David Kealhofer

In vivo quantification of spatially varying mechanical properties in developing tissues

The mechanical properties of the cellular microenvironment and their spatiotemporal variations are thought to play a central role in sculpting embryonic tissues, maintaining organ architecture and controlling cell behavior, including cell differentiation. However, no direct in vivo and in situ measurement of mechanical properties within developing 3D tissues and organs has yet been performed. Here we introduce a technique that employs biocompatible, magnetically responsive ferrofluid microdroplets as local mechanical actuators and allows quantitative spatiotemporal measurements of mechanical properties in vivo. Using this technique, we show that vertebrate body elongation entails spatially varying tissue mechanics along the anteroposterior axis. Specifically, we find that the zebrafish tailbud is viscoelastic (elastic below a few seconds and fluid after just 1 min) and displays decreasing stiffness and increasing fluidity toward its posterior elongating region. This method opens new avenues to study mechanobiology in vivo, both in embryogenesis and in disease processes, including cancer.

Observation of the Quantum Hall Effect in Confined Films of the Three-Dimensional Dirac Semimetal Cd3As2

The magnetotransport properties of epitaxial films of Cd_{3}As_{2}, a paradigm three-dimensional Dirac semimetal, are investigated. We show that an energy gap opens in the bulk electronic states of sufficiently thin films and, at low temperatures, carriers residing in surface states dominate the electrical transport. The carriers in these states are sufficiently mobile to give rise to a quantized Hall effect. The sharp quantization demonstrates surface transport that is virtually free of parasitic bulk conduction and paves the way for novel quantum transport studies in this class of topological materials. Our results also demonstrate that heterostructuring approaches can be used to study and engineer quantum states in topological semimetals.

Negative magnetoresistance due to conductivity fluctuations in films of the topological semimetal Cd3As2

RAPID COMMUNICATIONS PHYSICAL REVIEW B 95, 241113(R) (2017) Negative magnetoresistance due to conductivity fluctuations in films of the topological semimetal Cd 3 As 2 Timo Schumann, 1,* Manik Goyal, 1 David A. Kealhofer, 2 and Susanne Stemmer 1,† Materials Department, University of California, Santa Barbara, California 93106-5050, USA Department of Physics, University of California, Santa Barbara, California 93106-9530, USA (Received 9 April 2017; revised manuscript received 5 June 2017; published 26 June 2017) Recently discovered Dirac and Weyl semimetals display unusual magnetoresistance phenomena, including a large, nonsaturating, linear transverse magnetoresistance and a negative longitudinal magnetoresistance. The latter is often considered as evidence of fermions that have a defined chirality. Classical mechanisms, due to disorder or nonuniform current injection, can, however, also produce negative longitudinal magnetoresistance. Here, we report on magnetotransport measurements performed on epitaxial thin films of Cd 3 As 2 , a three-dimensional Dirac semimetal. Quasilinear positive transverse magnetoresistance and negative longitudinal magnetoresistance are observed. By evaluating films of different thickness and by correlating the temperature dependence of the carrier density and mobility with the magnetoresistance characteristics, we demonstrate that both the quasilinear positive and the negative magnetoresistance are caused by conductivity fluctuations. Chiral anomaly is not needed to explain the observed features. DOI: 10.1103/PhysRevB.95.241113 Recent experimental realizations of three-dimensional topological semimetals have opened up numerous exciting opportunities to test quasiparticle behavior that mimics mass- less, relativistic Dirac or Weyl fermions. A signature of Weyl fermions is the “chiral anomaly”, when charge flows from one Weyl node to one of opposite chirality under parallel electric and magnetic fields, thereby giving rise to an unusual negative longitudinal magnetoresistance (MR) [1,2]. In Dirac semimet- als the Dirac nodes split into Weyl nodes in the magnetic field and similar physics is expected. Recent observations [3–9] of longitudinal negative MR in Dirac and Weyl semimetals have therefore generated significant attention. While most studies consider the negative longitudinal MR to be evidence of the chiral anomaly, current jetting has been proposed as an alternative explanation [10,11]. Current jetting arises under inhomogeneous current injection into high-mobility materials that have a large conductivity anisotropy [12]. It leads to a strong preference of the current to flow in the direction of the magnetic field, resulting in a negative longitudinal MR [12]. Other studies claim to have ruled out current jetting caused by nonuniform current injection [7]. Negative longitudinal MR can also be due to conductivity fluctuations and has been reported for disordered semiconductors [13,14]. A recent study [15] showed that mobility fluctuations are a likely origin of linear, positive transverse MR in Cd 3 As 2 , a three-dimensional Dirac semimetal [16–20]. Most studies that attribute the negative longitudinal MR to chiral anomaly have been carried out on bulk materials. Thin films offer several important advantages towards resolving the origin of the negative longitudinal MR in Dirac and Weyl semimetals. In particular, thin-film-device geometries are much less susceptible to current jetting. Furthermore, the effects of nonuniformities can be studied by varying the film thickness and growth conditions. In this work, we study the magnetotransport properties of epitaxial Cd 3 As 2 thin films. schumann.timo@gmx.net stemmer@mrl.ucsb.edu We show that both the transverse positive MR and the negative longitudinal MR are due to conductivity fluctuations. Cd 3 As 2 films were grown by molecular beam epitaxy on relaxed, 180-nm-thick GaSb buffer layers on GaAs (111)B ¯ direction), as de- substrates (1 ◦ miscut towards the [ 1 ¯ 12] scribed elsewhere [21]. The beam equivalent pressure was 2 × 10 −6 Torr, the substrate temperatures ranged between 150 ◦ C and 170 ◦ C, and the growth time was between 5 min and 60 min (see Table I). Surface morphologies were investigated by optical, atomic force, and scanning electron microscopies. The film’s structure and alignment with the substrate were probed by x-ray diffraction (XRD) using Cu Kα radiation. The films were patterned into Hall bar structures with widths and lengths of 100 μm using standard optical lithography and Ar + ion milling. The thicknesses of the Cd 3 As 2 layers were deter- mined by scanning electron microscopy of the patterned Hall bar structures. Magnetoresistance measurements were carried out in a Quantum Design Dynacool PPMS, at temperatures between 300 K and 2 K and magnetic fields up to 9 T. DC excitation currents of 10 μA were used in all measurements. Shubnikov–de Haas oscillations were detected in the thinnest films (See Fig. S1 in the Supplemental Material [22]). Representative optical and atomic force micrographs of a Cd 3 As 2 film are shown in Figs. 1(a) and 1(b). In most areas, the film is smooth with atomically stepped surfaces ˚ matches the interplanar [Fig. 1(b)]. The step height (∼4 A) spacing of the Cd 3 As 2 (112) planes, which form the surface [21]. The defects in the optical micrograph [Fig. 1(a)] are likely areas that grew in a three-dimensional (island) growth mode. Their density was about 10 3 −10 4 cm −2 . Out-of-plane XRD confirmed single-phase, epitaxial Cd 3 As 2 [Fig. 1(c)]. High-resolution XRD showed Laue fringes from the GaSb buffer layer and for the thinnest Cd 3 As 2 layers [arrows in Fig. 1(d)]. The three-dimensional carrier densities determined from low-field Hall measurements were similar for all films grown at 150 ◦ C (see Table I) and ranged from ∼(0.8−1.8) × 10 17 cm −3 at 2 K to (5.0−8.0) × 10 17 cm −3 at 300 K. These carrier densities are lower than those of single crystals reported in the literature (typically > 10 18 cm −3 [15,17,23]), indicating ©2017 American Physical Society

