Quantum quench of Kondo correlations in optical absorption

The interaction between a single confined spin and the spins of an electron reservoir leads to one of the most remarkable phenomena of many-body physics--the Kondo effect. Electronic transport measurements on single artificial atoms, or quantum dots, have made it possible to study the effect in great detail. Here we report optical measurements on a single semiconductor quantum dot tunnel-coupled to a degenerate electron gas which show that absorption of a single photon leads to an abrupt change in the system Hamiltonian and a quantum quench of Kondo correlations. By inferring the characteristic power-law exponents from the experimental absorption line shapes, we find a unique signature of the quench in the form of an Anderson orthogonality catastrophe, induced by a vanishing overlap between the initial and final many-body wavefunctions. We show that the power-law exponent that determines the degree of orthogonality can be tuned using an external magnetic field, which unequivocally demonstrates that the observed absorption line shape originates from Kondo correlations. Our experiments demonstrate that optical measurements on single artificial atoms offer new perspectives on many-body phenomena previously studied using transport spectroscopy only.     

量子围栏中金属表面态电子概率密度分布

量子围栏是以原子为材料在金属表面构成的闭合图案,由于电子具有波动性,因此,在量子围栏内可明显观察到二维电子概率驻波.文中以圆形和方形量子围栏为例,应用有限元数学方法,借助MATLAB数学软件,用计算机求解出被束缚在围栏中金属表面态电子相应于不同本征值的波函数及概率密度分布,与实验结果一致.该方法可方便求得处在任意形状势阱中的微观粒子波函数及概率密度分布情况,为定量研究电子被微观散射物散射产生的干涉现象提供新的手段.     

Quantum phase transition in a single-molecule quantum dot

Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a many-particle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, is believed to govern many of the fascinating properties of strongly correlated systems such as heavy-fermion compounds or high-temperature superconductors. In contrast to bulk materials with very complex electronic structures, artificial nanoscale devices could offer a new and simpler means of understanding quantum phase transitions. Here we demonstrate this possibility in a single-molecule quantum dot, where a gate voltage induces a crossing of two different types of electron spin state (singlet and triplet) at zero magnetic field. The quantum dot is operated in the Kondo regime, where the electron spin on the quantum dot is partially screened by metallic electrodes. This strong electronic coupling between the quantum dot and the metallic contacts provides the strong electron correlations necessary to observe quantum critical behaviour. The quantum magnetic phase transition between two different Kondo regimes is achieved by tuning gate voltages and is fundamentally different from previously observed Kondo transitions in semiconductor and nanotube quantum dots. Our work may offer new directions in terms of control and tunability for molecular spintronics.     

Kondo effect in quantum dots and molecular devices

Kondo effect is a very important many-body phenomenon in condensed mailer physics, which explains why the resistance increases as the temperature is lowered （usually 〈10 K） in dilute magnetic alloy, and why the conductance increases as temperature is decreased in quantum dots. This paper simply introduces equilihrium and nonequilibrium Kondo effects in quantum dots together with the Kondo effect in quantum dots with even number of electrons （when the singlet and triplet states are degenerate）. Furthermore, Kondn effect in single aton/molecular transistorss is introduced, which indicates a new way in study Kondo effect.     

Kondo physics in carbon nanotubes

The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fundamental physics. For example, metallic carbon nanotubes are quantum wires that have been found to act as one-dimensional quantum dots, Luttinger liquids, proximity-induced superconductors and ballistic and diffusive one-dimensional metals. Here we report that electrically contacted single-walled carbon nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the interaction between a localized impurity magnetic moment and delocalized electrons in a metallic host, the Kondo effect has been used to explain enhanced low-temperature scattering from magnetic impurities in metals, and also occurs in transport through semiconductor quantum dots. The far greater tunability of dots (in our case, nanotubes) compared with atomic impurities renders new classes of Kondo-like effects accessible. Our nanotube devices differ from previous systems in which Kondo effects have been observed, in that they are one-dimensional quantum dots with three-dimensional metal (gold) reservoirs. This allows us to observe Kondo resonances for very large electron numbers (N) in the dot, and approaching the unitary limit (where the transmission reaches its maximum possible value). Moreover, we detect a previously unobserved Kondo effect, occurring for even values of N in a magnetic field.     

