Unconventional field-induced spin gap in an S=1/2 Chiral staggered chain

Physical Review Letters American Physical Society 122 (2019) 057207

Authors:

Jesse Liu, S Kittaka, Roger Johnson, T Lancaster, J Singleton, T Sakakibara, Y Kohama, J Van Tol, Arzhang Ardavan, BH Williams, SJ Blundell, ZE Manson, JL Manson, PA Goddard

Abstract:

We investigate the low-temperature magnetic properties of the molecule-based chiral spin chain ½CuðpymÞðH2OÞ4SiF6 · H2O (pym ¼ pyrimidine). Electron-spin resonance, magnetometry and heat capacity measurements reveal the presence of staggered g tensors, a rich low-temperature excitation spectrum, a staggered susceptibility, and a spin gap that opens on the application of a magnetic field. These phenomena are reminiscent of those previously observed in nonchiral staggered chains, which are explicable within the sine-Gordon quantum-field theory. In the present case, however, although the sineGordon model accounts well for the form of the temperature dependence of the heat capacity, the size of the gap and its measured linear field dependence do not fit with the sine-Gordon theory as it stands. We propose that the differences arise due to additional terms in the Hamiltonian resulting from the chiral structure of ½CuðpymÞðH2OÞ4SiF6 · H2O, particularly a uniform Dzyaloshinskii-Moriya coupling and a fourfold periodic staggered field.

Electric field control of spins in molecular magnets

Physical Review Letters American Physical Society 122:3 (2019) 037202

Authors:

J Liu, J Mrozek, WK Myers, GA Timco, B Kintzel, W Plass, Arzhang Ardavan

Abstract:

Coherent control of individual molecular spins in nanodevices is a pivotal prerequisite for fulfilling the potential promised by molecular spintronics. By applying electric field pulses during time-resolved electron spin resonance measurements, we measure the sensitivity of the spin in several antiferromagnetic molecular nanomagnets to external electric fields. We find a linear electric field dependence of the spin states in Cr7Mn, an antiferromagnetic ring with a ground-state spin of S ¼ 1, and in a frustrated Cu3 triangle, both with coefficients of about 2 rad s−1=V m−1. Conversely, the antiferromagnetic ring Cr7Ni, isomorphic with Cr7Mn but with S ¼ 1=2, does not exhibit a detectable effect. We propose that the spinelectric field coupling may be used for selectively controlling individual molecules embedded in nanodevices.

Electrically controlled nuclear polarization of individual atoms

Nature Nanotechnology Nature Publishing Group 13 (2018) 1120-1125

Authors:

K Yang, P Willke, Y Bae, A Ferrón, JL Lado, Arzhang Ardavan, J Fernández-Rossier, AJ Heinrich, CP Lutz

Abstract:

Nuclear spins serve as sensitive probes in chemistry1 and materials science2 and are promising candidates for quantum information processing3,4,5,6. NMR, the resonant control of nuclear spins, is a powerful tool for probing local magnetic environments in condensed matter systems, which range from magnetic ordering in high-temperature superconductors7,8 and spin liquids9 to quantum magnetism in nanomagnets10,11. Increasing the sensitivity of NMR to the single-atom scale is challenging as it requires a strong polarization of nuclear spins, well in excess of the low polarizations obtained at thermal equilibrium, as well as driving and detecting them individually4,5,12. Strong nuclear spin polarization, known as hyperpolarization, can be achieved through hyperfine coupling with electron spins2. The fundamental mechanism is the conservation of angular momentum: an electron spin flips and a nuclear spin flops. The nuclear hyperpolarization enables applications such as in vivo magnetic resonance imaging using nanoparticles13, and is harnessed for spin-based quantum information processing in quantum dots14 and doped silicon15,16,17. Here we polarize the nuclear spins of individual copper atoms on a surface using a spin-polarized current in a scanning tunnelling microscope. By employing the electron–nuclear flip-flop hyperfine interaction, the spin angular momentum is transferred from tunnelling electrons to the nucleus of individual Cu atoms. The direction and magnitude of the nuclear polarization is controlled by the direction and amplitude of the current. The nuclear polarization permits the detection of the NMR of individual Cu atoms, which is used to sense the local magnetic environment of the Cu electron spin.

Hyperfine interaction of individual atoms on a surface

Science American Association for the Advancement of Science 362:6412 (2018) 336-339

Authors:

Philip Willke, Yujeong Bae, Kai Yang, Jose L Lado, Alejandro Ferron, Taeyoung Choi, Arzhang Ardavan, Joaquín Fernández-Rossier, Andreas J Heinrich, Christopher P Lutz

Abstract:

Taking advantage of nuclear spins for electronic structure analysis, magnetic resonance imaging, and quantum devices hinges on knowledge and control of the surrounding atomic-scale environment. We measured and manipulated the hyperfine interaction of individual iron and titanium atoms placed on a magnesium oxide surface by using spin-polarized scanning tunneling microscopy in combination with single-atom electron spin resonance. Using atom manipulation to move single atoms, we found that the hyperfine interaction strongly depended on the binding configuration of the atom. We could extract atom- and position-dependent information about the electronic ground state, the state mixing with neighboring atoms, and properties of the nuclear spin. Thus, the hyperfine spectrum becomes a powerful probe of the chemical environment of individual atoms and nanostructures.

Molecular electronic spin qubits from a spin-frustrated trinuclear copper complex

Chemical Communications Royal Society of Chemistry 54:92 (2018) 12934-12937

Authors:

B Kintzel, M Bohme, Junjie Liu, A Burkhardt, Jakub Mrozek, A Buchholz, Arzhang Ardavan, W Plass

Abstract:

The trinuclear copper(II) complex [Cu3(saltag)(py)6]ClO4 (H5saltag = tris(2-hydroxybenzylidene)triaminoguanidine) was synthesized and characterized by experimental as well as theoretical methods. This complex exhibits a strong antiferromagnetic coupling (J = −298 cm−1) between the copper(II) ions, mediated by the N–N diazine bridges of the tritopic ligand, leading to a spin-frustrated system. This compound shows a T2 coherence time of 340 ns in frozen pyridine solution, which extends to 591 ns by changing the solvent to pyridine-d5. Hence, the presented compound is a promising candidate as a building block for molecular spintronics.