<html><head></head><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:12px"><div id="yui_3_16_0_ym19_1_1483444183019_19972">Dear friends</div><div id="yui_3_16_0_ym19_1_1483444183019_19972">We know that applying electric field will modify the hamiltonian by adding a term E.P. My question is why the application of perpendicular electric filed on z direction( our vacuum is in z direction) is logically allowed, but not about the parallel electric field in x or y direction( which we have periodic boundary condition)? Is it possible to apply in-plane electric field(which is made of saw-like potential) along x or y direction on freestanding periodic graphene.? If yes, how?</div><div id="yui_3_16_0_ym19_1_1483444183019_19972"><br></div><div id="yui_3_16_0_ym19_1_1483444183019_19972">I appreciate any help.</div><div id="yui_3_16_0_ym19_1_1483444183019_19972"><br></div><div id="yui_3_16_0_ym19_1_1483444183019_19972"><br></div><div id="yui_3_16_0_ym19_1_1483444183019_19972">Thanks a lot</div><div id="yui_3_16_0_ym19_1_1483444183019_19972"><br></div><div id="yui_3_16_0_ym19_1_1483444183019_19972">Toktam Morshedloo, Physics Dept., PNU university</div><div id="yui_3_16_0_ym19_1_1483444183019_19972"><br></div></div></body></html>