纳米材料锰掺杂量子点中的近藤效应

采用非平衡格林函数方法,探讨了量子点掺杂锰原子纳米材料的低温输运特性。结果表明,输运受到掺杂锰原子的强烈影响。具体来说,在微分电导中出现（2S＋1）个近藤劈裂峰,锰原子表现为一个多值量子化有效磁场,导致这种劈裂。     

Kondo effect in an integer-spin quantum dot

The Kondo effect--a many-body phenomenon in condensed-matter physics involving the interaction between a localized spin and free electrons--was discovered in metals containing small amounts of magnetic impurities, although it is now recognized to be of fundamental importance in a wide class of correlated electron systems. In fabricated structures, the control of single, localized spins is of technological relevance for nanoscale electronics. Experiments have already demonstrated artificial realizations of isolated magnetic impurities at metallic surfaces, nanoscale magnets, controlled transitions between two-electron singlet and triplet states, and a tunable Kondo effect in semiconductor quantum dots. Here we report an unexpected Kondo effect in a few-electron quantum dot containing singlet and triplet spin states, whose energy difference can be tuned with a magnetic field. We observe the effect for an even number of electrons, when the singlet and triplet states are degenerate. The characteristic energy scale is much larger than in the ordinary spin-1/2 case.     

Emerging local Kondo screening and spatial coherence in the heavy-fermion metal YbRh2Si2

The entanglement of quantum states is both a central concept in fundamental physics and a potential tool for realizing advanced materials and applications. The quantum superpositions underlying entanglement are at the heart of the intricate interplay of localized spin states and itinerant electronic states that gives rise to the Kondo effect in certain dilute magnetic alloys. In systems where the density of localized spin states is sufficiently high, they can no longer be treated as non-interacting; if they form a dense periodic array, a Kondo lattice may be established. Such a Kondo lattice gives rise to the emergence of charge carriers with enhanced effective masses, but the precise nature of the coherent Kondo state responsible for the generation of these heavy fermions remains highly debated. Here we use atomic-resolution tunnelling spectroscopy to investigate the low-energy excitations of a generic Kondo lattice system, YbRh(2)Si(2). We find that the hybridization of the conduction electrons with the localized 4f electrons results in a decrease in the tunnelling conductance at the Fermi energy. In addition, we observe unambiguously the crystal-field excitations of the Yb(3+) ions. A strongly temperature-dependent peak in the tunnelling conductance is attributed to the Fano resonance resulting from tunnelling into the coherent heavy-fermion states that emerge at low temperature. Taken together, these features reveal how quantum coherence develops in heavy 4f-electron Kondo lattices. Our results demonstrate the efficiency of real-space electronic structure imaging for the investigation of strong electronic correlations, specifically with respect to coherence phenomena, phase coexistence and quantum criticality.     

Orbital Kondo effect in carbon nanotubes

Progress in the fabrication of nanometre-scale electronic devices is opening new opportunities to uncover deeper aspects of the Kondo effect--a characteristic phenomenon in the physics of strongly correlated electrons. Artificial single-impurity Kondo systems have been realized in various nanostructures, including semiconductor quantum dots, carbon nanotubes and individual molecules. The Kondo effect is usually regarded as a spin-related phenomenon, namely the coherent exchange of the spin between a localized state and a Fermi sea of delocalized electrons. In principle, however, the role of the spin could be replaced by other degrees of freedom, such as an orbital quantum number. Here we show that the unique electronic structure of carbon nanotubes enables the observation of a purely orbital Kondo effect. We use a magnetic field to tune spin-polarized states into orbital degeneracy and conclude that the orbital quantum number is conserved during tunnelling. When orbital and spin degeneracies are present simultaneously, we observe a strongly enhanced Kondo effect, with a multiple splitting of the Kondo resonance at finite field and predicted to obey a so-called SU4 symmetry.     

Kondo resonance in a single-molecule transistor

When an individual molecule, nanocrystal, nanotube or lithographically defined quantum dot is attached to metallic electrodes via tunnel barriers, electron transport is dominated by single-electron charging and energy-level quantization. As the coupling to the electrodes increases, higher-order tunnelling and correlated electron motion give rise to new phenomena, including the Kondo resonance. To date, all of the studies of Kondo phenomena in quantum dots have been performed on systems where precise control over the spin degrees of freedom is difficult. Molecules incorporating transition-metal atoms provide powerful new systems in this regard, because the spin and orbital degrees of freedom can be controlled through well-defined chemistry. Here we report the observation of the Kondo effect in single-molecule transistors, where an individual divanadium molecule serves as a spin impurity. We find that the Kondo resonance can be tuned reversibly using the gate voltage to alter the charge and spin state of the molecule. The resonance persists at temperatures up to 30 K and when the energy separation between the molecular state and the Fermi level of the metal exceeds 100 meV.     

