmetry is fundamental to the multitude of observed effects in LAO/STO.

Supplementary information

S. Ilani and P. McEuen,

Annual Review of Condensed Matter Physics 1, 1 (2010)

Technological breakthroughs in nanotube device fabrication and electronic measurement have made possible experiments of unprecedented precision that reveal new and surprising phenomena. In this review, we present the fundamental properties of nanotubes along with recent discoveries and discuss the most exciting emerging research directions.

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