Control of magnetism on the atomic scale is becoming essential as data storage devices are miniaturized. We show that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, exhibit two magnetic states, the Néel states, that are stable for hours at low temperature. For the smallest structures, we observed transitions between Néel states due to quantum tunneling of magnetization. We sensed the magnetic states of the designed structures using spin-polarized tunneling and switched between them electrically with nanosecond speed. Tailoring the properties of neighboring antiferromagnetic nanostructures enables a low-temperature demonstration of dense nonvolatile storage of information.

Frank Gaitan and Lane Clark Ramsey Numbers and Adiabatic Quantum Computing Phys. Rev. Lett. 108, 010501 (2012) The graph-theoretic Ramsey numbers are notoriously difficult to calculate. In fact, for the two-color Ramsey numbers R(m,n) with m,n≥3, only nine are currently known. We present a quantum algorithm for the computation of the Ramsey numbers R(m,n). We show how the computation of R(m,n) can be mapped to a combinatorial optimization problem whose solution can be found using adiabatic quantum evolution. We numerically simulate this adiabatic quantum algorithm and show that it correctly determines the Ramsey numbers R(3,3) and R(2,s) for 5≤s≤7. We then discuss the algorithm’s experimental implementation, and close by showing that Ramsey number computation belongs to the quantum complexity class quantum Merlin Arthur.

Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resolution of spin relaxation measurements to the atomic scale has been a challenge. We show how a scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers were found to vary between 50 and 250 nanoseconds. Our method can in principle be generalized to monitor the temporal evolution of other dynamical systems.

1) IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120, USA 2) Institute of the Physics of Nanostructures, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland Experimental Setup: We use an ultra-high-vacuum scanning tunneling microscope (STM) operating at an adjustable temperature of 0.6 K to 10 K. Magnetic fields up to 7 T were applied perpendicular to the sample surface. The fast voltage pulses are applied to the STM tip, but throughout this report we use the commonly employed convention of specifying the voltage of the sample with respect to the tip. For pump-probe measurements the tip-sample distance was fixed by setting the tunnel current to 1 nA at +10.0 mV sample voltage prior to opening the feedback loop. The pump and probe voltage pulses were generated as continous pulse trains by a pulse pattern generator (Agilent 81110A). The pump-probe cycle was repeated every 2 µs and the probe pulse was chopped at 810 Hz. The pump and probe pulses were summed, attenuated by 20 dB and applied to the tip of the STM. The tunnel current is detected at the sample and fed to a current preamplifier (Femto DLPCA-200) with ~1 kHz bandwidth and from there to a lockin amplifier to selectively detect the 810 Hz component of the current corresponding to the tunnel current of the probe pulse. For the measurements shown in Fig. 2C, Fig. 4 and Fig. S2 the pump and probe pulses were 100 ns FWHM long with 50 ns linear-ramp rise and fall times. The amplitude of the pump pulse was −36.5 mV and thereby well above the −16.7 mV threshold for spin excitation. The probe pulse voltage was −4.0 mV leading to a baseline of N = 341 electrons per probe pulse as measured when the probe precedes the pump. The pulse parameters used in Fig. 3A are the same except for a lowered probe-pulse voltage of −1.2 mV and the variable pump-pulse voltage. Spin-polarized STM measurements on Fe-Cu dimers: The spin-polarized tip used in this work has one magnetic atom, Mn, attached to the otherwise non-magnetic Cu-coated apex. The magnetic atom that was used to create spin-polarization in the tip was picked up after the Fe-Cu dimers were assembled by vertical atom manipulation (S1, S2). The magnetic moment of the attached atom is aligned parallel to the external magnetic field. This also determines the direction of the tip's spin-polarization (S3). Since the magnetic atom at the tip apex is adsorbed directly on the metal surface of the tip (without a decoupling layer such as Cu

Reich discusses the Schön case and how it was handled by the journals that had been publishing his high-impact papers and by scientists in the affected fields.

Reich discusses the Schön case and how it was handled by the journals that had been publishing his high-impact papers and by scientists in the affected fields.

: Highlights of the full reporting period include the construction of a custom-built low-temperature high magnetic field STM with ultra high vacuum sample preparation, the first of its kind. Key experimental results were obtained in STM studies of Kondo lattice and in behavior of Kondo effect in magnetic fields. IBM achieved its milestone of demonstrating single-atom spin-flip spectroscopy, published in Science (2004). IBM finalized its work on the spectroscopic properties of physisorbed molecular hydrogen in nanoscopic junctions, finding unusual excitations. Finally, IBM found when Mn atoms were placed near the edge of thin oxide islands, an enhanced zero bias conductance emerged, a notably clear manifestation of the Kondo effect.

