Abstract
Snake states are trajectories of charge carriers curving back and forth along an interface. There are two types of snake states, formed by either inverting the magnetic field direction or the charge carrier type at an interface. The former has been demonstrated in GaAs–AlGaAs heterostructures, whereas the latter has become conceivable only with the advance of ballistic graphene where a gapless p–n interface governed by Klein tunnelling can be formed. Such snake states were hidden in previous experiments due to limited sample quality. Here we report on magnetoconductance oscillations due to snake states in a ballistic suspended graphene p–n junction, which occur already at a very small magnetic field of 20 mT. The visibility of 30% is enabled by Klein collimation. Our finding is firmly supported by quantum transport simulations. We demonstrate the high tunability of the device and operate it in different magnetic field regimes.
Introduction
A magnetic field fundamentally modifies the transport properties of an electronic conductor by acting on its charge carriers via the Lorenz force. The most prominent magnetotransport effect is the quantum Hall effect in a twodimensional electron gas. A strong perpendicular magnetic field forces the charge carriers into onedimensional conduction channels in which they flow along the edges of a sample. At moderate magnetic fields, however, electron trajectories can be understood in a quasiclassical picture where the Lorenz force bends charge carriers into cyclotron orbits. In the bulk this leads to localization, whereas at the boundary charge carriers can propagate via socalled skipping orbits. Magnetic focusing experiments^{1,2} represent a direct proof of the skipping orbit picture. In such experiments, an increase of conductance is observed if the distance between two contacts is an integer multiple of the diameter of a cyclotron orbit. One condition for the observation of such trajectories is ballistic transport over the relevant device dimensions. This has limited the observation of skipping orbits to the cleanest available semiconductor samples.
Since 2004, graphene as a new twodimensional conductor has moved into the focus of condensedmatter research and its behaviour in magnetic field has been intensely investigated. The quality of graphene devices has improved over the recent years and ballistic transport over distances of several microns have been demonstrated recently^{3,4}. Since graphene is a gapless semiconductor, it offers the possibility of creating internal interfaces with opposite charge carrier polarity. These socalled p–n interfaces^{5,6,7} are formed by local electrostatic gating.
If electrons that propagate via skipping orbits encounter such a p–n interface, they will turn into snake states. These states consist of alternating half circles with opposite chirality and they transport current along the interface. Similar snake states have first been realized in GaAs/AlGaAs twodimensional electron gases by defining regions of alternating magnetic field direction^{8}. These states share the condition of commensurability (similar to the abovedescribed magnetic focusing experiments) with p–n snake states but they do not propagate along a single and tunable interface. Snake states in graphene p–n junctions were claimed to have been observed in disordered substratesupported samples^{9} but the experiment lacked direct evidence for snaking trajectories.
In this article, we investigate ballistic transport across a graphene p–n junction in different magnetic field regimes and identify magnetoconductance oscillations as a direct signature of snake states. These findings are supported by detailed tightbinding simulations that allow us to visualize the alternating cyclotron orbits of the snake states.
Results
Evolution of electron trajectories in graphene p–n junction
In Fig. 1a–d, we illustrate schematically how trajectories under increasing perpendicular magnetic field evolve in such a device. Figure 1a describes the lowfield situation where transport is still dominated by Fabry–Pérot oscillations with slightly bent trajectories^{10}. As the field is increased, resonant scar states (Fig. 1b) may occur, as observed in semiconductor quantum dots^{11}. At higher fields (Fig. 1c), snake states at the p–n interface govern the electronic properties. Finally (Fig. 1d) the system enters the quantum Hall regime^{12,13} where transport is dominated by edge states and Landau level mixing at the p–n interface can occur^{14,15}. Even though this article is focusing on snake states, we will discuss the phenomenology of the mentioned magnetic field regimes. By doing so, we present an integral picture of graphene p–n physics in magnetic field.
