UCoAl belongs to a UTX family (T is a late transition metal of 3d, 4d or 5d series, X is a p-metal Al, Ga, In or Sn) with the hexagonal crystal structure of the ZrNiAl type. In magnetic field of 0.7 T applied along the c-axis, UCoAl undergoes a metamagnetic transition to a forced ferromagnetic state [1]. The ground state of UCoAl is paramagnetic [2], so it has to be classified as itinerant metamagnet with uniquely low critical field Hcr of the transition. Due to so low Hcr value, UCoAl is extremely sensitive to any influence like dilution of the U sublattice, external pressure or substitution. When Co is substituted by T = Fe, Ru, Rh and Ir in the UCo1-xTxAl systems, Hcr rapidly decreases and already 1-2% doping stabilizes spontaneous ferromagnetism. Opposite, for T = Ni, Pd and Pt, Hcr increases and paramagnetism is stabilized.
　Influence of only one of late d metal, Os, on the metamagnetism in UCoAl was not studied previously due to metallurgical problems (melting temperature of Os exceeds evaporation temperature of Al). Now we solved these problems, prepared UCo1-xOsxAl solid solutions and studied their crystal structure and magnetic properties. Terminal compound UOsAl does not form the ZrNiAl-type structure but hexagonal Laves phase of the MgZn2 type. It is temperature-independent paramagnet, similar to the isostructural compound UFeAl. No field-induced transition is observed in fields up to 60 T. UCo1-xOsxAl solid solutions with x < 0.2 have the ZrNiAl crystal structure. The compound with x = 0.2 has traces (~2%) of an impurity phase, so we consider this Os content as a homogeneity limit.
　Magnetization measurements performed on single crystals (except for x = 0.2, this alloy was studied on isotropic polycrystal) showed that compounds with x ≥ 0.01 are ferromagnets. Spontaneous moment increases from 0.32 B/f.u. at x = 0.01 to 0.55 B/f.u. at x = 0.20, which is almost twice larger than the magnetic moment induced at the metamagnetic transition in UCoAl. TC increases from 16 K at x = 0.01 to 54 K at x = 0.20. All the compounds within the homogeneity range exhibit strong field and thermal hysteresis. At very low Os content, UCo0.998Os0.002Al, the ground state of the compound is still metamagnetic like in UCoAl. Transition field at 2 K is 0.35 T, which is already as twice as lower than that in UCoAl. Hcr increases with increasing temperature in the same way as in UCoAl. UCo0.995Os0.005Al has almost purely ferromagnetic ground state with small metamagnetic component. It has Ms = 0.3 B and TC = 8 K. The ferromagnetism can be suppressed completely by external hydrostatic pressure of only 0.1 GPa. UCo0.99Os0.01Al is already a pure ferromagnet with TC = 16 K and Ms = 0.32 B. All compounds within the homogeneity range exhibit huge uniaxial magnetic anisotropy. The anisotropy field, defined as the field where the hard-axis curve reaches Ms value (or M just above metamagnetic transition in the case of metamagnets), is at least 120-130 T.

[1] A.V. Andreev, R.Z. Levitin, Yu.F.Popov, R.Yu. Yumaguzhin, Sov. Phys. Solid State, 27, 1145 (1985).
[2] V. Sechovský, L. Havela, F.R. de Boer, J.J.M. Franse, P.A. Veenhuizen, J. Šebek, J. Stehno, A.V. Andreev, Physica B, 142, 283 (1986).