Coulomb blockade and the Kondo effect in single-atom transistors

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems. Experiments to date have examined a multitude of molecules conducting in parallel, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions or scanning probes as well as three-terminal single-molecule transistors made from carbon nanotubes, C(60) molecules, and conjugated molecules diluted in a less-conducting molecular layer. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect.     

Thermal radiation scanning tunnelling microscopy

In standard near-field scanning optical microscopy (NSOM), a subwavelength probe acts as an optical 'stethoscope' to map the near field produced at the sample surface by external illumination. This technique has been applied using visible, infrared, terahertz and gigahertz radiation to illuminate the sample, providing a resolution well beyond the diffraction limit. NSOM is well suited to study surface waves such as surface plasmons or surface-phonon polaritons. Using an aperture NSOM with visible laser illumination, a near-field interference pattern around a corral structure has been observed, whose features were similar to the scanning tunnelling microscope image of the electronic waves in a quantum corral. Here we describe an infrared NSOM that operates without any external illumination: it is a near-field analogue of a night-vision camera, making use of the thermal infrared evanescent fields emitted by the surface, and behaves as an optical scanning tunnelling microscope. We therefore term this instrument a 'thermal radiation scanning tunnelling microscope' (TRSTM). We show the first TRSTM images of thermally excited surface plasmons, and demonstrate spatial coherence effects in near-field thermal emission.     

Observation of the two-channel Kondo effect

Some of the most intriguing problems in solid-state physics arise when the motion of one electron dramatically affects the motion of surrounding electrons. Traditionally, such highly correlated electron systems have been studied mainly in materials with complex transition metal chemistry. Over the past decade, researchers have learned to confine one or a few electrons within a nanometre-scale semiconductor 'artificial atom', and to understand and control this simple system in detail(3). Here we combine artificial atoms to create a highly correlated electron system within a nano-engineered semiconductor structure. We tune the system in situ through a quantum phase transition between two distinct states, each a version of the Kondo state, in which a bound electron interacts with surrounding mobile electrons. The boundary between these competing Kondo states is a quantum critical point-namely, the exotic and previously elusive two-channel Kondo state, in which electrons in two reservoirs are entangled through their interaction with a single localized spin.     

磁性杂质对量子点内电子态密度Kondo峰的影响

讨论了一个耦合于量子点的磁性杂质,当两边是铁磁性导线时量子点上电子的态密度Kondo峰的变化情况。用格林函数运动方程的方法和特定的自洽方法得到了态密度的解析表达式。杂质与量子点上电子的耦合使得原本简并的电子能级分裂。数值计算结果表明当两边的铁磁导线极化反平行时,态密度的Kondo峰几乎不随着磁性杂质方位角的变化而变化。当极化平行时,会出现3个Kondo峰,并且峰之间的间隔随着杂质的方位角的增大而增大。如果选取适当极化率的铁磁导线,由导线的铁磁性引起的Kondo峰的分裂可以被杂质的耦合作用抵?消掉。?     

Detecting excitation and magnetization of individual dopants in a semiconductor

An individual magnetic atom doped into a semiconductor is a promising building block for bottom-up spintronic devices and quantum logic gates. Moreover, it provides a perfect model system for the atomic-scale investigation of fundamental effects such as magnetism in dilute magnetic semiconductors. However, dopants in semiconductors so far have not been studied by magnetically sensitive techniques with atomic resolution that correlate the atomic structure with the dopant's magnetism. Here we show electrical excitation and read-out of a spin associated with a single magnetic dopant in a semiconductor host. We use spin-resolved scanning tunnelling spectroscopy to measure the spin excitations and the magnetization curve of individual iron surface-dopants embedded within a two-dimensional electron gas confined to an indium antimonide (110) surface. The dopants act like isolated quantum spins the states of which are governed by a substantial magnetic anisotropy that forces the spin to lie in the surface plane. This result is corroborated by our first principles calculations. The demonstrated methodology opens new routes for the investigation of sample systems that are more widely studied in the field of spintronics-that is, Mn in GaAs (ref. 5), magnetic ions in semiconductor quantum dots, nitrogen-vacancy centres in diamond and phosphorus spins in silicon.     