Device architecture
Figure 1e shows the design of the measured device and Fig. 1f a scanning electron microscope picture of a similar sample. In a suspended 2 × 2 μm graphene sheet, a p–n junction is formed by applying different voltages on the left (V_{left}) and right (V_{right}) bottom gates, resulting in different charge carrier concentrations n_{left} and n_{right}. The fabrication follows partly ref. 16, which we combined with a wet transfer process allowing us to align the graphene with the bottom gates. For details, see ref. 17 and Methods.
Measurements in the Fabry–Pérot and quantum Hall regime
In the following, we characterize the measured device in the zero, low and highfield regimes. Figure 2a shows a twodimensional colour map of the electrical conductance G(V_{right},V_{left}). As soon as a p–n interface is formed, G is lowered drastically. Regular Fabry–Pérot resonances in the left/right cavity are visible as oscillations perpendicular to the zero density line in the left/right cavity (horizontal/vertical white dashed line in Fig. 2a), indicating ballistic transport^{3,4}. The inset in Fig. 2a is a slice along the pp–nn diagonal (blue dashed). We estimate the mobility to be at carrier density n=1.1 × 10^{9} cm^{−2}, which we calculated by a simple parallel plate capacitor model. The mobility is mainly limited by scattering at the contacts^{3,18}.
In Fig. 2b, we show cuts along the pp–nn diagonal at different magnetic fields B and obtain quantum Hall plateaus at G=G_{0} × ν, where ν=2,6,10,... is the filling factor^{12,13} and G_{0}=e^{2}/h is the conductance quantum. Even at fields as low as 60 mT, the ν=2 plateau is visible. The colourscale map in Fig. 2c taken at 200 mT shows that plateaus develop in the unipolar region with conductance values given by the lowest number of edge modes in the left or right cavity, that is, G=G_{0} × min(ν_{right},ν_{left})=2, 6, 10 e^{2}/h. This observation compares well to experiments of ref. 14, even though we apply only 0.2 T instead of 4 T. In the bipolar region, the conductance stays well below 2e^{2}/h due to the smoothness of the p–n interface.
In a further step, we study the dispersion of the Fabry–Pérot interference pattern in low magnetic field^{10}. Figure 2d shows the numerical derivative as a function of B and , where represents the magnitude of gate voltage in the situation of antisymmetric charge density (red dashed line in Fig. 2a, that is, the np–pn diagonal). In this configuration, the device consists of two Fabry–Pérot cavities of equal length L≈0.8 μm. The darkened region where the cyclotron radius R_{c}<L will be discussed later. In Fig. 2e, a tightbinding transport calculation is shown, which reproduces the measured interference pattern very well (for details see ref. 19 and Methods). We highlight the quality of the measured graphene and the ability of the simulation to capture the complex oscillation pattern of this micronsized system in the magnetic field. The dispersion of the Fabry–Pérot oscillations can be described by bent electron trajectories such as the one sketched in the inset of Fig. 2e (refs 3, 10). The condition for constructive interference is met if the accumulated phase along such a trajectory is a multiple of 2π (see ‘Methods’). The yellow lines in Fig. 2e are numerical solutions based on such a condition.
Measurement of snake states
We now discuss the regime where snake states emerge. In Fig. 3a, a snake state at a sharp p–n junction is sketched. Consider a charge carrier trajectory starting at the grey cross with momentum k in −x direction. Due to the magnetic field, the trajectory is bent towards the p–n interface within the cyclotron diameter . If the trajectory hits the p–n interface, the hole will be transmitted to the n side with high probability due to Klein tunnelling^{20}. At the upper edge of the sample, the snake trajectory scatters at the left side, resulting in a current towards the left contact. At lower n, R_{c} is reduced. As sketched in Fig. 3b, the snake trajectory scatters to the right at the upper edge, resulting in a net current towards the right contact. With this mechanism, one expects conductance oscillations that depend on B and n, and constant conductance along curves where R_{c} is constant. In Fig. 3c, we display calculated functions n(B) for constant R_{c}. Snake states occur once the cyclotron diameter 2R_{c} is smaller than the sample width W (green dashed curve) and can be described by quasiclassical trajectories as long as R_{c} is larger than the magnetic length (red dashed line). In the regime W>2R_{c}>l_{B}, additional parabolic lines show the condition for which the number of oscillations in the snake pattern is fixed and commensurate with W, that is, m × 2R_{c}=W with m=1,2,3,....