 Effects of the Co substitution for Fe on the magnetism of the strongly anisotropic ferrimagnets DyFe5Al7 and HoFe5Al7 (tetragonal crystal structure of the ThMn12-type) are studied using magnetization measurements in static (up to 14 T) and pulsed (up to 58 T) fields at 2-300 K on single-crystalline samples grown in a triarc furnace by modified Czochralski method. Since the Co analogue of RFe5Al7 does not form, the homogeneity range of RFe5-xCoxAl7 is expected to be limited. Solubility limit is found as x = 2.5 for both R = Dy and Ho. Within the homogeneity range the lattice parameter a decreases by ~0.7% whereas the parameter c stays constant. The easy-magnetization direction in all compounds is found to be the [110] axis, the [001] axis is the hardest direction.
 In the ground state of DyFe5Al7, the magnetic moment of the Dy sublattice dominates. The spontaneous magnetic moment is 2.1 μB in DyFe5Al7 and increases linearly to 3 μB for x = 2 due to a decrease of the moment of the 3d-metal sublattice. With increasing temperature, a compensation of the Dy and 3d-metal sublattices is observed, above Tcomp the total moment is along the 3d metal sublattice. The Curie temperature TC linearly falls from 231 K for x = 0 to 120 K for x = 2.5. The observed strong decrease of TC with increasing Co content is unexpected because the Co substitution for Fe in 3d-4f intermetallic compounds practically always leads to increase of the magnetic ordering temperatures due to the strengthening of exchange interactions. At the same time, Tcomp = 93 K does not change within the homogeneity range. The compounds display anomalies for magnetic field applied along the easy magnetization direction. Two phase transitions are observed for DyFe5Al7 at the critical fields μ0Hcr1 = 30 T and μ0Hcr2 = 53 T. Both transitions display hysteresis and are of the first order. The pronounced difference between the curves measured for field applied along the [100] and [110] axes reflects a large anisotropy within the basal plane. DyFe4.5Co0.5Al7 also displays two field-induced phase transitions of the first order, μ0Hcr1 is also 30 T, μ0Hcr2 shifts to 43 T. For the compounds with x = 1 and 2, the first-order transitions transform to a wide S-shape curve centered at 35 T. No field-induced transitions are observed along the [100] and [001] axes in all DyFe5-xCoxAl7 compounds. 
 Qualitatively similar results were observed in HoFe5-xCoxAl7 solid solutions: unusual strong decrease of TC (from 216 K for x = 0 to 67 K for x = 2.5) and almost unchanged Tcomp. (65 K - 72 K). Along the easy axis, two first-order field-induced magnetic transitions (at 17 T and 37 T) are observed for HoFe5Al7 and one transition at 27 T for HoFe4CoAl7. The magnetization curve has an S-shape for HoFe3Co2Al7. 
For HoFe5Al7, we studied substitution of Fe atoms also by Cr ones. In contrast to Co substitution where the number of 3d electrons increases, a substitution of Fe by Cr leads to a smaller number of 3d electrons. Nevertheless, TC also decreases, moreover, very drastically. In HoFe4CrAl7 it is only 22 K.

 Intermetallic compounds RCo5 (R is a rare-earth metal) have hexagonal crystal structure of the CaCu5 type. Due to lanthanide contraction, the stability of this structure decreases along the R series. The compounds with R = Er and Tm can be obtained only in very fast-cooled samples, moreover, only as a second off-stoichiometric phase in ingots with R2Co17 as a main phase. A partial replacement of Co by Al with a larger atomic radius compensates the effect of lanthanide contraction, and compounds RCo4Al with the CaCu5 crystal structure exist for all rare-earth elements including R = Er and Tm. Therefore, a systematic study of the whole RCo4Al series can be performed.
We studied high-field magnetization (up to 60 T) on single crystals of ErCo4Al and TmCo4Al which can be considered as Al solid solutions in the non-existing compounds ErCo5 and TmCo5. Single crystals were prepared by a modified Czochralski method in triarc furnace in Prague. High-field magnetization was measured in Dresden-Rossendorf High-Field Laboratory.
ErCo4Al is a ferrimagnet with TC = 503 K and Ms = 3.6 B/f.u. (at 2 K) directed along the c axis. Magnetic moment of the Co sublattice can be found as 5.4 B from the difference between moment of the Er3+ ion and total Ms. The Er sublattice dominates at low temperatures but weakens with temperature faster than the Co sublattice, so above the compensation point Tcomp = 127 K the total moment is along the Co sublattice.
ErCo4Al exhibits two metamagnetic transitions in field applied along the c axis, at 45 and 52 T (at 2 K). Magnetic moment above transitions, 9 B, is equal to MEr, so the transitions can be attributed to demagnetization of the Co sublattice. Therefore, the state is on halfway to the forced ferromagnet and next transitions should occur above 60 T. Ratio between magnetization changes at the transitions, 3:1, shows that at the first transition the Co atoms in the 3g positions are demagnetized and at the second transition - in the 2c positions where one of Co atom is substituted by Al. Critical field of the first transition decreases with increasing temperature and is extrapolated to zero at Tcomp, no transition is observed above this temperature. Second transition is not visible above 40 K. The transitions are accompanied by pronounced magnetostrictive and magnetoelastic effects. No field-induced transitions are observed in the basal plane of the crystal.
TmCo4Al is a ferrimagnet with TC = 490 K and Ms = 2.1 B/f.u. (2 K) with, as the Er analogue, uniaxial anisotropy. Below compensation point Tcomp = 87 K the total moment is along the Tm sublattice, above Tcomp along Co. Magnetic moment of the Co sublattice in assumption of collinear antiparallel arrangement of the Tm and Co sublattices can be determined as 5 B. Therefore, the observed magnetic moment 12 B after the metamagnetic transition corresponds to the forced ferromagnetic arrangement of the sublattices (M = MTm+MCo) and no more transitions should be expected in higher fields (in difference with Er analogue).
Due to high coercivity at low temperatures (in particular, at Tcomp), a phenomenon of “negative magnetization” is observed in both compounds when the magnetization of the sample, passing through Tcomp, becomes oriented against the applied field. At heating, the magnetization becomes “normal” when the coercive field decreases below value of applied field.
The high-field behavior of ErCo4Al and TmCo4Al is discussed in comparison with results on other R-Co intermetallics recently studied on the single crystals (Er2Co17 and Tm2Co17). In these compounds, the total moment in the ground state is along the Co sublattice, not along the R sublattice as in ErCo4Al.

