In second-order quantum phase transitions from magnetically ordered to paramagnetic states at T = 0, tuned by pressure or chemical substitution, a quantum critical point is expected to appear with critical behavior manifesting in the slowing down of spin fluctuations in the paramagnetic state and a continuous development of the order parameter in the ordered state. Quantum criticality is discussed widely as a possible driving force for unconventional superconductivity and other exotic phenomena in correlated electron systems. In the real world, however, quantum critical points and quantum criticality are often masked by a preceding first-order transition and/or the development of competing states. Pressure tuning of the itinerant-electron helical magnet MnSi is a well-known example of the suppression of a quantum critical point due to a first-order phase transition and resulting destruction of the ordered state. Utilizing muon spin relaxation experiments, here we report that 15% Fe-substituted (Mn,Fe)Si exhibits completely different behavior with pressure tuning, including the restoration of second-order quantum critical behavior and a quantum critical point at p QPC ~ 21–23 kbar, which coincides with the T = 0 crossing point of the extrapolated phase boundary line of pure MnSi. This result is quantitatively consistent with the recent theory of itinerant-electron ferromagnets by Sang, Belitz, and Kirkpatrick, who argued that disorder would restore a quantum critical point which is otherwise hidden by a first-order transition.

Analysis of the spatial dependence of current-voltage characteristics obtained from scanning tunneling microscopy experiments indicates that the charge density wave (CDW) occurring in NbSe_{2} is subject to locally strong pinning by a non-negligible density of defects, but that on the length scales accessible in this experiment the material is in a "Bragg glass" phase where dislocations and antidislocations occur in bound pairs and free dislocations are not observed. An analysis based on a Landau theory is presented showing how a strong local modulation may produce only a weak long range effect on the CDW phase.

In order to realize significant benefits from the assembly of solid-state materials from molecular cluster superatomic building blocks, several criteria must be met. Reproducible syntheses must reliably produce macroscopic amounts of pure material; the cluster-assembled solids must show properties that are more than simply averages of those of the constituent subunits; and rational changes to the chemical structures of the subunits must result in predictable changes in the collective properties of the solid. In this report we show that we can meet these requirements. Using a combination of magnetometry and muon spin relaxation measurements, we demonstrate that crystallographically defined superatomic solids assembled from molecular nickel telluride clusters and fullerenes undergo a ferromagnetic phase transition at low temperatures. Moreover, we show that when we modify the constituent superatoms, the cooperative magnetic properties change in predictable ways.

Submitted for the MAR13 Meeting of The American Physical Society Magnetic Origin of Electronic Nematicity in NaFeAs (Part I) CARLOS J. ARGUELLO, ETHAN ROSENTHAL, ERICK ANDRADE, Department of Physics, Columbia University, RAFAEL FERNANDES, School of Astronomy and Physics, University of Minnesota, ANDREW MILLIS, Department of Physics, Columbia University, CHANGQING JIN, Institute of Physics, Chinese Academy of Sciences, ABHAY PASUPATHY, Department of Physics, Columbia University — Several experiments have shown that the parent states of the iron pnictides display electronic nematicity at high temperature, where the electronic states spontaneously break the rotational symmetry of the crystal lattice. A common feature displayed by many pnictide systems is a tetragonal to orthorhombic distortion on cooling down the system below TS and a magnetically ordered phase below TSDW . In particular, NaFeAs has a structural to orthorhombic transition (TS=54K) and a SDW transition (TSDW= 39K). This wide temperature difference between transitions makes it an excellent testing ground for the characterization of the electronic states in each one of these regimes. The electronic states of this material can be directly visualized as a function of temperature using atomic-resolution scanning tunneling microscopy/spectroscopy. Real-space images of the electronic states show domains on the micron scale, with a strong unidirectional character persisting to temperatures well above TS . These unidimensional features are found to be localized around defects in the system. We will discuss the details of the energy and temperature dependence of these features in both real space and Fourier space, as well as draw differences with the structurally similar LiFeAs. Carlos J. Arguello Department of Physics, Columbia University Date submitted: 17 Dec 2012 Electronic form version 1.4