In Fig. 3d, we show the measured conductance (bottom) and its numerical derivative dG/dB (top). The measurement exhibits strong oscillations that follow parabolalike curves. We notice that the oscillations occur on a background of strongly decreasing conductance from G≈6e^{2}/h to G<2e^{2}/h. The steep decrease indicates that the transport becomes dominated by the lowdensity region close to the p–n interface and this happens when 2R_{c}<W.
In a real p–n interface, the density does not sharply jump from the p to the n side but evolves smoothly. An electron trajectory in such a smooth p–n interface is sketched in Fig. 3e. Here the density gradient leads to an electric field E_{x} and its interplay with the perpendicular magnetic field results in E × B drift (here along y), leading to elongated cyclotron orbits. The condition R_{y}=const. can be studied in the measurement of Fig. 3f, where we show at B=120 mT. The measured oscillation pattern follows curves of constant E_{x} at the p–n interface (obtained from electrostatic simulations) as shown in the inset.
Tightbinding theory of snake states
So far, we have seen that the oscillation pattern occurs in the regime where snake states are expected (that is, 2R_{c}<W) and that the oscillations are related to transport along the p–n interface. We now present a quantitative comparison between experiment and theory. Figure 4a shows a quantum transport simulation of and on the basis of a scalable tightbinding model^{19} that fully takes into account the device geometry. The simulation compares very well with the measurement shown in Fig. 3d. The parabolalike patterns are reproduced and a similarly steep decrease of conductance is obtained. In Fig. 4b, a slice following the white dashed line in Fig. 4a is shown. The visibility ΔG/G of the oscillations reaches 30% in theory and experiment and is enabled by the strong Klein collimation at the smooth p–n interface.
Next we apply the Keldysh–Green’s function method to extract local current density profiles (see Methods) at high and low conductance along this line. In Fig. 4c, we show the x component of the current density j_{x}, taken at (dashed circle in Fig. 4b). The current is injected from the left contact using a small d.c. offset. In the left cavity, a complex resonance pattern appears, given by socalled ‘bubbling’ trajectories^{21}, which are reflected before reaching the p–n interface and do not contribute to current between the contacts. The pattern relevant for transport is located at the p–n interface (dashed line). We observe that j_{x} changes sign along the p–n interface and that the blue and red regions penetrate the dashed line, indicating that transport is dominated by Kleincollimated snake trajectories. As a guide to the eye, we added a curve in Fig. 4d–g that follows the snake state. This is done for different values for which G is maximal/minimal at a fixed magnetic field of 90 mT (coloured circles in Fig. 4b). In Fig. 4d, for example, the current density profile corresponds to a conductance maximum where the current points to the right at the upper edge of the p–n interface. One period is added by changing from one maximum to the next. More current density profiles evolving with at fixed B=90 mT are shown in Supplementary Movie 1. By tracing along one of the parabolalike patterns, the current density profile of the snake state stays constant (an example is given in Supplementary Movie 2).
The conductance oscillates as a function of the ratio W/2R_{y}, the exact snake period corresponding to 4R_{y} is, however, difficult to determine using quasiclassical trajectories, since current is injected from many points under various angles resulting in a complex cusp structure similar to what was predicted in refs 22, 23, 24, 25. The excellent agreement between measurement and calculated conductance for which we could determine local current density profiles clearly indicates that the oscillations result from snake state trajectories.
There are additional parabolalike structures at lower magnetic field indicated by arrows in Fig. 4a, these structures are, however, less pronounced in the experimental data. Those resonances occur in a regime where scar states (Fig. 1b) would be expected. The parabolalike behaviour indicates that the states are commensurate with a cavity dimension. In the model, the resonances disappear for nonreflective contacts as expected for scar states.