The Dresden High Magnetic Field Laboratory (HLD) is a pulsed-field user facility which provides external and in-house researchers with the possibility to perform a broad range of scientific experiments in pulsed magnetic fields [1]. Two independent, modular capacitor banks deliver energy for ten mono- and dual-coil non-destructive pulsed magnets generating magnetic fields up to 95 T available for a wide variety of advanced experiments.
Application of high magnetic fields allows the investigation, modification and control of different states of matter. Frustrated magnets provide a promising avenue for realizing exotic quantum states of matter. We report on selected high magnetic field studies of low-dimensional and frustrated magnetic materials that have been shown to host a broad range of fascinating new and exotic phases. This includes robust magnetization and magnetostructural plateaus, spin-nematic state, superfluid phases, and supersolidity. Typically, the spin-strain interactions play an important role in these unusual states. Here, we present some recent ultrasound, magnetization, and magnetocaloric results obtained in selected frustrated and low-dimensional spin systems in high magnetic fields [2-3].
We acknowledge support by the HLD at HZDR, member of the European Magnetic Field Laboratory (EMFL) and by the DFG via SFB 1143.
[1] http://www.hzdr.de/hld
[2] V. Tsurkan et al., Sci. Adv. 3, e1601982 (2017).
[3] Zhe Wang et al., Phys. Rev. Lett. 120, 207205 (2018).

　近年, Yb系金属化合物において, これまでよく理解されてきた磁気量子臨界現象に従わない, 非従来型の量子臨界現象が複数発見され, 強相関電子系において大きな問題となっている。最近, Yb系準結晶Yb15Al34Au51が上記と共通の非従来型の量子臨界現象を発現することが発見された。驚くべきことに, この物質は常圧・ゼロ磁場で量子臨界性を発現しており、さらに驚くべきことに, 圧力を1.5GPaかけても臨界性が保たれることが観測された。また、温度Tと磁場Bの比の一つのスケーリングで表されるT/Bスケーリングの振る舞いも観測された。圧力に対してrobustな量子臨界性、およびT/Bスケーリングの振る舞いは、周期結晶-YbAlB4でも観測されている。このような新しいタイプの量子臨界現象を統一的に理解する機構として、Ybの臨界価数ゆらぎの理論が提案されている。最近、-YbAlB4の姉妹物質-YbAl1-xFexB4 (x=0.014)やYbNi3Ga9、および準結晶Yb15(Al1-xGax)34(Au1-yCuy)51において価数転移の臨界点および価数量子臨界性を示唆する実験が報告された。講演では、非従来型の量子臨界現象を統一的に説明する機構として、Ybの臨界価数ゆらぎの理論を紹介し、最近までの理論と実験の発展を議論する。

　希土類元素のイッテルビウム（以下Yb）は、4f閉殻に対して一つ電子が不足した電子配置4f 13（4f –1正孔）を持ち、電子配置が4f 1（4f –1電子）のセリウム（以下Ce）と対称な電子配置を持つ。これまでにCe化合物は数多く合成されており、磁気相互作用（局在性）と近藤効果（遍歴性）が拮抗した量子臨界点近傍の特異な電子状態、例えば重い電子系超伝導などが注目されてきた。Yb化合物においても同様の電子状態が期待されるが、その物質開拓はCeの場合に比べて簡単ではない。その理由として、Ybの蒸気圧が高く合成方法に制約があること、Ceと比べてYbの4f軌道の広がりが狭く混成が単純に原子間距離によらないことなどをあげることができる。 　我々は、原材料の蒸発を抑えた物質合成が可能なフラックス法を用いて、三元系Yb化合物探査を行っている。この講演では我々が発見した、重い電子系かつキラルらせん磁性を示すYbNi3Al9、圧力9GPa以上で磁気秩序が誘起される価数揺動物質YbNi3Ga9、極低温の0.3Kで磁気秩序を示す正方晶YbNi2Si3を中心に、派生物質も含め、その磁性や電子状態について紹介する。 　これらの研究の多くを広島大学に所属する研究者と共同してすすめており、ここに感謝する。また、日本学術振興会の研究拠点形成事業（A.先端拠点形成型）「スピンキラリティを軸にした先端材料コンソーシアム（広島大学、井上克也）」および科研費 JP16H01073、JP18H04315の助成を得ている。