Fe掺杂对Zn1 xCoxO纳米晶体结构及磁性行为的影响

利用溶胶-凝胶法制备了Fe掺杂Zn1 xCoxO(x=001~002)纳米晶体.通过XRD,TEM和VSM对其结构、晶格参数和磁性进行了表征和分析.所有样品都没有检测到非晶态产物以及Fe,Co团簇等第二相,即Fe掺杂没有改变Zn1 xCoxO晶体的纤锌矿型ZnO结构、晶粒度大小以及室温铁磁性.随着Fe掺杂浓度的提高,饱和磁化强度、剩余磁化强度和矫顽力等磁性参数都有相应的改善.Fe掺杂还引起了交换偏置,并且,该现象随着Fe掺杂浓度的提高而增强;但是,单原子Fe对饱和磁化强度的贡献低于单原子Co对自发磁化强度的贡献,这表明Fe,Co共掺杂ZnO材料与Co掺杂ZnO材料的磁性机制有所不同.     

Quantum tunnelling of the magnetization in a monolayer of oriented single-molecule magnets

A fundamental step towards atomic- or molecular-scale spintronic devices has recently been made by demonstrating that the spin of an individual atom deposited on a surface, or of a small paramagnetic molecule embedded in a nanojunction, can be externally controlled. An appealing next step is the extension of such a capability to the field of information storage, by taking advantage of the magnetic bistability and rich quantum behaviour of single-molecule magnets (SMMs). Recently, a proof of concept that the magnetic memory effect is retained when SMMs are chemically anchored to a metallic surface was provided. However, control of the nanoscale organization of these complex systems is required for SMMs to be integrated into molecular spintronic devices. Here we show that a preferential orientation of Fe(4) complexes on a gold surface can be achieved by chemical tailoring. As a result, the most striking quantum feature of SMMs-their stepped hysteresis loop, which results from resonant quantum tunnelling of the magnetization-can be clearly detected using synchrotron-based spectroscopic techniques. With the aid of multiple theoretical approaches, we relate the angular dependence of the quantum tunnelling resonances to the adsorption geometry, and demonstrate that molecules predominantly lie with their easy axes close to the surface normal. Our findings prove that the quantum spin dynamics can be observed in SMMs chemically grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.     

A multi-component Fermi surface in the vortex state of an underdoped high-Tc superconductor

To understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high-transition-temperature (high-T(c)) copper oxide superconductors have revealed the existence of a Fermi surface akin to that in normal metals, comprising fermionic carriers that undergo orbital quantization. The unexpectedly small size of the observed carrier pocket, however, leaves open a variety of possibilities for the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we report experiments on quantum oscillations in the magnetization (the de Haas-van Alphen effect) in superconducting YBa(2)Cu(3)O(6.51) that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave model with spiral or related modulated magnetic order, consistent with experimental observations.     

锕系元素基态与激发态性质的量子拓扑化学

在基态原子价壳层电子隐核图的基础上,将拓扑理论和量子化学方法相结合,引入价电子的主量子数、角量子数、自旋磁量子数以及价电子的轨道能量等结构参数,得到一个新的量子拓扑指数Ym.利用线性回归方法分别建立了锕系元素16种基态与激发态性质的定量结构-性质关系（QSPR）模型,采用留一法和Jackknife检验相结合的方法,对定量结构-性质相关模型的稳定性和预测能力进行检验.研究结果表明,用新的量子拓扑指数Ym能较好地表征锕系元素的分子结构信息,且所建模型具有良好的稳定性和预测能力.     

Tunable quantum tunnelling of magnetic domain walls

Perhaps the most anticipated, yet experimentally elusive, macroscopic quantum phenomenon is spin tunnelling in a ferromagnet, which may be formulated in terms of domain wall tunnelling. One approach to identifying such a process is to focus on mesoscopic systems where the number of domain walls is finite and the motion of a single wall has measurable consequences. Research of this type includes magnetotransport measurements on thin ferromagnetic wires, and magnetization experiments on single particles, nanomagnet ensembles and rare-earth multilayers. A second method is to investigate macroscopic disordered ferromagnets, whose dynamics are dominated by domain wall motion, and search the associated relaxation-time distribution functions for the signature of quantum effects. But whereas the classical, thermal processes that operate in these experiments are easily regulated via temperature, the quantum processes have so far not been tunable, making difficult a definitive interpretation of the results in terms of tunnelling. Here we describe a disordered magnetic system for which it is possible to adjust the quantum tunnelling probabilities. For this material, we can model both the classical, thermally activated response at high temperatures and the athermal, tunnelling behaviour at low temperatures within a unified framework, where the domain wall is described as a particle with a fixed mass. We show that it is possible to tune the quantum tunnelling processes by adjusting the 'mass' of this particle with an external magnetic field.