Discussion
We investigated the magnetoconductance of a ballistic graphene p–n junction in different magnetic field regimes. We have observed resonance patterns occurring in the intermediate quasiclassical regime in experiment and theory, which result from the formation of snake states at the p–n interface. Among many other possibilities, these states can be used to guide electrons on arbitrary paths with a high efficiency even at very low magnetic fields. This could be used to guide electrons away from sample edges to suppress uncontrolled momentum or spin scattering. The directional scattering at the sample boundaries could be used to implement multiterminal switches^{24,26}. Furthermore, the similarity between Andreev reflection and Klein tunnelling is stressed in theory^{27} leading to a correspondence of snake states and Andreev edge states which are of theoretical^{28} and experimental^{29} interest. Our work points out that snake states are highly tunable and occur at low fields and that ballistic graphene p–n junctions in a magnetic field reveal novel and intriguing phenomena.
Methods
Experimental methods
Highquality graphene is obtained by in situ current annealing^{30}. All the measurements were performed in a variable temperature Helium cryostat with a base temperature of 1.5 K. We measured differential conductance G=dI/dV by standard lockin technique applying an a.c. voltage of 0.1 mV at 77 Hz.
For the quantum Hall data of Fig. 2b,c, we subtracted a contact resistance of 1.2 kΩ.
We extracted the cavity length L used in Fig. 2d,e from the spacing Δn between resonant Fabry–Pérot peaks in the bipolar situation and obtained L≈0.8 μm.
Simulation methods
Realspace Green’s function method in the tightbinding framework using a scaled graphene Hamiltonian^{19} is applied to simulate ballistic quantum transport in the present device, taking into account the realistic onsite energy profile obtained by threedimensional electrostatic simulation for the selfpartial capacitances of the bottom gates. All the presented conductance simulations are obtained by calculating the transmission function at zero temperature, combined with the contact resistance 1.2 kΩ. Local current densities are imaged by applying the Keldysh–Green’s function method in the linear response regime^{31} on the basis of the same model Hamiltonian used for conductance simulation. At each lattice site n, the bond charge current density J_{n}=∑_{m}e_{n→m}‹J_{n→m}› is computed, where the summation runs over all the sites m nearest to n, e_{n→m} is the unit vector pointing from n to m, and ‹J_{n→m}› is the quantum statistical average of the bond charge current operator J_{n→m} (ref. 32). After computing for each site, the positiondependent current density profile J(x,y)=[j_{x}(x,y),j_{y}(x,y)] is imaged. In Fig. 4c–g, the x component j_{x}(x,y) is shown.
The lowfield Fabry–Pérot interference contours sketched in Fig. 2e are numerically obtained from solving the resonance condition ΔΦ=2jπ, arising from the path difference between the directly transmitted and twice reflected trajectories within the p cavity as sketched in the inset of Fig. 2e, which is found to be the major interference contribution. For such a simplified model, the phase difference is given by ΔΦ=Φ_{WKB}+Φ_{AB}+Φ_{Berry}+Φ_{0}, where is the kinetic WKBphase, Φ_{AB}=δA × eB/ℏ is the Aharanov–Bohm phase due to the flux enclosed by the bent orbit segments, is the Berry phase, and Φ_{0}=π is a constant phase due to reflections off the two p–n junction interfaces of the p cavity (smooth at the middle and sharper at the contact side). Here k_{F} is the numerical average of within the p cavity. The cavity size L is numerically determined and is about half of the flake length L/2=840 nm. The loop area is given by δA=R_{c}^{2}(φ−sin φ) with φ=2 arcsin (L/2R_{c}) and R_{c}=ℏ k_{F}/eB. The form of the Berry phase follows from the consideration of ref. 33 with the critical field estimated by , where the critical transmission value T_{c} is a parameter close to one and does not significantly influence the shape of the contours; T_{c}=0.95 is chosen. The contours sketched in Fig. 2e correspond to j=1,2,⋯,8.
Additional information
How to cite this article: Rickhaus, P. et al. Snake trajectories in ultraclean graphene p–n junctions. Nat. Commun. 6:6470 doi: 10.1038/ncomms7470 (2015).
