Science - USA (2020-01-03)

further emphasizes the important role played
by nontrivial topology in creating the observed
DW modes.
The topological nature of the DW modes
necessarily dictates a specific spatial and en-
ergy dependence of the DW states. In par-
ticular, with increasing energy, the Majorana
mode must continuously evolve from being
localized at the DW at zero energy to being
delocalized as their energy reaches the gap
edge. In other words, the modes’localization
length increases with increasing energy, lead-
ing to an increase in the modes’spatial extent
from the DW. To visualize this evolution, we
obtained spatial differential conductance maps
(dI/dVmaps) in the vicinity of the DW ranging
in energies fromEFup to 1.5 mV, where the
first set of coherent peaks is located (Fig. 4, A
to F). AtEF, the in-gap states are confined
within an ~3-nm width. These states begin
to expand in real space with increasing en-
ergy and become far less visible at 0.85 mV be-
cause of a lack of contrast in the intensity with
respect to the rest of the area. ThedI/dVmaps
show that the DW states are present at all
energies inside the gap. However, the spatial
extent of the states grows with increasing en-
ergy, which is consistent with a topological
origin for these modes.
FeSe0.45Te0.55may have provided the first
glimpse into linearly dispersing 1D Majorana
modes. Possible future experiments include the
measurement of the fractional (4p) Josephson
effect using a flux loop or depositing magnetic
layers to generate chiral Majorana modes or
localized MBS. Beyond the Majorana fermion
context, our experimental results have two
important implications. First, our observa-
tions provide supporting evidence for the ex-
istence of topological surface states and a
Fu-Kane proximitized superconducting state
in FeSe0.45Te0.55. Second, the connection be-

tween the crystal DW and the superconduct-

ingp-phase shift provides evidence in support

of a superconducting order parameter with a

real-space sign change within a unit cell ( 42 ).

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ACKNOWLEDGMENTS

The authors thank H. Ding, S. Kivelson, and C. Kane for useful

conversations.Funding:The work in Brookhaven is supported by

the Office of Science, U.S. Department of Energy, under contract

DE-SC0012704. V.M. gratefully acknowledges support from

U.S. Department of Energy, Office of Science, Basic Energy

Sciences, under award DE-SC0014335 (STM studies) and NSF

award DMR-1610143. T.L.H. thanks the U.S. National Science

Foundation under the Materials Research Science and Engineering

Center program under NSF award DMR-1720633 (SuperSEED) for

support. M.G. and D.K.M. acknowledge support from the U.S.

Department of Energy, Office of Science, Basic Energy Sciences,

under award DE-FG02-05ER46225.Author contributions:

V.M. and Z.W. conceived the experiments; Z.W., J.O.R., and

L.J. performed the experiments; J.O.R. and S.H. wrote software for

data analysis; Z.W., J.O.R., and S.H. carried out data analysis;

G.D.G. made the samples; M.G., T.L.H., and D.K.M. carried out the

theory; and V.M. and Z.W. wrote the paper, with contributions from

all authors.Competing interests:The authors declare no competing

interests.Data and materials availability:The experimental

data and theory code of this study are available at ( 48 ).

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/367/6473/104/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S13

Tables S1 and S2

References ( 49 , 50 )

29 January 2019; resubmitted 20 July 2019

Accepted 14 November 2019

10.1126/science.aaw8419

Wanget al.,Science 367 , 104–108 (2020) 3 January 2020 4of4

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