A fluid-to-solid jamming transition underlies vertebrate body axis elongation

Just as in clay moulding or glass blowing, physically sculpting biological structures requires the constituent material to locally flow like a fluid while maintaining overall mechanical integrity like a solid. Disordered soft materials, such as foams, emulsions and colloidal suspensions, switch from fluid-like to solid-like behaviours at a jamming transition1–4. Similarly, cell collectives have been shown to display glassy dynamics in 2D and 3D5,6 and jamming in cultured epithelial monolayers7,8, behaviours recently predicted theoretically9–11 and proposed to influence asthma pathobiology8 and tumour progression12. However, little is known about whether these seemingly universal behaviours occur in vivo13 and, specifically, whether they play any functional part during embryonic morphogenesis. Here, by combining direct in vivo measurements of tissue mechanics with analysis of cellular dynamics, we show that during vertebrate body axis elongation, posterior tissues undergo a jamming transition from a fluid-like behaviour at the extending end, the mesodermal progenitor zone, to a solid-like behaviour in the presomitic mesoderm. We uncover an anteroposterior, N-cadherin-dependent gradient in yield stress that provides increasing mechanical integrity to the presomitic mesoderm, consistent with the tissue transiting from a wetter to a dryer foam-like architecture. Our results show that cell-scale stresses fluctuate rapidly (within about 1 min), enabling cell rearrangements and effectively ‘melting’ the tissue at the growing end. Persistent (more than 0.5 h) stresses at supracellular scales, rather than cell-scale stresses, guide morphogenetic flows in fluid-like tissue regions. Unidirectional axis extension is sustained by the reported rigidification of the presomitic mesoderm, which mechanically supports posterior, fluid-like tissues during remodelling before their maturation. The spatiotemporal control of fluid-like and solid-like tissue states may represent a generic physical mechanism of embryonic morphogenesis.Cell collectives in embryonic tissues undergo a fluid-to-solid jamming transition, similar to those that occur in soft materials such as foams, emulsions and colloidal suspensions, to physically sculpt the vertebrate body axis.

Thickness dependence of the quantum Hall effect in films of the three-dimensional Dirac semimetal Cd3As2

Low-temperature magnetotransport studies are reported for (112)Cd3As2 films grown on (111)CdTe by molecular beam epitaxy as a function of the Cd3As2 film thickness. All films show Shubnikov-de Haas oscillations. An even-integer quantum Hall effect is observed for films thinner than 70 nm. For the thinnest films, the bulk is gapped and transport at low temperatures occurs only via the gapless, two-dimensional states. The lowest Landau level is reached at ∼10 T, and the longitudinal resistance nearly vanishes at the plateaus in the Hall resistance. The results are discussed in the context of the current theoretical understanding of topological surface states in three-dimensional Dirac semimetals.

Two-dimensional Dirac fermions in thin films of Cd3As2

Author(s): Galletti, Luca; Schumann, Timo; Shoron, Omor F; Goyal, Manik; Kealhofer, David A; Kim, Honggyu; Stemmer, Susanne