References
van Houten, H. et al. Coherent electron focusing with quantum point contacts in a twodimensional electron gas. Phys. Rev. B Condens. Matter 39, 8556–8575 (1989) .
Taychatanapat, T., Watanabe, K., Taniguchi, T. & JarilloHerrero, P. Electrically tunable transverse magnetic focusing in graphene. Nat. Phys. 9, 225–229 (2013) .
Rickhaus, P. et al. Ballistic interferences in suspended graphene. Nat. Commun. 4, 2342 (2013) .
Grushina, A. L., Ki, D.K. & Morpurgo., A. F. A ballistic pn junction in suspended graphene with split bottom gates. Appl. Phys. Lett. 102, 223102 (2013) .
Huard, B. et al. Transport measurements across a tunable potential barrier in graphene. Phys. Rev. Lett. 98, 236803 (2007) .
Özyilmaz, B. et al. Electronic transport and quantum hall effect in bipolar graphene pnp junctions. Phys. Rev. Lett. 99, 166804 (2007) .
Gorbachev, R. V., Mayorov, A. S., Savchenko, A. K., Horsell, D. W. & Guinea, F. Conductance of pnp graphene structures with "airbridge" top gates. Nano Lett. 8, 1995–1999 (2008) .
Ye, P. D., Weiss, D. & Gerhardts, R. R. Electrons in a periodic magnetic field induced by a regular array of micromagnets. Phys. Rev. Lett. 74, 3013 (1995) .
Williams, J. R. & Marcus, C. M. Snake states along graphene pn junctions. Phys. Rev. Lett. 107, 046602 (2011) .
Young, A. F. & Kim, P. Quantum interference and klein tunnelling in graphene heterojunctions. Nat. Phys. 5, 222–226 (2009) .
Bird, J. et al. Leadorientationdependent wave function scarring in open quantum dots. Phys. Rev. Lett. 82, 4691–4694 (1999) .
Novoselov, K. S. et al. Twodimensional gas of massless dirac fermions in graphene. Nature 438, 197–200 (2005) .
Zhang, Y., Tan, Y.W., Stormer, H. L. & Kim, P. Experimental observation of the quantum hall effect and berry's phase in graphene. Nature 438, 201 (2005) .
Williams, J. R., Dicarlo, L. & Marcus, C. M. Quantum hall effect in a gatecontrolled pn junction of graphene. Science 317, 638–641 (2007) .
Abanin, D. A. & Levitov, L. S. Quantized transport in graphene pn junctions in a magnetic field. Science 317, 641–643 (2007) .
Tombros, N. et al. Large yield production of high mobility freely suspended graphene electronic devices on a polydimethylglutarimide based organic polymer. J. Appl. Phys. 109, 093702 (2011) .
Maurand, R. et al. Fabrication of ballistic suspended graphene with localgating. Carbon 79, 486–492 (2014) .
Cayssol, J., Huard, B. & GoldhaberGordon, D. Contact resistance and shot noise in graphene transistors. Phys. Rev. B 79, 075428 (2009) .
Liu, M.H. et al. Scalable tightbinding model for graphene. Phys. Rev. Lett. 114, 036601 (2015) .
Cheianov, V. & Falko, V. Selective transmission of Dirac electrons and ballistic magnetoresistance of np junctions in graphene. Phys. Rev. B 74, 041403 (2006) .
Carmier, P., Lewenkopf, C. & Ullmo, D. Semiclassical magnetotransport in graphene np junctions. Phys. Rev. B 84, 195428 (2011) .
Davies, N. et al. Skipping and snake orbits of electrons: singularities and catastrophes. Phys. Rev. B 85, 155433 (2012) .
Patel, A. A., Davies, N., Cheianov, V. & Fal'ko, V. I. Classical and quantum magnetooscillations of current flow near a pn junction in graphene. Phys. Rev. B 86, 081413 (2012) .
Milovanovic, S. P., Ramezani Masir, M. & Peeters, F. M. Spectroscopy of snake states using a graphene Hall bar. Appl. Phys. Lett. 103, 233502 (2013) .