We have observed prominent nonlinearities in differential conductance spectra of H-2 on copper surfaces using a low-temperature scanning tunneling microscope. These nonlinearities result from transitions between states of H-2 with distinct conductances. Tunneling electrons drive these transitions by giving up energy to highly coverage-dependent excitations that do not correspond to known vibrational or rotational modes of H-2. The nonlinear conductance features can be modeled by extending the conventional framework for inelastic electron tunneling spectroscopy to include saturation effects.

We discuss two examples of novel information-transport and processing mechanisms in nanometre-scale structures. The local modulation and detection of a quantum state can be used for information transport at the nanometre length-scale, an effect we call a quantum mirage'. We demonstrate that, unlike conventional electronic information transport using wires, the quantum mirage can be used to pass multiple channels of information through the same volume of a solid. We discuss a new class of nanometre-scale structures called 'molecule cascades', and show how they may be used to implement a general-purpose binary-logic computer in which all of the circuitry is at the nanometre length-scale.

The motion of one molecule on a surface can cause the subsequent motion of another molecule, and so on in a cascade of molecular motion. We call such an engineered motion a “molecule cascade”. A molecule cascade is similar to a row of toppling dominoes. It offers a unique chance to study the physical mechanism of hopping molecules as well as interesting applications for information transport and computation [1]. Carbon monoxide molecules were arranged in atomically precise configurations on the copper (111) surface with a low-temperature scanning tunneling microscope (STM). We found that one configuration of three CO molecules at nearest neighbor sites is stable enough to be investigated but unstable enough to decay on the timescale of seconds. This “chevron” configuration is shown in Fig. 1. A cascade of motion is achieved by placing pairs of CO molecules such that a preceding hop sets up another chevron and so on in a chain of arbitrary length. We never saw molecule cascades moving backwards and therefore concluded that the energy loss per hopping molecule is larger than kBT. This triggered a thorough investigation of the hopping mechanism of chevron-based cascades. We studied the hopping rate as a function of temperature from 0.5 Kelvin to about 10 Kelvin. The lower temperature limit of 0.5K is given by our new STM. At low temperatures we found a hopping rate that is independent of temperature from 0.5K to about 6K (2.5s for CO) indicative of quantum tunneling. To further test this hypothesis we studied the hopping rate as a function of carbon and oxygen isotopes. We relied heavily on the use of inelastic vibrational spectroscopy to distinguish the different isotopes in-situ [2]. A molecule cascade built from heavy carbon proceeds about 4 times slower than light carbon whereas the oxygen mass plays a smaller role in this tunneling process. At temperatures above 6K we observed thermally activated hopping with very low exponential prefactors for all isotopes suggesting tunneling from thermally excited vibrational states. The Oxygen isotope does not influence the propagation speed nearly as much. A linked chevron cascade with O propagates about half as fast as the O cascade. Hence, a mass change on the Oxygen is less efficient in tuning the propagation speed of the cascade than a mass change on the Carbon. Cascades of any type can be used to transmit information from the beginning of the cascade to its end. In a molecule cascade this is inherently binary: either the molecule is in the initial or in the final state. In analogy, the dominoes are either standing upright or have fallen over. Figure 2 shows a molecule cascade based on the linked chevron design in the initial and final configuration. After triggering the cascade with the controlled motion of one molecule, all remaining hops occur spontaneously. Through the use of empirical rules we were able to engineer the intersection of molecule cascades to demonstrate logic functions such as AND, OR, fanout, and crossover. We were able to drive the input of one gate directly from the output of another, demonstrating that operating a nanoscopic circuit is possible, a first step towards integrated nanocircuits. The most complicated circuit built with molecule cascades is a three input sorter where three outputs count the number of inputs that have been triggered. By designing the same logic circuit in current CMOS technology the vast potential for nanotechnology becomes apparent: the molecule cascade circuit is 260,000 times smaller in surface area.

We demonstrate the ability to measure the energy required to flip the spin of single adsorbed atoms. A low-temperature, high–magnetic field scanning tunneling microscope was used to measure the spin excitation spectra of individual manganese atoms adsorbed on Al 2 O 3 islands on a NiAl surface. We find pronounced variations of the spin-flip spectra for manganese atoms in different local environments.