Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional ...superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.
Ordered assemblies of magnetic atoms on the surface of conventional superconductors can be used to engineer topological superconducting phases and realize Majorana fermion quasiparticles (MQPs) in a ...condensed matter setting. Recent experiments have shown that chains of Fe atoms on Pb generically have the required electronic characteristics to form a one-dimensional topological superconductor and have revealed spatially resolved signatures of localized MQPs at the ends of such chains. Here we report higher-resolution measurements of the same atomic chain system performed using a dilution refrigerator scanning tunnelling microscope (STM). With significantly better energy resolution than previous studies, we show that the zero-bias peak (ZBP) in Fe chains has no detectable splitting from hybridization with other states. The measurements also reveal that the ZBP exhibits a distinctive 'double eye' spatial pattern on nanometre length scales. Theoretically we show that this is a general consequence of STM measurements of MQPs with substantial spectral weight in the superconducting substrate, a conclusion further supported by measurements of Pb overlayers deposited on top of the Fe chains. Finally, we report experiments performed with superconducting tips in search of the particle-hole symmetric MQP signature expected in such measurements.
Abstract
In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted ...versus the doping level
p
. In most materials,
p
cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature,
T
c
, using the assumption that the
T
c
dependence on doping is universal. Here, we present angle-resolved photoemission studies of Bi
2
Sr
2
CaCu
2
O
8+
δ
, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to
T
c
= 91 K. We show that
p
can be determined from the underlying Fermi surfaces and that in-situ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side. Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of 2Δ
0
= (4 ± 1)
k
B
T
c
The pairing mechanism of cuprate superconductors is still under debate. Here, Valla et al. report that mass renormalization in Bi$$_{2}$$ 2 Sr$$_{2}$$ 2 CaCu$$_{2}$$ 2 O$$_{8+\delta }$$ 8+δ weakens ...with doping and disappears precisely where superconductivity disappears, eliminating phononic mechanism for pairing.
We propose an easy-to-build easy-to-detect scheme for realizing Majorana fermions at the ends of a chain of magnetic atoms on the surface of a superconductor. Model calculations show that such chains ...can be easily tuned between trivial and topological ground states. In the latter, spatially resolved spectroscopy can be used to probe the Majorana fermion end states. Decoupled Majorana bound states can form even in short magnetic chains consisting of only tens of atoms. We propose scanning tunneling microscopy as the ideal technique to fabricate such systems and to probe their topological properties.
In cuprate superconductors, superconductivity is accompanied by a plethora of orders and phenomena that complicate our understanding of superconductivity in these materials. Prominent in the ...underdoped regime, these orders weaken or vanish with overdoping. Here, we approach the superconducting phase from the more conventional overdoped side. We present angle-resolved photoemission spectroscopy studies of BiFormula: see textSrFormula: see textCaCuFormula: see textOFormula: see text, cleaved and annealed in ozone to increase the doping all the way to the non-superconducting phase. We show that the mass renormalization in the antinodal region of the Fermi surface that possibly reflects the pairing, weakens with doping and completely disappears precisely where superconductivity disappears. This is the evidence that in the overdoped regime, superconductivity is determined primarily by the coupling strength. A doping dependence and an abrupt disappearance above the transition temperature eliminate phononic mechanism of the observed renormalization and identify the onset of spin-fluctuations as its likely origin.
Borophene, a theoretically proposed two-dimensional (2D) boron allotrope
, has attracted much attention
as a candidate material platform for high-speed, transparent and flexible electronics
. It was ...recently synthesized, on Ag(111) substrates
, and studied by tunnelling and electron spectroscopy
. However, the exact crystal structure is still controversial, the nanometre-size single-crystal domains produced so far are too small for device fabrication and the structural tunability via substrate-dependent epitaxy is yet to be proven. We report on the synthesis of borophene monitored in situ by low-energy electron microscopy, diffraction and scanning tunnelling microscopy (STM) and modelled by ab initio theory. We resolved the crystal structure and phase diagram of borophene on Ag(111), but found that the domains remain nanoscale for all growth conditions. However, by growing borophene on Cu(111) surfaces, we obtained large single-crystal domains, up to 100 μm
in size. The crystal structure is a novel triangular network with a concentration of hexagonal vacancies of η = 1/5. Our experimental data, together with first principles calculations, indicate charge-transfer coupling to the substrate without significant covalent bonding. Our work sets the stage for fabricating borophene-based devices and substantiates the idea of borophene as a model for development of artificial 2D materials.
The western gray whale population is endangered with approximately 175 individuals and 33 known reproductive females. Photo-identification studies were conducted from 2002-2017 during the gray whale ...foraging season off northeastern Sakhalin Island, Russia. Despite abundant prey resources, significant variation in whales' body condition, inter-birth intervals and calf survival have been documented with limited understanding of factors that account for the observed variability. We examine sea ice concentrations at their known foraging grounds to define the maximum duration of a "foraging season". We explore the relationship between foraging season length during a female's pregnancy and post-weaning calf survival and reproduction. Approximately 77% of the variation in calf survival, which ranged annually from 10-80%, was associated with the duration of the feeding season while the mother was pregnant. Poor body conditions and prolonged inter-birth intervals of western gray whales have also been documented to coincide with shorter duration in feeding seasons found in this study. These results imply that shorter foraging seasons are associated with reduced energy intake by physically limiting the number of days gray whales can forage, and thus sea ice conditions may be one limiting factor affecting growth rates of this endangered population of baleen whales.
The composition of metabolites in the coelomic fluid of the starfish
Asterina pectinifera
and sea urchin
Mesocentrotus nudus
was studied under normal and hypoxic conditions using NMR spectroscopy. ...Their change after keeping animals under hypoxia was shown. Sea urchins revealed the presence of anaerobic metabolism, which intensifies under stress in conditions of hypoxia. No similar mechanism of energy metabolism has been found for the starfish. Under conditions of hypoxia, a significant increase in lactate, which is observed in sea urchins, is not observed in the composition of the coelomic liquid of starfish.
Asterine
tissues contain a mixture of carotenoids in large quantities, the most well-known of which is astaxanthin. The starfish
Asterina pectinifera
can be used as a raw material for new medicinal, cosmetic, and food products.
Abstract
We report a Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) study of a DyBa
2
Cu
3
O
7-δ
(DBCO) thin film (
T
c
~ 79 K) synthesized by the molecular beam epitaxy (MBE). We ...observed an unusual transfer of spectral weight in the local density of states (LDOS) spectra occurring only within the superconducting gap. By a systematic control of the tip-sample distance and the junction resistance, we demonstrate that the spectral weight transfer can be switched at a nano-meter length scale. These results suggest that an interaction between the STM tip and the sample alters the electronic configurations in the film. This probably originates from a combination of an intrinsic band bending at the interface between the surface and the bulk, and a tip-induced band bending. These results may open a new avenue for band engineering and applications of thin films of high-
T
c
cuprates.
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