Finite-bias conductance anomalies at a singlet-triplet crossing

Molecular systems can exhibit a complex, chemically tailorable inner structure which allows for targeting of specific mechanical, electronic and optical properties. At the single-molecule level, two major complementary ways to explore these properties are molecular quantum-dot structures and scanning probes. This article outlines comprehensive principles of electron-transport spectroscopy relevant to both these approaches and presents a new, high-resolution experiment on a high-spin single-molecule junction exemplifying these principles. Such spectroscopy plays a key role in further advancing our understanding of molecular and atomic systems, in particular the relaxation of their spin. In this joint experimental and theoretical analysis, particular focus is put on the crossover between resonant regime [single-electron tunneling (SET)] and the off-resonant regime [inelastic electron (co)tunneling (IETS)]. We show that the interplay of these two processes leads to unexpected mirages of resonances not captured by either of the two pictures alone. Although this turns out to be important in a large fraction of the possible regimes of level positions and bias voltages, it has been given little attention in molecular transport studies. Combined with nonequilibrium IETSfour-electron pump-probe excitations -these mirages provide crucial information on the relaxation of spin excitations. Our encompassing physical picture is supported by a master-equation approach that goes beyond weak coupling. The present work encourages the development of a broader connection between the fields of molecular quantum-dot and scanning probe spectroscopy.

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“…These effects are included in Eq. (23), which was shown 201 to reproduce the experimental data of Ref. 199 in detail.…”

Section: Breaking the Rules Of Transport Spectroscopysupporting

confidence: 62%

Transport mirages in single-molecule devices

Gaudenzi

Misiorny

Burzurí

et al. 2017

The Journal of Chemical Physics

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“…30, the causal field superoperators G ± [Eq. (46)] generate a basis for the Liouville space of the quantum dot, in close analogy with the construction of the usual fermion Fock-basis in the many-particle Hilbert space. The 4-dimensional Hilbert-Fock space of the quantum dot is spanned by the orthogonal state vectors…”

Section: Causal Basis For Liouville-fock Space: Super-pauli Principlementioning

confidence: 99%

“…(54) holds for the partial traces of the corresponding dot or reservoir superoperators G q 1 [Eq. (46)] and J q 1 [Eq. (47)].…”

Section: Appendix A: Field Superoperatorsmentioning

confidence: 99%

“…If one uses in Eq. (46) and (47) instead of d 1 and b 1 mutually anticommuting sets of fermion operators d ′ 1 and b ′ 1 = Γ rσ /2πa ′ 1 , constructed in Sec. II A, then one obtains the same anticommutation relations (50), (51) for the resulting field superoperators.…”

Section: Appendix A: Field Superoperatorsmentioning

confidence: 99%

“…Only recently, this idea has been combined with the real-time diagrammatic theory targeting stationary transport. 30 By itself, the real-time diagrammatic theory is already a very successful framework for the calculation of transport properties of nanoscale, strongly interacting systems, 35 allowing various levels of approximations to be systematically formulated and worked out, both analytically [36][37][38] and numerically, [39][40][41] which have found application to transport experiments [42][43][44][45][46][47] . Any general progress in simplifying or clarifying the general structure of this theory is therefore ultimately of experimental relevance since more accurate approximations come within reach.…”

Section: Introduction a Experimental Motivationmentioning

confidence: 99%

See 2 more Smart Citations

Time-dependent quantum transport: Causal superfermions, exact fermion-parity protected decay modes, and Pauli exclusion principle for mixed quantum states

Saptsov

Wegewijs

2014

Phys. Rev. B

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We extend the recently developed causal superfermion approach to the real-time diagrammatic transport theory to time-dependent decay problems. Its usefulness is illustrated for the Anderson model of a quantum dot with tunneling rates depending on spin due to ferromagnetic electrodes and / or spin polarization of the tunnel junction. This approach naturally leads to an exact result for one of the time-dependent decay modes for any value of the Coulomb interaction compatible with the wideband limit. We generalize these results to multilevel Anderson models and indicate constraints they impose on renormalization-group schemes in order to recover the exact noninteracting limit.(i) We first set up a second quantization scheme in the space of density operators constructing "causal" field superoperators using the fundamental physical principles of causality / probability conservation and fermion-parity superselection (univalence). The time-dependent perturbation series for the time-evolution is renormalized by explicitly performing the wideband limit on the superoperator level. As a result, the occurrence of destruction and creation superoperators are shown to be tightly linked to the physical short-and long-time reservoir correlations, respectively. This effective theory takes as a reference a damped local system, which may also provide an interesting starting point for numerical calculations of memory kernels in real time.(ii) A remarkable feature of this approach is the natural appearance of a fermion-parity protected decay mode which can be measured using a setup proposed earlier [Phys. Rev. B 85, 075301 (2012)]. This mode can be calculated exactly in the fully Markovian, infinite-temperature limit by leading order perturbation theory, but surprisingly persists unaltered for finite temperature, for any interaction and tunneling spin polarization.(iii) Finally, we show how a Liouville-space analog of the Pauli principle directly leads to an exact expression in the noninteracting limit for the time evolution, extending previous works by starting from an arbitrary initial mixed state including spin-and pairing coherences and two-particle correlations stored on the quantum dot. This exact result is obtained already in finite-order renormalized perturbation theory, which surprisingly is not quadratic but quartic in the field superoperators, despite the absence of Coulomb interaction. The latter fact we relate to the time-evolution of the two-particle component of the mixed state, which is just the fermion-parity operator, a cornerstone of the formalism. We illustrate how the super-Pauli principle also simplifies problems with nonzero Coulomb interaction.

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Magnetically Tuned Kondo Effect in a Molecular Double Quantum Dot: Role of the Anisotropic Exchange

et al. 2019

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We investigate theoretically and experimentally the singlet-triplet Kondo effect induced by a magnetic field in a molecular junction. Temperature dependent conductance, G(T ), is calculated by the numerical renormalization group, showing a strong imprint of the relevant low energy scales, such as the Kondo temperature, exchange and singlet-triplet splitting. We demonstrate the stability of the singlet-triplet Kondo effect against weak spin anisotropy, modeled by an anisotropic exchange. Moderate spin anisotropy manifests itself by lowering the Kondo plateaus, causing the G(T ) to deviate from a standard temperature dependence, expected for a spin-half Kondo effect. We propose this scenario as an explanation for anomalous G(T ), measured in an organic diradical molecule coupled to gold contacts. We uncover certain new aspects of the singlet-triplet Kondo effect, such as coexistence of spin-polarization on the molecule with Kondo screening and nonperturbative parametric dependence of an effective magnetic field induced by the leads.

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Finite-bias conductance anomalies at a singlet-triplet crossing

Cited by 5 publications

References 23 publications

Transport mirages in single-molecule devices

Transport mirages in single-molecule devices

Time-dependent quantum transport: Causal superfermions, exact fermion-parity protected decay modes, and Pauli exclusion principle for mixed quantum states

Magnetically Tuned Kondo Effect in a Molecular Double Quantum Dot: Role of the Anisotropic Exchange

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