Diluted magnetic semiconductor (DMS) exhibits unique magnetic and transport properties. The well known DMS is (Ga,Mn)As that is in thin film form so far only. However Mn dopant brings both localized spins and carriers in Ga1-xMnxAs, result in the lack of independent control of local moment and carrier densities. It is therefore difficult to study or control charge and spin variable quantitatively. Here we report that an individual control of carrier and spin can be realized in a new type of DMS for Mn-doped I-II-V type semiconductor Li1+yZn1-xMnxAs in bulk. The Li(Zn,Mn)As polycrystalline samples showed ferromagnetic transition temperature (T-C) up to 50K and spontaneous magnetization up to 2.9 mu B per Mn. The other advantage of new DMS over III-V DMSs is the availability of bulk specimens so far for these type of DMS. Moreover the new DMS shows a soft magnetic behavior with the coercive force < 100Oe which would be promising for prospective application. Muon spin relaxtion (mu SR) probed static magnetic order of full volume in the ferromagnetic region. Electrical transport properties showed Li1+y(Zn,Mn)As (y = 0.05-0.1) compounds have p-type carriers since excess Li substitutes Zn sites presumably[1].

We respond to the comment of Bramwell et al (arXiv:1111.4168v1) to our original publication (S. R. Dunsiger et al, Phys. Rev. Lett. 107, 207207 (2011)), detailing muon spin rotation measurements of the Spin Ice compound Dy2Ti2O7.

In a prototypical ferromagnet (Ga,Mn)As based on a III–V semiconductor, substitution of divalent Mn atoms into trivalent Ga sites leads to severely limited chemical solubility and metastable specimens available only as thin films. The doping of hole carriers via (Ga,Mn) substitution also prohibits electron doping. To overcome these difficulties, Masek et al. theoretically proposed systems based on a I–II–V semiconductor LiZnAs, where isovalent (Zn,Mn) substitution is decoupled from carrier doping with excess/deficient Li concentrations. Here we show successful synthesis of Li1+y(Zn1−xMnx)As in bulk materials. Ferromagnetism with a critical temperature of up to 50 K is observed in nominally Li-excess (y=0.05–0.2) compounds with Mn concentrations of x=0.02–0.15, which have p-type metallic carriers. This is presumably due to excess Li in substitutional Zn sites. Semiconducting LiZnAs, ferromagnetic Li(Zn,Mn)As, antiferromagnetic LiMnAs, and superconducting LiFeAs systems share square lattice As layers, which may enable development of novel junction devices in the future. Ferromagnetic systems produced by the transition metal doping of semiconductors may be used as components of spintronic devices. Here, a new ferromagnet, Li1+y(Zn1-xMnx)As, is prepared in bulk quantities and shown to have a critical temperature approaching 50 K.

In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.

We have performed detailed $^{57}$Fe Mössbauer spectroscopy measurements on Ba$_{0.78}$K$_{0.22}$Fe$_2$As$_2$ and BaFe$_{2-x}$Ni$_x$As$_2$ single crystal mosaics showing antiferromagnetic ordering below $T_N$ with superconductivity below $T_C$. Analysis of the Mössbauer spectra shows a decrease in the magnetic hyperfine (hf) field but no change in the magnetic volume fraction below $T_C$. This clearly indicates the coexistence of magnetism and superconductivity in these compounds. The decrease in the magnetic hf field below $T_C$ depends on the difference between $T_N$ and $T_C$, being the largest for $T_N$ close to $T_C$. Two different explanations for this observation are given. We also find that the non-magnetic volume fraction below $T_N$ correlates with the Ni doping $x$, being large for high $T_C$ and small for high $T_N$.