Physics in two dimensions in the lab

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1 Physics in two dimensions in the lab Nanodevice Physics Lab David Cobden PAB 308 Collaborators at UW Oscar Vilches (Low Temperature Lab) Xiaodong Xu (Nanoscale Optoelectronics Lab) Jiun Haw Chu (Quantum Materials Lab) Theorists: Anton Andreev, Boris Spivak, Marcel den Nijs Elsewhere: Josh Folk (UBC), Neil Wilson (Warwick), Yongtao Cui (UC Riverside) Nutshell history of 2D: Electrons on surfaces, eg of He (1950s) Thin metal films (1950s) Atoms/molecules adsorbed on surfaces (1960s) Silicon/oxide interface (1970s) Semiconductor heterojunctions (1980s) Graphene (2004) Surfaces of topological insulators (2006) Other layered materials: MoS 2, CrI 3, WTe 2, hbn,

2 Some electronic states found in 2D monolayer materials Good old fashioned insulators (hbn) Semimetals (graphene) Semiconductors Charge density waves Quantum Hall, fractional quantum Hall Wigner crystal, stripe/bubble phases Superconductivity Magnetism (various kinds) Ferroelectricity 2D topological insulator Topological superconductor Excitonic insulator Anomalous metals Plain metals? superconductor All kinds of combinations Wouldn t it be nice if we could switch between them electrically? CDW insulator magnet strange

3 Example of device fab Dry transfer technique - Zomer P et al. Appl Phys Lett 105, (2014) Platinum Few layer graphene (FLG) top gate WTe 2 Hexagonal boron nitride - hbn FLG bottom gate 2 5 nm nm ~5 nm nm 2 5 nm SiO nm p++ Si substrate 0.5 mm In the glove box, <0.5 ppm O 2 and H 2 O Exfoliate WTe 2 Pick up on hbn Put down on contacts

4 Nanotube nanoguitar the most sensitive mass balance Current (a.u.) f res Frequency (MHz) mass unit length adsorbed molecules per C atom 1 f 0 77 K Precision 1 atom, << 1 electron f (MHz) Kr gas pressure Adsorption can also be detected via the conductance

5 Conductance isotherms : studying 2D phase transitions 1 to 2 μm BN After low V anneal After high V anneal Cross section µm µm 0.2 µm AA AB

6 2D semiconductor heterojunctions a WSe 2 Optical SEM Mo W MoSe 2 Photoluminescence TEM 1D edge as substrate WSe 2 lateral heteroepitaxy MoSe 2 Monolayer MoSe 2 formation WSe 2 Mo W Se (x2) WSe 2 /MoSe 2 lateral heterostructure

7 Directly measuring electronic bands Angle resolved photoemission spectroscopy, k Binding Energy, E binding (ev) Vg= -5V Vg= -4V Vg= -3V Vg= -2V Vg= -1V Vg= 0V Vg= +1V Vg= +2V Vg= +3V Vg= +4V Vg=+5V -0.4 k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) -0.4 k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 ) k (Å -1 )

8 2D magnetism in monolayers CrI 3 15K monolayer bulk 61K Intralayer ferromagnetic Interlayer antiferromagnetic T Kerr rotation bilayer trilayer See Huang et al, Nature (2017) Electric field/strain tunable?

9 2D monolayer semiconductors MoS 2, WSe 2 Electrons are massive Dirac particles with added valley pseudospin Ambipolar monolayer WSe 2 transistor 1 V bg = -4 V V bg = 4 V V sd = 10 mv AC G ( S) V cg (V)

10 3D WTe 2 a van der Waals layered topological semimetal Orthorhombic T d structure W form chains Theory: Type II Weyl points Soluyanov et al. Nature 527, 495 (2015) Huge nonsaturating magnetoresistance at low T Ali, Ong et al, Nature 514, 205 (2014) Monolayer is quantum spin Hall candidate Qian, Junwei Liu, Fu, Li (Science, 2014) 0.53 K

11 The quantum spin Hall (QSH) effect A 2D insulator is topologically nontrivial if invariant 1 Kane & Mele; Kane & Fu, PRL Time-reversal invariant points Λ occupied bands Parity Λ 1implies band inversion 1 Bulk States Edge States If 1there exists at least one gapless mode on the edge +k and k not mixed by time symmetric perturbations possible / quantization Momentum along edge Mode is helical (spin locked to k) quantum spin Hall effect First evidence for QSH reported in HgCdTe (Konig et al, Science 2007) Majorana mode Science News

12 Few layer WTe 2 devices The first 2D semimetal to be studied Z. Fei et al, Nature Physics (10 April 2017) ~ 300 n p (10 13 cm 2 ) T (K) G ( S) V g (V) e 2 /h T (K) V g (V) e 2 /h V g (V) T (K)

13 Differentiating edge and bulk conduction WTe 2 5 m optical image K AFM image I/V ( S) edge conduction only bulk conduction V g (V)

14 Seeing the edge scanning microwave impedance microscopy Measurements by Yongtao Cui, Stanford and UC Riverside Technique: Rev Sci Instr 87, (2016) Brighter = higher conductivity conducting edge Bilayer region 9 GHz 300 mk 0T 12T

15 B field suppresses edge conduction characteristic of QSH G ( S) B B 1.6 K B (T) 120 Conductance ( S) difference =14 T 10 K Gate voltage (V)

16 Weird and wonderful properties of edge conduction T = 1.6 K, L = 150 nm 0 T 1 T 3 T / 5 T 10 T 15 T Bias, Gate voltage,

17 Phase diagram of monolayer WTe 2 (under construction) 100 strange semimetal Temperature (K) 10 1 metal 0.1 Correlated 2D topological insulator 2D superconductor n p (10 13 cm 2 ) Excitonic gap superconducting gap Quantum phase transition

18 Unpublished data deleted CONCLUSION 2D physics in the lab is going places!