電子のもつ軌道モーメントはMn酸化物の超巨大磁気抵抗や鉄系高温超伝導の形成に関与していると考えられており、その輸送特性は興味深い。しかし上述のような遷移金属の場合、原理的にスピンと軌道を分離できないため、軌道の寄与のみを純粋に調べることは難しい。一方、希土類元素Prイオンが立方対称の結晶電場中にあるとき、磁気モーメントを持たずにf電子軌道である多極子の自由度のみを持つ非磁性の基底二重項が実現することがある。 中でも、立方晶PrV2Al20はPrの持つ局在f電子と伝導電子の混成が強い系として注目されている。本系は0.6 - 0.7 Kで多極子秩序を示し、さらに0.05 K以下で重い電子超伝導を示す。この多極子秩序は相転移温度が低く、かつ非磁性であるため、その全容を明らかにするには高磁場と極低温を両立した実験が必要である。 私はPrV₂Al₂₀に対して1 K以下の極低温で、最高磁場15 Teslaの磁化測定および、30 Teslaを超える高磁場下での磁気抵抗測定を主に行ってきた。その結果、B ||[100] 、12 Tesla以上でのみ新たな高磁場相が現れることを明らかにした。本講演ではこの高磁場相で観測された、大きく異方的な磁気抵抗効果を中心に議論する。

非エルミート系の研究が世界的に非常に盛んになっていますが、日本国内では長年、様子見状態が続いてきました。ようやく最近になって国内でも若手研究者が台頭してきたものの、残念ながら全体的な研究レベルは世界に立ち後れていると言わざるを得ません。そのような状況を打破すべく、本講演では、まず世界的な非エルミート系の研究の現状を概観します。その後、私が関わった非エルミート系の話題として、非エルミート・ゲージ場とアンダーソン局在について述べます。ランダム系のハミルトニアンに虚数のベクトルポテンシャルを導入すると、アンダーソン局在が壊れると同時に、対応する固有値が複素数に転移します。その仕組みを説明し、主に１次元系における結果を紹介します。

Neutron scattering is an important method for characterization of materials. Neutrons have several unique properties: (i) the scattering from light elements is similar to heavy elements in the periodic table; (ii) neutrons have a magnetic moment and unique for characterization of magnetic materials; (iii) neutrons penetrate far into many materials, and thus unique to study bulk properties and easy to use complex sample environments; and (iv) the scattering between neighboring elements and isotopes of same element can be very big. This presentation will cover different advantages of neutron scattering methods, different types of neutron-based instrumentation and examples of applications. IFE is running a reactor, JEEP II, that is used for neutron scattering. The present and coming instrumentation in our upgrade program, NcNeutron – Norwegian Center for Neutron Research (www.ncneutron.no), will be presented.

The Dirac-cone surface states of 3D topological insulators are characterized by a chiral spin texture in k-space with the electron spin locked to its parallel momentum. Ultrafast laser excitation can induce spin-polarized currents in such a topological surface state either by optical transitions between the occupied and unoccupied part of the Dirac cone or directly by accelerating the electrons in a strong THz field. We monitor the resulting asymmetric electron population in momentum space by time- and angle-resolved photoelectron spectroscopy. I will demonstrate that the surface photocurrents can be controlled on the femtosecond timescale by the strength and the polarization of the excitation pulses. The elastic scattering times of the electrons carrying the current reach values up to 2.5 ps corresponding to mean-fee pathways of 0.75 μm. Our results suggest that topological insulators are a promising platform for a novel, light-wave driven electronics.

Recent theoretical studies of topologically nontrivial electronic states have pointed to the importance of spin-orbit coupling (SOC) for stabilizing these states. The assertion is held even in Kondo insulators, however, its systematic experimental study remains elusive [1, 2].
Here we present the successful growth of the substitution series Ce₃Bi₄(Pt_1-xPd_x)₃ (0≤x≤1) of the archetypal noncentrosymmetric Kondo insulator Ce₃Bi₄Pt₃. The Pt-Pd substitution is isostructural, isoelectronic, and isosize. It therefore is likely to leave the Kondo coupling and the chemical potential essentially unchanged. By contrast, the large mass difference between the 5d element Pt and the 4d element Pd leads to a large difference in the SOC, which thus is the dominating tuning parameter in the series.  With increasing x we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce₃Bi₄Pd₃ show thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.

[1] Hsin-Hua Lai et al., PNAS 115, 93-97 (2018).
[2] S. Dzaber et al., Phys. Rev. Lett. 118, 246601 (2017).