Milovanovic, S. P., Ramezani Masir, M. & Peeters, F. M. Magnetic electron focusing and tuning of the electron current with a pnjunction. J. Appl. Phys. 115, 043719 (2014) .
Chen, J.C., Xie, X. C. & Sun, Q.f. Current oscillation of snake states in graphene pn junction. Phys. Rev. B 86, 035429 (2012) .
Beenakker, C., Akhmerov, A., Recher, P. & Tworzydlo, J. Correspondence between Andreev reflection and Klein tunneling in bipolar graphene. Phys. Rev. B 77, 075409 (2008) .
Hoppe, H., Zulicke, U. & Schon, G. Andreev reflection in strong magnetic fields. Phys. Rev. Lett. 84, 1804–1807 (2000) .
Rickhaus, P., Weiss, M., Marot, L. & Schonenberger, C. Quantum hall effect in graphene with superconducting electrodes. Nano Lett. 12, 1942–1945 (2012) .
Moser, J., Barreiro, A. & Bachtold., A. Currentinduced cleaning of graphene. Appl. Phys. Lett. 91, 163513 (2007) .
Cresti, A., Grosso, G. & Parravicini, G. Electronic states and magnetotransport in unipolar and bipolar graphene ribbons. Phys. Rev. B 77, 115408 (2008) .
Nikolic, B., Zarbo, L. & Souma, S. Imaging mesoscopic spin Hall flow: spatial distribution of local spin currents and spin densities in and out of multiterminal spinorbit coupled semiconductor nanostructures. Phys. Rev. B 73, 075303 (2006) .
Shytov, A., Rudner, M. & Levitov., L. Klein backscattering and FabryPerot interference in graphene heterojunctions. Phys. Rev. Lett. 101, 156804 (2008) .
Acknowledgements
M.H.L. and K.R. acknowledge financial support by the Deutsche Forschungsgemeinschaft (SFB 689 and SPP 1666). This work was further funded by the Swiss National Science Foundation, the Swiss Nanoscience Institute, the Swiss NCCR QSIT, the Graphene Flagship, the ESF programme Eurographene, the EU FP7 project SE2ND, the ERC Advanced Investigator Grant QUEST and ERC grant 258789 and OTKA grant K112918. We thank Andreas Baumgartner, Laszlo Oroszlany and Szabolcs Csonka for fruitful discussions.
Author information
Authors and Affiliations
Contributions
P.R., P.M. and E.T. fabricated the devices. The measurements were performed by P.R., P.M., R.M., E.T. and M.W. M.H.L. performed the simulations. C.S. and K.R. guided the work. All the authors worked on the manuscript.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing financial interests.
Supplementary information
Supplementary Movie 1
In this movie we show the local current density distribution in x (jx) and ydirection (jy) for a fixed magnetic field of 90 mT. By tuning the densities in the p and n cavities (via \tilde V) the number of nodes of the snake state along the interface is changing, resulting in conductance oscillations. (MOV 9652 kb)
Supplementary Movie 2
In this movie the density and magnetic field are changed such, that the conductance stays constant, i.e. we sweep along one of the parabolalike lines. Even though the pattern in the left cavity is strongly changing, the snake state along the interface remains unchanged. (MOV 8771 kb)
Rights and permissions
This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/
About this article
Cite this article
Rickhaus, P., Makk, P., Liu, MH. et al. Snake trajectories in ultraclean graphene p–n junctions. Nat Commun 6, 6470 (2015). https://doi.org/10.1038/ncomms7470
Received:
Accepted:
Published:
DOI: https://doi.org/10.1038/ncomms7470
This article is cited by

Electronic materials with nanoscale curved geometries
Nature Electronics (2022)

Upstream modes and antidots poison graphene quantum Hall effect
Nature Communications (2021)

Electrostatic superlattices on scaled graphene lattices
Communications Physics (2020)

Interplay of filling fraction and coherence in symmetry broken graphene pn junction
Communications Physics (2020)

Fabry–Pérot resonances and a crossover to the quantum Hall regime in ballistic graphene quantum point contacts
Scientific Reports (2019)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.